Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Study of the temperature-dependence of the far-infrared spectrum of (TMA) (I) (TCNQ) Fortier, Normand 1982

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Notice for Google Chrome users:
If you are having trouble viewing or searching the PDF with Google Chrome, please download it here instead.

Item Metadata

Download

Media
831-UBC_1982_A6_7 F68.pdf [ 2.55MB ]
Metadata
JSON: 831-1.0085031.json
JSON-LD: 831-1.0085031-ld.json
RDF/XML (Pretty): 831-1.0085031-rdf.xml
RDF/JSON: 831-1.0085031-rdf.json
Turtle: 831-1.0085031-turtle.txt
N-Triples: 831-1.0085031-rdf-ntriples.txt
Original Record: 831-1.0085031-source.json
Full Text
831-1.0085031-fulltext.txt
Citation
831-1.0085031.ris

Full Text

STUDY OF THE TEMPERATURE-DEPENDENCE OF THE FAR-INFRARED SPECTRUM OF (TMA)(I)(TCNQ) by NORMAND FORTIER B.A.Sc, Ecole Polytechnique de Montreal, 1980 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Physics We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1982 © Normand F o r t i e r , 1982 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y of B r i t i s h C o l u m b i a , I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g of t h i s t h e s i s f o r s c h o l a r l y p u rposes may be g r a n t e d by the Head of my Department or by h i s or her r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g or p u b l i c a t i o n of t h i s t h e s i s 'for f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department of P h y s i c s The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada . V6T 1W5 Date: September 1, 1982 i i Abstract The temperature-dependence (20-300K) of the f a r - i n f r a r e d spectrum (20-1000cm"1) of Trimethylammonium Iodide Tetracyanoquinodimethane (TMA)(I)(TCNQ) in i t s ordered phase, has been measured for the f i r s t time. Our measurments confirmed the existence of two phase tr a n s i t i o n s near 150 and 100K. At least three absorption bands are activated below the 150K phase t r a n s i t i o n . We have assigned two of them to the t o t a l l y symmetric mode Ag 9 and Ag 1 0 of the TCNQ molecule. We have also observed evidence of B r i l l o u i n zone s p l i t t i n g , where one band above 150K, s p l i t into three, below 150K. Since the conduction band of (TMA)(I)(TCNQ) i s 1/3 f i l l e d , we have interpreted t h i s result as suggesting that the 150K t r a n s i t i o n i s a Pe i e r l s d i s t o r t i o n . We also observe a sharp increased in infrared absorption at 365cm"1 and a weaker one at 100cm"'1. Since the D.C. conductivity measurments indicates that a 1000cm"1 band gap is present at a l l temperatures, we interpret these absorption "edges" in terms of t r a n s i t i o n s between l o c a l i s e d states in the band gap. These l o c a l i s e d states would be introduced by either chemical impurities or by some randomness in the potential at each TCNQ s i t e . T a b l e o f C o n t e n t s A b s t r a c t i i L i s t o f T a b l e s i v L i s t o f F i g u r e s v Acknowledgement v i I . INTRODUCTION 1 1.1 The P o s s i b i l i t y Of H i g h T e m p e r a t u r e S u p e r c o n d u c t i v i t y 1 1.2 A p p l i c a t i o n s 3 1.3 S t r u c t u r a l And P h y s i c a l P r o p e r t i e s Of 1-D C o n d u c t o r s 4 1.3.1 S t r u c t u r a l A s p e c t s Of 1-D C o n d u c t o r s 4 1.3.2 The C o n c e p t s Of C h a r g e T r a n s f e r 5 1.3.3 An Example: The TCNQ M o l e c u l e 5 1.3.4 The C o n c e p t Of P e r i o d i c L a t t i c e D i s t o r t i o n ....6 1.3.5 The C o n c e p t Of C h a r g e D e n s i t y Wave 7 1.4 N a t u r e Of The Phase T r a n s i t i o n In (TMA)(I)(TCNQ) ...9 I I . PHYSICAL AND STRUCTURAL PROPERTIES OF (TMA ) ( I ) ( T C N Q ) 12 2.1 C r y s t a l l o g r a p h i c S t r u c t u r e 12 2.2 T e m p e r a t u r e - d e p e n d e n c e Of The U n i t - c e l l P a r a m e t e r s 15 2.3 T e m p e r a t u r e - d e p e n d e n c e Of The D.C. C o n d u c t i v i t y ..16 2.4 The T h e r m o e l e c t r i c Power 18 2.5 O p t i c a l Measurments 19 2.6 M a g n e t i c S u s c e p t i b i l i t y 22 2.7 S p e c i f i c Heat Measurment 23 2.8 C o n c u s i o n s 24 I I I . EXPERIMENTAL METHOD 25 3.1 E x p e r i m e n t a l Method 25 IV. RESULTS AND DISCUSSION 28 4.1 E v i d e n c e Of E l e c t r o n - p h o n o n I n t e r a c t i o n s 28 4.2 A c t i v a t i o n Of The T o t a l l y Symmetric Modes A g / 9 And Ag Y"1 0 35 4.3 The S p l i t t i n g Of The D o u b l e t A t 172 And 176cm" 1 ...39 4.4 D e t e c t i o n Of A Second Phase T r a n s i t i o n 39 4.5 N a t u r e Of The 150K Phase T r a n s i t i o n 40 4.6 N a t u r e Of The Second Phase T r a n s i t i o n 41 4.7 T e m p e r a t u r e - d e p e n d e n c e Of The I n t e g r a t e d I n t e n s i t y Of The 96, 113 And 135cm- 1 Bands 41 4.8 E v i d e n c e Of H y d r o g e n Bond S t r e t c h i n g 45 4.9 D e t e c t i o n Of An A c t i v a t i o n E n e r g y Of 3 6 5 c m - 1 48 4.10 S m a l l - p a r t i c l e S c a t t e r i n g 54 4.11 C o n c l u s i o n s 56 BIBLIOGRAPHY 57 i v L i s t of Tables I . Observed frequencies of hydrogen bond s t r e t c h i n g modes (21) 46 I I . Force constants f o r phenol-trimethylamines complexes (21) 47 V L i s t of F i g u r e s 1. The TCNQ molecu le 6 2. Band s t r u c t u r e of a 1-D compound 8 3. Band s t r u c t u r e of a 1-D compound a f t e r a p e r i o d i c l a t t i c e d i s t o r t i o n 8 4. Temperature-dependence of the D . C . c o n d u c t i v i t y (6). . 1 1 5. Temperature-dependence of the D . C . c o n d u c t i v i t y ( 7 ) . . 1 1 6. (TMA)(I ) (TCNQ): p r o j e c t i o n p e r p e n d i c u l a r to the b a x i s (9) 14 7. (TMA)(I ) (TCNQ): p r o j e c t i o n p a r a l e l l to the b a x i s (11) • 1 4 8. Temperature-dependence of the u n i t - c e l l parameters (12) 15 9. The t h e r m o e l e c t r i c power (5) . . . . . 1 8 10. Frequency-dependent c o n d u c t i v i t y (15) 20 11. D i e l e c t r i c f u n c t i o n (15) 20 12. D e n s i t y of s t a t e s i n a 1-D compound 21 13. Magnet i c s u s c e p t i b i l i t y and D . C . c o n d u c t i v i t y 22 14. S p e c i f i c heat (16) 23 15. F . I . R Spectrum of (TMA)(I)(TCNQ) between 20 and 400cm" 1 29 16. Temperature-dependence of the 226cm - 1 band 32 17. Temperature-dependence of the 313cm" 1 band 33 18. Temperature-dependence of the 155,164 172 and 176cm"1 bands 34 19. Temperature-dependence of the i n t e g r a t e d i n t e n s i t y of the 313cm" 1 band . . . 3 7 20. Temperature-dependence of the i n t e g r a t e d i n t e n s i t y of the 226cm" 1 band .38 21. Temperature-dependence of the i n t e g r a t e d i n t e n s i t y of the 155, 164 and 172 bands .'. 42 22. B r i l l o u i n zone s p l i t t i n g 43 23. Temperature-dependence of the i n t e g r a t e d i n t e n s i t y of the 96, 113, 125 and 135cm"1 bands 44 24. Hydrogen bond 45 25. Temperature-dependence of the Background I n t e n s i t y B .51 26. P l o t of l n ( B ) as a f u n c t i o n of 1/T 52 27. I n f r a r e d Spectrum of (TMA)(I)(TCNQ) Between 20 and 1000cm" 1 53 28. P l o t of l o g(J» / I ) as a f u n c t i o n of l o g(A ) 55 v i Acknowledgement I w o u l d 1-ike t o t h a n k D r . J o h n E . E l d r i d g e and D r . F r a n c e s E. B a t e s f o r t h e i r h e l p and a d v i c e d u r i n g t h e c o u r s e o f t h i s work. I a l s o w o u l d l i k e t o thank D r . E l d r i d g e and t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a f o r f i n a n c i a l s u p p o r t . 1 I . I N T R O D U C T I O N 1.1 T h e P o s s i b i l i t y O f H i g h T e m p e r a t u r e S u p e r c o n d u c t i v i t y T h e s e a r c h f o r h i g h t e m p e r a t u r e s u p e r c o n d u c t i v i t y h a s b e e n o n e o f t h e d r i v i n g f o r c e s o f t h e f i e l d o f o n e - d i m e n s i o n a l o r g a n i c c o n d u c t o r s . S u p e r c o n d u c t i v i t y i n a m e t a l a r i s e s w h e n t h e v a l e n c e e l e c t r o n s o f t h e a t o m s f o r m i n g t h e l a t t i c e c o n d e n s e i n t o p a i r s , c a l l e d C o o p e r p a i r s . T h e e x i s t e n c e o f a b o u n d s t a t e b e t w e e n t w o n e g a t i v e l y c h a r g e p a r t i c l e s i s d u e t o a l a t t i c e d e f o r m a t i o n m e d i a t e d b y t h e e l e c t r o n s a s t h e y m o v e i n t h e c r y s t a l . A n e l e c t r o n p a s s i n g n e a r t w o p o s i t i v e l y c h a r g e d i o n s , w i l l p u l l t h e m t o g e t h e r , p r o d u c i n g a r e g i o n w i t h a n e t p o s i t i v e c h a r g e , a b l e t o a t t r a c t a s e c o n d e l e c t r o n . A C o o p e r p a i r i s t h e r e b y f o r m e d . I n a p e r f e c t m e t a l l i c c r y s t a l , e l e c t r i c a l r e s i s t i v i t y i s c a u s e d b y t h e s c a t t e r i n g o f c o n d u c t i o n e l e c t r o n s b y l a t t i c e v i b r a t i o n s . E a c h s c a t t e r i n g e v e n t r e d u c e s t h e m o m e n t u m o f t h e e l e c t r o n s , l e a d i n g t o a d e c a y i n g e l e c t r i c a l c u r r e n t a n d J o u l e h e a t i n g . A t T=0K i t s r e s i s t a n c e g o e s t o z e r o . T h e e l e c t r o n i c e n e r g y o f a m e t a l i n i t s s u p e r c o n d u c t i n g s t a t e i s m i n i m i s e d w h e n a l l t h e C o o p e r p a i r s h a v e t h e s a m e m o m e n t u m . T h e c h a n g e i n m o m e n t u m o f o n e p a i r r e q u i r e s t h e c h a n g e i n m o m e n t u m o f a l l t h e p a i r s . A t t h e t e m p e r a t u r e a t w h i c h t h e C o o p e r p a i r s c o n d e n s e , t h e r e i s n o t e n o u g h v i b r a t i o n a l e n e r g y s t o r e d i n t h e l a t t i c e t o a c h i e v e s u c h a r e d i s t r i b u t i o n o f t h e m o m e n t u m o f t h e p a i r s . T h u s , t h e y f l o w i n t h e c r y s t a l w i t h a c o n s t a n t m o m e n t u m . A s t h e t e m p e r a t u r e i s r a i s e d , t h e t h e r m a l a g i t a t i o n o f t h e 2 i o n s becomes l a r g e r t h a n t h e l a t t i c e d i s t o r t i o n i n d u c e d by t h e t r a v e l l i n g e l e c t r o n s , d e s t r o y i n g t h e s u p e r c o n d u c t i n g s t a t e . T h i s phenomena i s t h o u g h t t o become dominant a t a r o u n d 30K and would e x p l a i n why s u p e r c o n d u c t i v i t y has n o t been o b s e r v e d a t t e m p e r a t u r e s h i g h e r t h a n 22.3K. In 1964 W.A. L i t t l e (1) p r o p o s e d a new mechanism f o r t h e f o r m a t i o n o f Cooper p a i r s , by w h i c h t h e a t t r a c t i v e p o t e n t i a l would be i n d u c e d by an e l e c t r o n i c d i s t o r t i o n i n s t e a d o f a l a t t i c e d i s t o r t i o n , l e a d i n g t o t h e p o s s i b i l i t y o f h i g h t e m p e r a t u r e s u p e r c o n d u c t i v i t y . He i m a g i n e d a m o l e c u l a r c r y s t a l f o r m e d by a c h a i n o f c a r b o n atoms a l o n g w h i c h t h e c o n d u c t i o n e l e c t r o n s c o u l d t r a v e l , and h i g h l y p o l a r i s a b l e o r g a n i c m o l e c u l e s w o u l d be a t t a c h e d t o t h e c h a i n . As an e l e c t r o n t r a v e l l i n g a l o n g t h e c h a i n e n c o u n t e r e d an o r g a n i c m o l e c u l e , i t w o u l d p o l a r i s e i t , and t h e r e b y c r e a t e a r e g i o n of p o s i t i v e c h a r g e n e a r t h e c h a i n , w h i c h w o u l d be a b l e t o a t t r a c t a s e c o n d e l e c t r o n . T h i s t y p e o f d i s t o r t i o n , i n d u c e d by t h e t r a v e l l i n g e l e c t r o n s , i n v o l v e s t h e d i s p l a c e m e n t of l i g h t e l e c t r o n s i n s t e a d o f heavy i o n s . C o n s e q u e n t l y t h i s f o r m e r d i s p l a c e m e n t i s l a r g e r t h e n t h e l a t t e r . T h e r m a l v i b r a t i o n s would not " e r a s e " t h e d i s t o r t i o n as e a s i l y and, t h e r e f o r e , t h e C o o p e r p a i r s c o u l d e x i s t a t h i g h t e m p e r a t u r e s . O r g a n i c c h e m i s t r y has n o t a s y e t been s u c c e s f u l l i n p r o d u c i n g s u c h a m o l e c u l a r c r y s t a l , but i t m i g h t o n l y be a q u e s t i o n o f t i m e . A l r e a d y some o f t h e f a m i l y of m o l e c u l a r c r y s t a l s b a s e d on t h e m o l e c u l e T e t r a m e t h y l - t e t r a s e l e n a f u l v a l e n e (TMTSF) have become s u p e r c o n d u c t i n g a t low t e m p e r a t u r e s . 3 ( T M T S F ) a ( P F f c ) becomes a s u p e r c o n d u c t o r n e a r 1K, a t a p r e s s u r e o f 10 KBar (2). ( T M S F ^ ( C L O A ) becomes a s u p e r c o n d u c t o r a t 1.2K and a t m o s p h e r i c p r e s s u r e (3). U n t i l now, o r g a n i c s u p e r c o n d u c t i n g compounds have been one-d i m e n s i o n a l . In o r d e r t o a c h i e v e a s t a t e o f h i g h t e m p e r a t u r e s u p e r c o n d u c t i v i t y , b r i d g e s h a v e been e s t a b l i s h e d beween t h e TMTSF c o l u m n s of TMTSF compounds, i n o r d e r t o enhance t h e m o t i o n s o f t h e c o n d u c t i n g e l e c t r o n s . The r e s u l t s c o u l d be e x c e p t i o n a l ; t h r e e - d i m e n s i o n a l o r g a n i c c o n d u c t o r s , r e s e m b l i n g i n t h i s s e n s e t o n o r m a l m e t a l s , b u t w i t h a h i g h t e m p e r a t u r e s u p e r c o n d u c t i v i t y . 1.2 A p p l i c a t i o n s O r g a n i c c h e m i s t r y has t h e p r o v e n a b i l i t y of c r e a t i n g compounds w i t h s p e c i f i c a c h i t e c t u r a l s t r u c t u r e s . U n d e r s t a n d i n g t h e r e l a t i o n s h i p s between p h y s i c a l p r o p e r t i e s and s t r u c t u r a l o r g a n i s a t i o n s c o u l d l e a d t o t h e d e s i g n o f m o l e c u l a r compounds w i t h s p e c i f i c p h y s i c a l p r o p e r t i e s . P r a c t i c a l a p p l i c a t i o n s c o u l d be numerous. These compounds a r e v e r y s e n s i t i v e t o t e m p e r a t u r e , p r e s s u r e , m a g n e t i c f i e l d and r a d i a t i o n . They c o u l d t h e r e f o r e be u s e d i n t h e f a b r i c a t i o n o f a l l k i n d s of d e t e c t o r s . S t i l l i n t h e e x p e r i m e n t a l s t a g e s , d i o d e s h a v e been p r o d u c e d by a d j o i n i n g an o r g a n i c c r y s t a l (TMCLO4 ) t o an n o n - o r g a n i c m a t e r i a l (GaSb) (1). R i g h t now t h e f a b r i c a t i o n o f i n t e g r a t e d c i r c u i t s ( I C ) r e q u i r e s t h e d e p o s i t i o n of a p o l y m e r f i l m on a s e m i c o n d u c t i n g s u b s t r a c t . P a r t o f t h e f i l m i s t h e n removed, c r e a t i n g v a l l e y s t h a t w i l l be f i l l e d w i t h a m e t a l l i c m a t e r i a l . The r e m a i n d e r o f 4 t h e p o l y m e r f i l m i s t h e n w a s h e d a w a y , l e a v i n g o n l y t h e m e t a l l i c n e t w o r k o n t h e s e m i c o n d u c t i n g s u b s t r a c t . T h e p r o c e s s c o u l d b e g r e a t l y s i m p l i f i e d w i t h t h e u s e d o f o r g a n i c s e m i c o n d u c t o r s . I t h a s b e e n s h o w n (1) t h a t s o m e m o l e c u l a r c r y s t a l s - c a n b e a f f e c t e d b y a b e a m o f e l e c t r o n s . B y s e n d i n g a b e a m o f 20A i n d i a m e t e r a t t h e s u r f a c e o f a n o r g a n i c s e m i c o n d u c t o r , i t i s p o s s i b l e t o c h a n g e t h e s t a t e o f t h e m o l e c u l e s f r o m a n i n s u l a t i n g s t a t e t o a m e t a l l i c o n e , t h e r e b y e l i m i n a t i n g t h e n e e d s f o r t h e s u c c e s s i v e d e p o s i t i o n s o f p o l y m e r a n d m e t a l l i c f i l m s . I n t h e i r s u p e r c o n d u c t i n g s t a t e , t h e r e s u l t s c o u l d b e e v e n m o r e i n t e r e s t i n g : h i g h m a g n e t i c f i e l d f o r m a g n e t i c l e v i t a t i o n , u l t r a - f a s t e l e c t r o n i c d e v i c e s f o r c o m p u t e r s . A l l t h i s c o u l d b e a c h i e v e d w i t h o u t t h e u s e o f l i q u i d h e l i u m , a r a r e a n d e x p e n s i v e f l u i d . 1.3 S t r u c t u r a l A n d P h y s i c a l P r o p e r t i e s O f 1 -D C o n d u c t o r s 1.3 .1 S t r u c t u r a l A s p e c t s O f 1 -D C o n d u c t o r s T h e r e i s now a g r o w i n g n u m b e r o f 1 -D o r g a n i c c o n d u c t o r s . D e s p i t e t h i s d i v e r s i t y , a c a r e f u l e x a m i n a t i o n s h o w s t h a t c r y s t a l s e x h i b i t i n g h i g h e l e c t r o n i c c o n d u c t i v i t y , a l l m e e t s p e c i f i c s t r u c t u r a l r e q u i r e m e n t s . F i r s t o f a l l , a n y s u c e s s f u l l o r g a n i c c o n d u c t o r s m u s t b e f o r m e d b y m o l e c u l a r b l o c k s t h a t f i t c l o s e l y t o g e t h e r . T h e c l o s e - p a c k e d a r r a n g e m e n t a l l o w s t h e c o n d u c t i n g e l e c t r o n s t o m o v e e a s i l y f r o m o n e m o l e c u l e t o t h e n e x t . 5 1 . 3.2 T h e C o n c e p t s O f C h a r g e T r a n s f e r A s e c o n d a s p e c t t h a t h a s p r o v e n t o b e v e r y i m p o r t a n t i s t h e c o n c e p t o f c h a r g e t r a n s f e r . T h i s e f f e c t a r i s e s i n a c r y s t a l w h i c h i s f o r m e d b y t w o t y p e s o f m o l e c u l e s , o n e t y p e b e i n g e l e c t r o n e g a t i v e a n d t h e o t h e r e l e c t r o p o s i t i v e . T h e s e l a t t e r m o l e c u l e s w i l l r e a d i l y g i v e e l e c t r o n s t o t h e f o r m e r , p a r t l y f i l l i n g a new v a l e n c e b a n d o n t h e a c c e p t o r m o l e c u l e s , w h i l e p a r t l y e m p t y i n g t h e v a l e n c e b a n d o n t h e d o n o r m o l e c u l e s . T h i s t r a n s f e r o f e l e c t r o n s c o u l d t h e r e f o r e l e a d t o a m e t a l l i c c o n d u c t i o n s a l o n g t h e s t a c k i n g d i r e c t i o n o f t h e o r g a n i c m o l e c u l e s . I n s o m e c a s e s , t h e t r a n s f e r m a y b e c o m p l e t e d ( e a c h d o n o r a t o m g i v i n g o n e e l e c t r o n t o e a c h a c c e p t o r a t o m ) , o r i n c o m p l e t e d ( o n t h e a v e r a g e d e a c h a c c e p t o r m o l e c u l e r e c e i v i n g l e s s t h a n o n e e l e c t r o n ) . 1 . 3 . 3 A n E x a m p l e ; T h e T C N Q M o l e c u l e T h e T C N Q m o l e c u l e ( f i g u r e - 1 ) e x h i b i t s a l l t h e c h a r a c t e r i s t i c s m e n t i o n n e d a b o v e . I t i s l a r g e a n d p l a n a r w i t h a s u b s t a n t i a l e l e c t r o n i c a f f i n i t y . V e r y l i t t l e e n e r g y i s n e e d e d t o s t a r t f i l l i n g a new v a l e n c e b a n d . I t s p l a n a r s t r u c t u r e a l l o w s a v e r y d e n s e s t a c k i n g o f t h e m o l e c u l e s o n t o p o f e a c h o t h e r . T h e v a l e n c e o r b i t a l s p r o t r u d e o u t o f t h e p l a n e o f t h e m o l e c u l e , p e r m i t t i n g a s u b s t a n t i a l o v e r l a p o f t h e s e o r b i t a l s , r e s u l t i n g i n a m e t a l l i c c o n d u c t i o n a l o n g t h e s t a c k i n g d i r e c t i o n . I n m o s t 1 - D c o m p o u n d s i n v o l v i n g t h e T C N Q m o l e c u l e , t h e i n t e r c h a i n d i s t a n c e i s l a r g e c o m p a r e d t o t h e i n t e r p l a n a r 6 d i s t a n c e . 1 T h e r e f o r e , t h e t r a n s v e r s e c o n d u c t i v i t y i s weak, t h e v a l e n c e e l e c t r o n s h a v i n g t o hop from one c h a i n t o t h e n e x t , a n d t h e o n e - d i m e n s i o n a l i t y of t h e e l e c t r i c a l c o n d u c t i v i t y i s s o l e l y due t o t h e s t r u c t u r a l o r g a n i s a t i o n o f t h e s e c r y s t a l s . F i g u r e 1 - The TCNQ m o l e c u l e 1.3.4 The C o n c e p t Of P e r i o d i c L a t t i c e D i s t o r t i o n In t h e p r e s e n c e o f an e x t e r n a l p e r i o d i c p o t e n t i a l , d i s c o n t i n u i t i e s a p p e a r i n t h e E ( k ) r e l a t i o n s h i p o f a f r e e -e l e c t r o n g a s . The f i r s t d i s c o n t i n u i t y a p p e a r s a t t h e e d g e s o f 7. t h e B r i l l o u i n z o n e . F i g u r e 2 ' shows t h e h a l f f i l l e d , band s t r u c t u r e o f a 1-D m a t e r i a l w i t h a l a t t i c e s p a c i n g a. I f we assume t h a t a s m a l l d i s t o r t i o n o f t h e l a t t i c e o c c u r s s u c h t h a t 1 T y p i c a l l y , t h e i n t e r p l a n a r d i s t a n c e i s 3A w h i l e t h e i n t e r c h a i n d i s t a n c e i s 10A. *• The s h a d e d a r e a i n d i c a t e s t h e p o r t i o n of t h e E ( k ) " c u r v e " w h i c h i s c o m p l e t e l y f i l l e d by e l e c t r o n s . 7 t h e p e r i o d i c i t y g o e s f r o m a t o 2a , t h e f i r s t d i s c o n t i n u i t y w i l l b e m o v e d f r o m K= 1f/a t o K= 1?/(2a), i . e . , a t t h e F e r m i l e v e l . A s s h o w n i n f i g u r e 3, t h e s t a t e s j u s t b e l o w t h e F e r m i l e v e l h a v e b e e n l o w e r e d i n e n e r g y w h i l e t h e s t a t e s a b o v e i t h a v e s e e n t h e i r e n e r g i e s i n c r e a s e . B e c a u s e a l l t h e s t a t e s a b o v e KF a r e e m p t y , t h i s new c o n f i g u r a t i o n l o w e r s t h e e l e c t r o n i c e n e r g y o f t h e s y s t e m . O b v i o u s l y , s u c h a d i s t o r t i o n i n c r e a s e s t h e p o t e n t i a l e n e r g y s t o r e d i n t h e l a t t i c e a n d w i l l o c c u r o n l y i f t h e e l e c t r o n i c e n e r g y s a v e d i s l a r g e r t h a n t h e m e c h a n i c a l e n e r g y s p e n t t o c r e a t e t h e d i s t o r t i o n . I t c a n b e s h o w n t h a t t h i s i s a l w a y s t h e c a s e i n 1 -D c o m p o u n d s (4 ) . 1.3.5 T h e C o n c e p t O f C h a r g e D e n s i t y W a v e T h e d i s t o r t i o n o f t h e l a t t i c e w i l l m o d u l a t e t h e d i s t r i b u t i o n o f e l e c t r o n s s u r r o u n d i n g i t , g i v i n g r i s e t o a " C H A R G E D E N S I T Y W A V E " ( C . D . W ) . I f t h e p e r i o d i c i t y o f t h e C . D . W i s n o t c o m m e n s u r a t e w i t h t h e p e r i o d i c i t y o f t h e u n d i s t o r t e d l a t t i c e , i t w i l l " s l i d e " a l o n g t h e c o n d u c t i n g d i r e c t i o n , p r o d u c i n g a n e n h a n c e m e n t o f t h e D . C c o n d u c t i v i t y . A t l o w t e m p e r a t u r e s , t h e C . D . W w i l l b e e i t h e r p i n n e d b y C o u l o m b a t t r a c t i o n t o a n o p p o s i t i v e l y c h a r g e d c h a i n , i n a m u l t i - c h a i n c o m p o u n d , o r b y i m p u r i t i e s , i n a o n e - c h a i n c o m p o u n d . T h e p i n n i n g w i l l g i v e r i s e t o a n a b s o r p t i o n b a n d i n t h e f a r -i n f r a r e d . 8 F i g u r e 2 - Band s t r u c t u r e o f a 1-D compound 9 1.4 N a t u r e Of The Phase T r a n s i t i o n In (TMA)(I)(TCNQ) • The m o l e c u l a r c r y s t a l T rimethylammonium I o d i d e T e t r a c y a n o q u i n o d i m e t h a n e ( TMA)(I)(TCNQ) has a h i g h l y a n i s o t r o p i c D.C. c o n d u c t i v i t y , t h e r a t i o o f t h e c o n d u c t i v i t y a l o n g t h e c o n d u c t i n g a x i s t o t h e c o n d u c t i v i t y p e r p e n d i c u l a r t o t h i s a x i s b e i n g A. 400. A l o n g t h e c o n d u c t i n g a x i s , t h e v a l u e o f t h e c o n d u c t i v i t y r a n g e s between 20-40Ucm)"1 a t room t e m p e r a t u r e ( 5 ) . I t c a n t h e r e f o r e be c o n s i d e r e d a s a 1-D m e t a l . The t e m p e r a t u r e - d e p e n d e n c e o f t h e D.C. c o n d u c t i v i t y a l o n g t h i s a x i s e x h i b i t e i t h e r a s e m i c o n d u c t i n g b e h a v i o r (6) ( f i g u r e 4) o r has a b r o a d maximum n e a r 240K (7) ( f i g u r e 5) w i t h a m e t a l l i c b e h a v i o r above t h a t t e m p e r a t u r e and a s e m i c o n d u c t i n g one below i t . In e v e r y c a s e s a phase t r a n s i t i o n i s o b s e r v e d a t 150K. A s e c o n d t r a n s i t i o n i s a l s o o b s e r v e d a t 89K i n t h e Young modulus d a t a ( 8 ) . The n a t u r e of t h e phase t r a n s i t i o n a t 150K i s s t i l l s u j b e c t t o c o n t r o v e r s y . Two d i s t i n c t p o i n t o f vi e w a r e p o s s i b l e : t h e t r a n s i t i o n c o u l d be e i t h e r a m e t a l - s e m i c o n d u c t o r one o r a s e m i c o n d u c t o r - s e m i c o n d u c t o r one. In b o t h c a s e s , e l e c t r o n -e l e c t r o n s i n t e r a c t i o n s and e l e c t r o n - p h o n o n s i n t e r a c t i o n s c o u l d d e t e r m i n e d t h e t e m p e r a t u r e - d e p e n d e n c e of t h e D.C. c o n d u c t i v i t y . In t h i s work, we p r e s e n t t h e f i r s t f a r - i n f r a r e d powder s p e c t r u m o f (TMA) ( I ) ( T C N Q ) i n i t s o r d e r e d p h a s e ( s e c t i o n 2 . 1 ) . The s p e c t r a were r e c o r d e d between 20 and 1000 cm" 1, a t 9 d i f f e r e n t t e m p e r a t u r e s between 20 and 300K. The main g o a l o f t h i s work i s t o c l a r i f y t h e n a t u r e o f t h e pha s e t r a n s i t i o n s a t 150 and 89K. In c h a p t e r I I we w i l l r e v i e w t h e c r y s t a l l o g r a p h i c 10 s t r u c t u r e a n d p h y s i c a l p r o p e r t i e s o f ( T M A ) ( I ) ( T C N Q ) . I n c h a p t e r I I I t h e e x p e r i m e n t a l t e c h n i q u e u s e d w i l l b e d i s c u s s e d . F i n a l l y i n c h a p t e r I V we w i l l p r e s e n t a n d d i s c u s s t h e r e s u l t s o b t a i n e d . F i g u r e 4 - T e m p e r a t u r e - d e p e n d e n c e o f t h e D.C. c o n d u c t i v i t y ( 6 ) . F i g u r e 5 - T e m p e r a t u r e - d e p e n d e n c e o f t h e D.C, c o n d u c t i v i t y (7). I-20, 120 m no (40 TEMPERATURE, T. "K MO 12 I I . PHYSICAL AND STRUCTURAL PROPERTIES OF (TMA)(I)(TCNQ) 2.1 C r y s t a l l o g r a p h i c S t r u c t u r e I n 1976, C o u g r a n e t a l . (9) r e p o r t e d t h a t T e t r a c y a n o q u i n o d i m e t h a n e (TCNQ) r e a c t s w i t h T r i m e t h y l a m o n i u m I o d i d e ( T M A ) ( I ) t o form t h e t e r n a r y c h a r g e t r a n s f e r compound ( T M A ) ( I ) ( T C N Q ) . E l e m e n t a l a n a l y s i s shows t h a t t h e compound has a 1:1:1 s t o c h i o m e t r y . The p r e s e n c e of t h e t r i h a l i d e i o n I 3 " i n t h e c r y s t a l has been d e t e r m i n e d by Raman measurments ( 5 ) , i n w h i c h t h e s t r o n g f e a t u r e o b s e r v e d a t 105cm" 1 was a s s i g n e d t o t h e sy m m e t r i c s t r e t c h 1/f o f I 3 " . T h e r e f o r e t h e s t o c h i o m e t r y and t h e c o n s e r v a t i o n o f c h a r g e s u g g e s t t h e f o l l o w i n g f o r m u l a t i o n . The c r y s t a l a t room t e m p e r a t u r e i s m o n o c l i n i c and b e l o n g s t o t h e s p a c e g r o u p C2/m (10) ( f i g u r e 6 ) . The l a r g e p l a n a r TCNQ m o l e c u l e s form r e g u l a r s t a c k s a l o n g t h e . b a x i s w i t h an i n t e r p l a n a r d i s t a n c e b/2 of 3.23 A. They a r e s e p a r a t e d by i o d i n e c h a i n s a l o n g t h e a. a x i s and by t h e TMA c a t i o n s a l o n g t h e c a x i s . The i o d i n e c h a i n s a r e a l s o p a r a l l e l t o t h e b a x i s and p r e s u m a b l y n o n - c o n d u c t i n g . A l l t h e atoms o f a TCNQ m o l e c u l e a r e i n t h e p l a n e (AOC). T h i s p l a n e c o n s t i t u t e s a m i r r o r p l a n e f o r t h e TMA c a t i o n s . The TCNQ m o l e c u l e s a r e c a r a c t e r i s e d by a z i g z a g s t a c k i n g , two n e i g h b o u r i n g TCNQ m o l e c u l e s b e i n g r e l a t e d by a screw a x i s symmetry o p e r a t i o n . T h e r e i s a weak h y d r o g e n bond between t h e TMA c a t i o n s and one o f t h e n i t r o g e n atoms o f t h e TCNQ m o l e c u l e . T h e s e c a t i o n s a r e t h e r e f o r e l i n k e d by t h e same symmetry o p e r a t i o n . The i o d i n e c h a i n s c o n s t i t u t e an a x i s 13 of symmetry C 2 . In most c a s e s , X - r a y measurments show a s t r o n g d i f f u s e n e s s due t o an i m p o r t a n t d i s o r d e r between t h e i o d i n e c h a i n s . On t h e o t h e r hand, t h e I 3 " i o n s a r e w e l l o r d e r e d w i t h i n a c h a i n , w i t h a c o r r e l a t i o n l e n g h t o f a b o u t 100 A ( 1 0 ) . More r e c e n t l y ( 1 1 ) , f u l l y o r d e r e d c r y s t a l s have been o b t a i n e d by c o o l i n g o f a s o l u t i o n o f a c e t o n i t r i l e a t room t e m p e r a t u r e o r below. T h e s e c r y s t a l s a r e l a r g e r t h a n t h e d i s o r d e r e d ones and do n o t e x h i b i t t h e d i f f u s e d i f f r a c t i o n p a t e r n a s s o c i a t e d w i t h t h e d i s o r d e r o f t h e i o d i n e l a t t i c e . By r a i s i n g t h e t e m p e r a t u r e o f t h e a c e t o n i t r i l e s o l u t i o n , v a r i o u s d e g r e e o f d i s o r d e r may be o b t a i n e d . F i g u r e 7 shows a p r o j e c t i o n of t h e o r d e r e d c r y s t a l a l o n g t h e b a x i s . Columns one, two and t h r e e o f f i g u r e 7 c o r r e s p o n d t o t h e c o l u m n s one, two and t h r e e r e s p e c t i v e l y of f i g u r e 6. 14 F i g u r e 6 - ( T M A ) ( I ) ( T C N Q ) : p r o j e c t i o n p e r p e n d i c u l a r t o t h e b a x i s (9) F i g u r e 7 - (TMA) (I )(TCNQ): p r o j e c t i o n p a r a l e l l t o t h e b a x i s (11) 15 2.2 Temperature-dependence Of The U n i t - c e l l Parameters The temperature-dependence of the u n i t - c e l l parameters have been studied by various authors. Coulon et a l . (12) have reported a change in slope at 150K (figure 8), a phenomenon very reminescent of what i s observed in TTF-TCNQ at the P e i e r l s t r a n s i t i o n . F i l h o l et a l . (10) observed a non-linear but continuous variation of the u n i t - c e l l parameters. Figure 8 - Temperature-dependence of the u n i t - c e l l parameters (12) e (»>r 13.90r-T ( K ) 16 2.3 T e m p e r a t u r e - d e p e n d e n c e Of The D.C. C o n d u c t i v i t y The room t e m p e r a t u r e D.C. c o n d u c t i v i t y a l o n g t h e c o n d u c t i n g b a x i s i s <*. 20 (JLCIII ) " 1 ( 5 ) . The c o n d u c t i v i t y i s h i g h l y a n i s o t r o p i c w i t h =400. The t e m p e r a t u r e - d e p e n d e n c e of t h e D.C. c o n d u c t i v i t y has been r e p o r t e d as b e i n g c h a r a c t e r i s t i c o f a s e m i c o n d u c t o r w i t h an a c t i v a t i o n e n e r g y p r e s e n t i n g a s i n g u l a r i t y a t 150K ( 6 ) : Above t h a t t e m p e r a t u r e , t h e a c t i v a t i o n e n e r g y was m e a s u r e d t o be e q u a l t o 0.05 eV (400cm- 1) w h i l e below i t , t o be e q u a l t o 0.15 eV (1200cm- 1) ( f i g u r e 4 ) . In o t h e r c a s e s t h e c o n d u c t i v i t y e x h i b i t e d a b r o a d maximum n e a r 240K w i t h a m e t a l l i c b e h a v i o r above t h a t t e m p e r a t u r e ( 7 ) . An anomaly was a l s o f o u n d , but t h i s t i m e a t 160K ( f i g u r e 5 ) . A b k o w i t z e t a l . (7) i n t e r p r e t t h e m e t a l l i c 1 b e h a v i o r i n t e r m s o f a s e m i c o n d u c t o r - s e m i c o n d u c t o r t r a n s i t i o n . F o l l o w i n g t h e i d e a o f E p s t e i n e t a l . ( 1 3 ) , t h e y a r g u e t h a t t h e m e t a l - l i k e v a r i a t i o n of <f (T) above 240K i s due t o t h e r m a l l y e x c i t e d c a r r i e r s h a v i n g a s t r o n g t e m p e r a t u r e - d e p e n d e n t m o b i l i t y . T h e i r r e s u l t s f o r •NMP-TCNQ i n d i c a t e s t h a t t h e m o b i l i t y s h o u l d have a T~* d e p e n d e n c e ( 1 3 ) . They have a l s o shown t h a t t h e s c a t t e r i n g o f c o n d u c t i o n e l e c t r o n s by o p t i c a l phonons y i e l d t h e T - * d e p endence ( 1 3 ) . The gap i n t h e d e n s i t y of s t a t e s would be i n d u c e d i n t h e f o l l o w i n g way: 1 The t e r m m e t a l l i c i s u s e d h e r e t o d e s c r i b e a m a t e r i a l i n w h i c h t h e c o n d u c t i v i t y i n c r e a s e s a s t h e t e m p e r a t u r e i s l o w e r e d . 1 7 The f o r m a t i o n o f t h e s a l t f o l l o w s t h e c h e m i c a l c h a r g e t r a n s f e r r e a c t i o n : 3 C T M A - f - J - ) + 3TG / V ? — o (TM A"*)3 CTCUQ)* X/ (2) T h e r e f o r e e a c h TCNQ m o l e c u l e a c e p t s on t h e a v e r a g e 2/3 o f an e l e c t r o n . In a s i m p l e o n e - e l e c t r o n band model, t h e number of p o s s i b l e v a l u e s of t h e wave v e c t o r K (and t h e r e f o r e t h e number of s t a t e s a v a i l a b l e ) i s e q u a l t o t h e number o f p r i m i t i v e c e l l s (N) t i m e t h e number o f s p i n s t a t e s ( 2 ) . The number o f s t a t e s a v a i l a b l e i s t h u s 4N. On t h e o t h e r hand, t h e t o t a l number o f e l e c t r o n s b e i n g t r a n s f e r e d i s ( N ) ( 2 ) ( 2 / 3 ) = 4 N / 3 . The upper v a l e n c e band o f t h e TCNQ m o l e c u l e s i s t h e r e f o r e 1/3 f i l l e d and one w o u l d e x p e c t t h e compound t o behave l i k e a m e t a l . But t h e i o d i n e c h a i n s , b e i n g commensurate w i t h t h e TCNQ c h a i n s , and h a v i n g a p e r i o d i c i t y o f 3b/2, c a n i n t r o d u c e a gap r i g h t a t t h e F e r m i l e v e l p r o v i d e d t h a t t h e i n t e r a c t i o n between t h e c o n d u c t i o n e l e c t r o n s and t h e i o d i n e c h a i n s i s s t r o n g enough. I f t h i s i s t h e c a s e , a gap i n t h e d e n s i t y of s t a t e s would be p r e s e n t a t a l l t e m p e r a t u r e s . 18 2.4 The T h e r m o e l e c t r i c Power The t h e r m o l e c t r i c power S o f a s e m i c o n d u c t o r i s g i v e n by where u e ' m c a n < ^ up ,mp d e n o t e t h e m o b i l i t y and e f f e c t i v e mass of t h e e l e c t r o n s a n d h o l e s r e s p e c t i v e l y . Above an d below t h e 150K t r a n s i t i o n , t h e t h e r m o e l e c t r i c power v a r i e s l i k e T" 1 s u g g e s t i n g t h a t b o t h r e g i o n s a r e s e m i c o n d u c t i n g (5) ( f i g u r e 9 ) . The change i n s l o p e o c c u r i n g a t 150K i n d i c a t e s t h a t t h e t e r m + I c h a n g e s a b r u p t l y f r o m a n e g a t i v e v a l u e (u <u p) t o a l a r g e r p o s i t i v e v a l u e (u^>Up). I t i s t h e r e f o r e c l e a r t h a t a t l e a s t u c and u^ , and p r e s u m a b l y Eg change a b r u p t l y a t 150K. (4) F i g u r e 9 - The t h e r m o e l e c t r i c power (5) A .5 ^ * I/T i trm 19 2.5 O p t i c a l Measurments O p t i c a l measurments made by T a n n e r e t a l . (15) i n d i c a t e t h e e x i s t e n c e o f a gap a t room t e m p e r a t u r e . By d o i n g a Kramer-K r o n i g a n a l y s i s of t h e i r r e f l e c t a n c e d a t a , t h e y have e v a l u a t e d b o t h t h e f r e q u e n c y - d e p e n d e n t c o n d u c t i v i t y ( f i g u r e 10) and t h e d i e l e c t r i c f u n c t i o n ( f i g u r e 1 1 ) . Below 200cm" 1, t h e y f o u n d a v a l u e o f w) o f 19cm" 1, i n good agreement w i t h t h e room-t e m p e r a t u r e D . C . v a l u e ( 5 ) . The b r o a d maximum o b s e r v e d i n w) was i n t e r p r e t e d a s t h e s i g n a t u r e o f a gap. The d e n s i t y of s t a t e s o f a t r u l y 1-D c o n d u c t o r v a r i e s l i k e (&.~£c) (and ) where Ec-Ev=Eg ( s e e f i g u r e l 2 ) . 3-D i n t e r a c t i o n s remove t h e s i n g u l a r i t y a t Ev a n d E c . B e c a u s e o f t h e h i g h v a l u e o f N ( E c ) and N ( E v ) , t h e t r a n s i t i o n s between s t a t e s o f e n e r g y Ec and Ev a r e h i g h l y f a v o r e d , g i v i n g r i s e t o t h e s t r o n g maximum i n w) a t Eg=Ec-Ev. They e s t i m a t e d t h e r o o m - t e m p e r a t u r e gap a t 0.1 eV. 20 Figure 10 - Frequency dependent conductivity (15) Photon Energy (oV) 05 (NMttH)(I)(TCNQ) SOOK Figure 11 - D i e l e c t r i c function (15) Photon Energy (tV) it 1 , _ —• 1 1 1 , — (NM«,H)(I)(TCNQ) SOOK • I E Ho _ — E Hb 1 1 — E lie • . -O 2000 ' i m ' 1 ' ' 1 >-=L Frtquoncy (em*1) 21 N(E) density of states: with 3-D interaction F i g u r e 12 - D e n s i t y o f s t a t e s i n a 1-D compound 22 2 . 6 M a g n e t i c S u s c e p t i b i l i t y T h e m a g n e t i c s u s c e p t i b i l i t y m e a s u r m e n t s i n d i c a t e t h a t ( T M A ) ( I ) ( T C N Q ) i s m e t a l l i c a b o v e T c . T h e r e s u l t s h o w n o n f i g u r e 13 ( 1 6 ) w e r e o b t a i n e d u s i n g t h e . F a r a d a y m e t h o d . T h e m e t h o d g i v e s t h e t o t a l s u s c e p t i b i l i t y o f t h e c o m p o u n d . T h e d i a m a g n e t i c c o n t r i b u t i o n i s t h e n s u b s t r a c t e d t o o b t a i n t h e p a r a m a g n e t i c s u s c e p t i b i l i t y ( ^ s p i n ) . A t h i g h t e m p e r a t u r e s , t h e p a r a m a g n e t i s m i s a l m o s t c o n s t a n t , o n l y s h o w i n g a s l i g h t d e c r e a s e w i t h d e c r e a s i n g t e m p e r a t u r e . T h i s b e h a v i o r i s t y p i c a l o f a n o n -m a g n e t i c f r e e e l e c t r o n m e t a l ( P a u l i p a r a m a g n e t i s m ) . T h e p r e s e n c e o f a g a p w o u l d p r o d u c e a n a c t i v a t e d s u s c e p t i b i l i t y t h a t w o u l d i n c r e a s e w i t h t e m p e r a t u r e , t h a t i s , t h e t e m p e r a t u r e -d e p e n d e n c e o f ^ s p i n w o u l d b e d o m i n a t e d b y t h e t e m p e r a t u r e d e p e n d e n c e o f t h e n u m b e r o f s i n g l e p a r t i c l e e x c i t a t i o n s . T h e d a t a a l s o s u g g e s t t h a t ^ s p i n g o e s t o z e r o a s t h e t e m p e r a t u r e g o e s t o z e r o . ( T h e a b s e n c e o f a l i n e a r t e r m i n t h e s p e c i f i c h e a t ( f i g u r e 14) c o n f i r m s t h a t ) . T h u s t h e l o w t e m p e r a t u r e s d a t a o f t h e s p i n s u s c e p t i b i l i t y e x h i b i t t h e p r o p e r t i e s o f a n o n -m a g n e t i c s m a l l b a n d g a p i n s u l a t o r , w h i l e t h e h i g h t e m p e r a t u r e s d a t a e x h i b i t t h e f e a t u r e s o f a m e t a l . F i g u r e 13 - M a g n e t i c s u s c e p t i b i l i t y a n d D . C . c o n d u c t i v i t y 23 2.7 S p e c i f i c Heat Measurment The low temperatures s p e c i f i c heat shows no l i n e a r term (16) ( f i g u r e 14). Thus the e l e c t r o n i c c o n t r i b u t i o n i s very s m a l l . This i s indeed expected of a small band gap i n s u l a t o r , s i n c e a n e g l i g i b l e amount of f r e e c a r r i e r s are present at low temperatures. The c o n t r i b u t i o n of long wavelength a c c o u s t i c phonons to the l a t t i c e s p e c i f i c heat gives a T 3 dependence. The curvature observed i m p l i e s that low energy o p t i c a l modes a l s o c o n t r i b u t e to the s p e c i f i c heat. Figure 14 - S p e c i f i c heat (16) CIT CmJ/mofelK'J TMA 200. 24 2.8 Concusions The behavior of the D.C. conductivity and magnetic s u s c e p t i b i l i t y above 150K can be seen as indicating that (TMA)(I)(TCNQ) i s me t a l l i c . The anomaly observed in the thermoelectric power indicates that the mobility does not vary as a smooth function of temperature (T" a) and that a phase t r a n s i t i o n occurs at 150K. The presence of an i n f l e c t i o n point in the thermal variations of the l a t t i c e parameters confirms the occurrence of a structural d i s t o r t i o n at 150K. 25 I I I . EXPERIMENTAL METHOD 3.1 E x p e r i m e n t a l Method The measurments were t a k e n u s i n g powdered samples o f (TM A ) ( I ) ( T C N Q ) c r y s t a l s . The c r y s t a l s were f i n e l y g r o u n d w i t h a m o r t a r and p e s t l e i n a s o l u t i o n o f c o m m e r c i a l n u j o l m u l l . The t h i c k m i x t u r e o b t a i n e d was t h e n s p r e a d on a TPX o r s i l i c o n window. The TPX windows were u s e d i n t h e s p e c t r a l r e g i o n between 20 and 400 cm-1, w h i l e s i l i c o n windows were u s e d f o r f r e q u e n c i e s above 400 cm" 1. V a r i o u s p a r t i c l e s i z e s and t h i c k n e s s o f m u l l were t r i e d . In t h e s p e c t r a l r e g i o n c o v e r e d ( 2 0 - 1 0 0 0 c m " 1 ) , no e f f e c t s r e l a t e d t o p a r t i c l e s i z e s were o b s e r v e d . On t h e o t h e r hand, t h e s p e c t r a were v e r y s e n s i t i v e t o t h e t h i c k n e s s o f t h e m u l l . F o r a t h i c k n e s s l a r g e r t h a n A. 1mm, o n l y b r o a d and weak f e a t u r e s were o b s e r v e d w h i l e f o r a t h i c k n e s s s m a l l e r t h a n A^0..25mm no a b s o r p t i o n t o o k p l a c e and c o n s e q u e n t l y no s t r u c t u r e s were s e e n . W i t h a t h i c k n e s s o f A- 0.5mm and p a r t i c l e d i a m e t e r s r a n g i n g between 0.3 and 1.0um, s h a r p and s t r o n g a b s o r p t i o n bands were o b s e r v e d . The s i z e o f t h e p a r t i c l e s was e s t i m a t e d by c o m p a r i n g t h e n u j o l powder w i t h a s e c o n d powder o f known s i z e . A l t h o u g h d i f f i c u l t t o e v a l u a t e , t h e l e a k a g e t h r o u g h t h e powder i s b e l i e v e d t o be s m a l l e r t h a n 10 %. The s p e c t r a were r e c o r d e d u s i n g a M i c h e l s o n i n t e r f e r o m e t e r (Beckmann model F S - 7 2 0 ) . The samp l e s were mounted i n a J a n i s S u p e r v a r i t e m p dewar (model 8 CNDT). The c o o l i n g was a c h i e v e d by v a p o r i z i n g h e l i u m w i t h a h e a t e r l o c a t e d b e n e a t h t h e sample. The t e m p e r a t u r e was me a s u r e d u s i n g a c a l i b r a t e d s i l i c o n d i o d e 26 embeded i n t h e sample h o l d e r . An a c c u r a c y o f ± 1K was a c h i e v e d f o r a l l t h e measurments u s i n g a PAR 152 t e m p e r a t u r e c o n t r o l l e r . The i n s t r u m e n t c o n t r o l s t h e c u r r e n t f l o w i n g i n t h e h e a t e r . A s e c o n d s i l i c o n d i o d e l o c a t e d on t h e v a p o r i z e r a r r a n g e m e n t m e a s u r e s t h e t e m p e r a t u r e and f e e d s back t h e r e a d i n g t o t h e PAR 152, w h i c h a d j u s t t h e c u r r e n t a c c o r d i n g l y . The sample and v a p o r i z e r a r r a n g e m e n t l o c a t e d on t h e dewar t a i l were c o n t a i n e d w i t h i n a r e c t a n g u l a r c e l l p r o v i d e d w i t h windows on a l l s i d e s . F o r t h i s e x p e r i m e n t , m y l a r windows o f 0.75 i n c h i n d i a m e t e r and 0.025mm t h i c k n e s s were u s e d . Below 100K, t h e m y l a r windows were h e l i u m t i g h t . Above i t , h e l i u m s t a r t e d t o d i f f u s e t h r o u g h t h e windows and i t was n e c e s s a r y t o pump on t h e sample chamber. Between 20 and 400 cm" 1 a b o l o m e t e r u s i n g a c o m p o s i t e germanium e l e m e n t was u s e d as d e t e c t o r . Above 400cm" 1 a G o l a y d e t e c t o r gave a b e t t e r s i g n a l - t o - n o i s e r a t i o and was u s e d i n s t e a d . The l o n g t i m e i n v o l v e d i n r e c o r d i n g h i g h r e s o l u t i o n s p e c t r a o v e r a l a r g e f r e q u e n c y r a n g e , combined w i t h t h e l o n g t i m e t a k e n by t h e s y s t e m t o r e a c h h i g h t e m p e r a t u r e s , f o r c e d us t o s p r e a d t h e e x p e r i m e n t o v e r s e v e r a l d a y s . C a r e was t h e r e f o r e t a k e n t o a l w a y s r e p e a t some p r e v i o u s measurments b e f o r e g o i n g t o any new t e m p e r a t u r e , a l l o w i n g c o n s i s t e n c y i n t h e r e s u l t s from one day t o t h e n e x t . B a c k g r o u n d measurments were r e c o r d e d a t t h e same t e m p e r a t u r e s a s t h e sample measurments, and w i t h . i d e n t i c a l windows. A l l t h e s p e c t r a p r e s e n t e d i n t h i s work a r e t h e l o g a r i t h m o f 27 t h e r a t i o o f t h e b a c k g r o u n d i n t e n s i t y ( I D ) t o t h e s a m p l e i n t e n s i t y ( I ) . T h e t r a n s m i s s i o n t h r o u g h a s a m p l e o f t h i c k n e s s d i s g i v e n b y : w h e r e R i s t h e r e f l e c t a n c e a n d « \ t h e a b s o r p t i o n c o e f i c i e n t . c o e f i c i e n t \ . T h e c u r v e s h o w i n g t h e t e m p e r a t u r e - d e p e n d e n c e o f t h e i n t e g r a t e d i n t e n s i t i e s o f a b s o r p t i o n b a n d s a r e t h e a r e a o f t h e l o g a r i t h m i c b a c k g r o u n d t o s a m p l e r a t i o . T h e a r e a w e r e m e a s u r e d u s i n g a p l a n o m e t e r t o e n l a r g e s p e c t r a . P o l y n o m i a l s o f v a r i o u s d e g r e e s w e r e t h e n f i t t e d t h r o u g h t h e s e e x p e r i m e n t a l d a t a p o i n t s . T h e s c a l e u s e d f o r t h e s e g r a p h s i s a n a r b i t r a r y o n e , 100 r e f e r i n g t o t h e l a r g e s t i n t e n s i t y m e a s u r e d f o r a p a r t i c u l a r b a n d . (5) i s t h e n p r o p o r t i o n a l t o t h e a b s o r p t i o n 28 IV. RESULTS AND DISCUSSION 4.1 E v i d e n c e Of E l e c t r o n - p h o n o n I n t e r a c t i o n s F i g u r e (15) shows a f a r - i n f r a r e d s p e c t r u m o f (TMA)(I)(TCNQ) between 20 a n d 400 cm" 1, o b t a i n e d a t 20 and 300K r e p e c t i v e l y . The 20K s p e c t r u m i s t h e a v e r a g e r e s u l t o f two r u n s , r e c o r d e d a t a r e s o l u t i o n o f 2 cm" 1, w h i l e t h e 300K s p e c t r u m i s t h e a v e r a g e o f t h r e e r u n s r e c o r d e d a t a r e s o l u t i o n of 4cm" 1. In o r d e r t o e m p h a s i s e t h e d i f f e r e n c e s i n t h e two s p e c t r a , we have s u p e r p o s e d them on t o p o f e a c h o t h e r , u s i n g t h e same s c a l e . The a b s o r p t i o n l e v e l i s t h e r e f o r e a r b i t r a r y . The room t e m p e r a t u r e s p e c t r u m i s a l m o s t f e a t u r e l e s s . We o b s e r v e t h e p r e s e n c e o f two b r o a d a b s o r p t i o n bands c e n t e r e d a t 120 and 160cm" 1, s u p e r i m p o s e d on a b a c k g r o u n d w h i c h r e a c h e s a maximum a t 300cm" 1. These b r o a d s t r u c t u r e s a r e t h e room t e m p e r a t u r e c o u n t e r p a r t s of t h e v e r y s t r o n g v i b r a t i o n a l s t r u c t u r e s o b s e r v e d i n t h e 20K s p e c t r u m , n e a r 113 and 164cm" 1. A n o t h e r s t r o n g r i s e i n a b s o r p t i o n i s s e e n a t 365cm" 1. I t s e x i s t e n c e was c o n f i r m e d by e x t e n d i n g t h e measurments up t o 1000cm" 1 ( s e e s e c t i o n 4 . 9 ) . The n a t u r e o f t h e s e a b s o r p t i o n " e d g e s " w i l l be d i s c u s s e d i n s e c t i o n 4.9. The 20K s p e c t r u m shows numerous a b s o r p t i o n b a n d s . We have i n d i c a t e d t h e s t r o n g e s t o n e s . A l l of t h e s t r u c t u r e s have a s t r o n g t e m p e r a t u r e - d e p e n d e n c e . We o b s e r v e a t l e a s t t h r e e bands t h a t do n o t have c o u n t e r p a r t s i n t h e 300K s p e c t r u m : 1 1 i i i — I — T — i — i — i — i — r — ' — i — i 1 — l — l — I — I i l I i i t » « i i i 20 60 100 140 180 220 260 300 340 38t> F i g u r e 15 - F.I.R Spectrum of (TMA)(I)(TCNQ) between 20 and 400cm"1 30 The 226cm" 1 s t r u c t u r e i s c l e a r l y n ot o b s e r v e d i n t h e 300K s p e c t r u m . In f i g u r e 16 we show t h e t e m p e r a t u r e - d e p e n d e n c e o f t h e band between 20 and 180K. The weak s t r u c t u r e s o b s e r v e d a t 230cm" 1 (160K) and 229cm" 1 (180K) c a n n o t be t h e 226cm" 1 band s i n c e t h e y a r e s h i f t e d i n t h e wrong d i r e c t i o n . 1 - F i g u r e 17 shows t h e t e m p e r a t u r e - d e p e n d e n c e o f t h e 313cm" 1 band between 20 and 180K. A g a i n t h e 320cm" 1 f e a t u r e a t 180K c a n n o t be t h e 313cm" 1 band. On t h e o t h e r hand, t h e 313cm" 1 i s o n l y w e a k l y s e e n a t 140R. We t h e r e f o r e c o n c l u d e t h a t t h e b r o a d s t r u c t u r e o b s e r v e d i n t h e 160K s p e c t r u m i s n o i s e . - F i g u r e 18 shows t h e t e m p e r a t u r e - d e p e n d e n c e o f t h e bands a t 155 and 164cm"1, and t h e d o u b l e t a t 172 and 176cm" 1 (20K v a l u e s ) . The 164 band i s not o b s e r v e d a t 160, 180 and 300K. The b r o a d s t r u c t u r e s a t 153 and 169cm" 1 (160K v a l u e s ) p e r s i s t up t o 300K. The i n t e n s e s t r u c t u r e a t 113cm" 1 a l s o e x h i b i t a s t r o n g t e m p e r a t u r e - d e p e n d e n c e . N e v e r t h e l e s s t h e band i s c l e a r l y n o t a c t i v a t e d . The t e m p e r a t u r e - d e p e n d e n c e o f i t s i n t e g r a t e d i n t e n s i t y w i l l be d i s c u s s e d i n s e c t i o n 4.7 and a p r e l i m i n a r y a s s i g n m e n t g i v e n i n s e c t i o n 4.8 We a t t r i b u t e t h e s e new s t r u c t u r e s t o e l e c t r o n - p h o n o n i n t e r a c t i o n s . More s p e c i f i c a l l y , t o t h e c o u p l i n g o f t h e c o n d u c t i o n e l e c t r o n s w i t h t o t a l l y s y mmetric modes o f v i b r a t i o n . 1 Upon i n c r e a s i n g t h e t e m p e r a t u r e , t h e c h e m i c a l bonds become weaker. One t h e r e f o r e e x p e c t s t h e bands t o s h i f t t o l o w e r f r e q u e n c i e s . 31 I t h a s b e e n s h o w n ( 1 8 ) t h a t t h i s c o u p l i n g g i v e s r i s e t o a new s e t o f i n f r a r e d f e a t u r e s o f f r e q u e n c i e s c l o s e t o t h e f r e q u e n c y o f t o t a l l y s y m m e t r i c m o d e s ( n o r m a l l y i n f r a r e d i n a c t i v e ) . T h e c o u p l i n g m a y b e d r i v e n b y o n e o f t h e f o l l o w i n g m e c h a n i s m : 1) I n d i m e r i s e d s y s t e m s , t o t a l l y s y m m e t r i c v i b r a t i o n s t h a t a r e o u t o f p h a s e o n t h e t w o m o l e c u l e s f o r m i n g a d i m e r m a y d r i v e t h e r a d i c a l e l e c t r o n i n t o a n o s c i l l a t o r y m o t i o n o f f r e q u e n c y c l o s e t o t h e f r e q u e n c y o f t h e t o t a l l y s y m m e t r i c m o d e . T h e a b s o r p t i o n i s e l e c t r o n i c i n n a t u r e a n d p o l a r i s e d a l o n g t h e t h e d i m e r i s e d d i r e c t i o n . 2 ) I n 1 - D P e i e r l s d i s t o r t e d c o m p o u n d s , e a c h c o m p o n e n t o f t h e i n c o m m e n s u r a t e C . D . W . m a y c o u p l e w i t h a t o t a l l y s y m m e t r i c m o d e , g i v i n g r i s e t o i n f r a r e d a c t i v i t y p o l a r i s e d a l o n g t h e c o n d u c t i n g a x i s . I n f i g u r e - 1 5 a l l t h e s t r u c t u r e s o f t h e 2 0 K s p e c t r u m a p p e a r a s a b s o r p t i o n b a n d s , i n d i c a t i n g t h a t t h e g a p b e t w e e n t h e v a l e n c e a n d c o n d u c t i o n b a n d i s w e l l a b o v e 4 0 0 c m " 1 . 32 C o {_ I_ O z O «— Q_ or O CO m < 180 200 220 240 260 280 cm F i g u r e 16 - Temperature-dependence of the 226cm'1 band •1 33 C Z3 >> (_ 2 n t_ a z O (-C L cr O LO CD < 270 290 310 330 350 370 cm F i g u r e 17 - T e m p e r a t u r e - d e p e n d e n c e of t h e 313cm" 1 band-34 c ZJ o O i— Q. rr O co < 140 160 180 2 0 0 2 2 0 2 4 0 cm" 1 Figure 18 - Temperature-dependence of the 155,164 172 and 176cm - 1 bands 35 4.2 A c t i v a t i o n Of The T o t a l l y Symmetric Modes Ag 9 And Ag 1 0 Among t h e 10 t o t a l l y s y m m e t r i c modes o f t h e TCNQ m o l e c u l e , o n l y A g / 9 and A g i ^ ° a r e n o r m a l l y f o u n d below 400cm" 1. F i g u r e 19 shows t h e t e m p e r a t u r e - d e p e n d e n c e o f t h e i n t e g r a t e d i n t e n s i t y of t h e 313cm" 1 band. We have a l s o p l o t t e d on t h e same f i g u r e t h e t e m p e r a t u r e - d e p e n d e n c e o f t h e b a c k g r o u n d a b s o r p t i o n measured a t t h e same f r e q u e n c y . The band i s c l e a r l y a c t i v a t e d i n t h e n e i g h b o u r h o o d o f 140K. We o b s e r v e a s h a r p i n c r e a s e between 140 and 120K, f o l l o w e d by a weaker one between 120 and 20K. T h i s k i n d o f t e m p e r a t u r e - d e p e n d e n c e i s t y p i c a l o f what has been o b s e r v e d by B o z i o and P e c i l e (19) ( c a p t i o n o f f i g u r e 19) and E l d r i d g e and B a t e s ( 2 0 ) , f o r t h e a c t i v a t i o n o f Agl^ 9 and A g / 1 0 i n TTF-TCNQ. We t e n t a t i v e l y a s s i g n t h e 313cm* 1 s t r u c t u r e t o t h e t o t a l l y s y m m e t r i c mode A g / 9 . We have a l s o drawn on t h e f i g u r e e r r o r b a r s f o r t y p i c a l d a t a p o i n t s . The e r r o r on t h e i n t e n s i t y a x i s i s ± 3 u n i t s . T h i s number was r e a c h e d by c a l c u l a t i n g t h e s t a n d a r d d e v i a t i o n of t h e d a t a p o i n t s o b t a i n e d w i t h t h e p l a n o m e t e r . The e r r o r on t h e t e m p e r a t u r e a x i s i s of t h e o r d e r o f ± 1 K ( s e c t i o n 3 . 1 ) . U s i n g t h e . c r i t e r i a o f B o z i o and P e c i l e (19) f o r t h e t r a n s i t i o n t e m p e r a t u r e , we f i n d a v a l u e of 145 ± 3 K . F i g u r e 20 shows t h e t e m p e r a t u r e - d e p e n d e n c e o f t h e i n t e g r a t e d i n t e n s i t y o f t h e 226cm" 1 band. A g a i n a sudden enhancement t a k e s p l a c e between 140 and 100K. Below 100K, t h e i n t e g r a t e d i n t e n s i t y r e m a i n s c o n s t a n t . T h i s a c t i v a t e d band c o u l d be t h e t o t a l l y s y m m e t r i c mode U s i n g t h e c r i t e r i a 36 o f B o z i o a n d P e c i l e (19), we f i n d a t r a n s i t i o n t e m p e r a t u r e o f 159+9K. S i n c e we h a v e o n l y o n e d a t a p o i n t a t e v e r y 20K, t h e d e t e r m i n a t i o n o f t h e t r a n s i t i o n t e m p e r a t u r e b y e x t r a p o l a t i n g t h e l i n e a r p o r t i o n o f t h e i n t e g r a t e d i n t e n s i t y c u r v e o n t h e t e m p e r a t u r e a x i s , i s r a t h e r i n a c c u r a t e . T h i s i n a c c u r a c y i s r e s p o n s i b l e f o r t h e t w o d i f f e r e n t t r a n s i t i o n t e m p e r a t u r e s f o u n d (145 a n d 159K). N e v e r t h e l e s s , f r o m o u r r e s u l t s ( f i g u r e 19 a n d 20), a t r a n s i t i o n t e m p e r a t u r e o f 150±. 5K° s e e m s t o b e t h e m o s t a c c e p t a b l e v a l u e . 3 7 F i g u r e 20 - T e m p e r a t u r e - d e p e n d e n c e o f t h e i n t e g r a t e d i n t e n s i t y o f t h e 2 2 6 c m - 1 band 3 9 4.3 The S p l i t t i n g Of The D o u b l e t A t 172 And 176cm' 1 F i g u r e 18 shows t h e t e m p e r a t u r e - d e p e n d e n c e o f t h e g r o u p o f s t r u c t u r e s a t 155, 164, 172 a n d 176cm" 1 (20K v a l u e s ) . Between 300 and 160K, we o b s e r v e o n l y two weaker bands a t 153 and 169cm" 1 r e s p e c t i v e l y . Upon r e d u c i n g t h e t e m p e r a t u r e t o 140K, a weak s h o u l d e r a p p e a r s a t 164cm" 1. A t 100K two s t r u c t u r e s a r e o b s e r v e d a t 164 and 172cm" 1. A s e c o n d s p l i t t i n g o c c u r s when t h e t e m p e r a t u r e i s f u r t h e r r e d u c e d : A t 100K, t h e 172cm" 1 band do e s n o t show any s i g n o f d i s t o r t i o n . At 60K i t becomes b r o a d e r t h a n i t s 100K c o u n t e r p a r t and l o o s e i t s symmetry. Between 60 and 40K, t h e w i d t h o f t h e band s t i l l i n c r e a s e s and a c l e a r s h o u l d e r a p p e a r s a t 175cm" 1. A t 20K, t h e two s t r u c t u r e s a r e c l e a r l y s een a t 172 and 176cm" 1. S t a r t i n g w i t h a s i n g l e band a t room-t e m p e r a t u r e , we t h e r e f o r e end up w i t h a t r i p l e t a t 20K. 4.4 D e t e c t i o n Of A Second Phase T r a n s i t i o n F i g u r e (21) shows t h e t e m p e r a t u r e - d e p e n d e n c e o f t h e i n t e g r a t e d i n t e n s i t y of t h e bands a t 155, 164cm" 1 and t h e i n t e g r a t e d d o u b l e t a t 172 and 176cm" 1 (20K v a l u e s ) . T h e i r s i m i l a r i t y below 140K i s s t r i k i n g . Between 140 and 120K, t h e i r i n t e n s i t i e s a r e s t r o n g l y e n h a n c e d and r e m a i n c o n s t a n t between 120 and 100K. They a r e t h e n f u r t h e r e n h a n c e d between 100 and 60K, and s a t u r a t e below t h a t t e m p e r a t u r e . The a b s e n c e o f d a t a p o i n t s between 60 and 100K does not a l l o w us t o p r e c i s e l y d e t e r m i n e t h e o n s e t o f t h i s s e c o n d enhancement b u t i t i s c l e a r l y r e l a t e d t o t h e anomaly o b s e r v e d i n t h e Young modulus d a t a a t 89K ( 8 ) . We n o t e t h a t t h i s s e c o n d t r a n s i t i o n d o e s not show up i n 40 t h e D.C. c o n d u c t i v i t y , m a g n e t i c s u s c e p t i b i l i t y , t h e r m o e l e c t r i c power and s p e c i f i c h e a t measurments. However, our i n f r a r e d measurments p r o v i d e good e v i d e n c e o f t h e e x i s t e n c e o f t h e t r a n s i t i o n . 4.5 N a t u r e Of The 150K Phase T r a n s i t i o n The a c t i v a t i o n o f a b s o r p t i o n bands i n t h e i n f r a r e d s p e c t r u m o f ( T M A ) ( I ) ( T C N Q ) p r o v i d e s good e v i d e n c e t h a t a p e r i o d i c l a t t i c e d i s t o r t i o n t a k e s p l a c e a t t h e 150K t r a n s i t i o n . The a c t i v a t i o n o f t h e s u s c e p t i b i l i t y ( w h i c h g o e s t o z e r o as t h e t e m p e r a t u r e goes t o z e r o ) i s i n agreement w i t h s u c h a p o s s i b i l i t y , s i n c e t h e s u s c e p t i b i l i t y of a 1-D a r r a y o f e q u a l l y s p a c e d e l e c t r o n i c s p i n s s h o u l d r e m a i n f i n i t e a t a l l f i n i t e t e m p e r a t u r e s . M o r e o v e r , t h e s p l i t t i n g o f t h e 169cm" 1 band i n t o t h r e e bands s u g g e s t s t h a t t h e p e r i o d i c l a t t i c e d i s t o r t i o n m i g h t be a P e i e r l s d i s t o r t i o n . S i n c e t h e c o n d u c t i o n band of ( T M A ) ( I ) ( T C N Q ) i s 1/3 f i l l e d , s u c h a d i s t o r t i o n would push t h e B r i l l o u i n zone f r o m fT/a t o l7?(3a), where a i s t h e s t a c k i n g s p a c i n g o f t h e c o n d u c t i n g TCNQ m o l e c u l e s . The mechanism i s i n d i c a t e d i n f i g u r e 22. T h i s p a r t i c u l a r zone f o l d i n g g i v e r i s e t o t h r e e b r a n c h e s a t q=0 1 and would e x p l a i n t h e f o r m a t i o n o f t r i p l e t s a s t h e t e m p e r a t u r e i s r e d u c e d below t h e p h a s e t r a n s i t i o n t e m p e r a t u r e . We n o t e t h a t t h e 313cm" 1 band ( f i g u r e 17 a t 20K) a l s o seems t o a p p e a r as a t r i p l e t . 1 The two d e g e n a r a t e l o w e r b r a n c h e s a r e s p l i t t e d a p p a r t upon i n t e r a c t i n g w i t h one a n o t h e r . 41 4.6 N a t u r e Of The S e c o n d Phase T r a n s i t i o n R e t u r n i n g t o f i g u r e s 19 a n d 20, we - n o t e t h a t t h e t e m p e r a t u r e - d e p e n d e n c e of Ag/"9 and AgK' 1 0 i s v e r y weak below 100K. T h i s s u g g e s t t h a t t h e TCNQ c h a i n s do n o t p l a y a m a j o r r o l e i n t h e s e c o n d d i s t o r t i o n . T h i s r e s u l t i s i n agreement w i t h t h e i n t e r p r e t a t i o n g i v e n by B r i l l e t a l . ( 8 ) , who have s u g g e s t e d t h a t t h e t r a n s i t i o n i s a s s o c i a t e d w i t h t h e " f r e e z e o u t " o f t h e m e t h y l r o t a t i o n g r o u p s . 4.7 T e m p e r a t u r e - d e p e n d e n c e Of The I n t e g r a t e d I n t e n s i t y Of The  96, 113 And 135cm" 1 Bands. F i g u r e 23 shows t h e i n t e g r a t e d i n t e n s i t y o f t h e band a t 96, 113 and 135 cm" 1. No a t t e m p t w i l l be made t o a s s i g n them a l l . T h e s e t h r e e bands show an enhancement o f t h e i r i n t e g r a t e d i n t e n s i t y n e a r 140K. The i n t e g r a t e d i n t e n s i t y o f t h e 96 cm" 1 band shows a s e c o n d enhancement between 60 and 100K, p r e s u m a b l y a s s o c i a t e d w i t h t h e o n s e t o f t h e s e c o n d d i s t o r t i o n . F o r t h e 113 cm-1 band, t h e s e c o n d t r a n s i t i o n i s not c l e a r l y o b s e r v e d . I n s t e a d t h e i n t e g r a t e d i n t e n s i t y i n c r e a s e s a t a c o n s t a n t r a t e as t h e t e m p e r a t u r e i s r e d u c e d from 120 t o 60K. The band a t 135 cm" 1 i s t h e o n l y one f o r w h i c h t h e o n s e t of t h e s e c o n d d i s t o r t i o n d e c r e a s e s t h e v a l u e o f t h e i n t e g r a t e d i n t e n s i t y of t h e band. The c o m p l i c a t e d t e m p e r a t u r e - d e p e n d e n c e e x h i b i t e d by t h e s e l a s t bands i s p r o b a b l y n o t o n l y d e t e r m i n e d by t h e o n s e t s of t h e f i r s t and s e c o n d d i s t o r t i o n s , b ut a l s o by i n t e r a c t i o n s among t h e m s e l v e s . CM 100 >- 8 0 LU > 20 u c: free carrier absorption 15QE 172 cm-1 155 cm-1 1 1 1 20 4 0 6 0 8 0 100 120 140 160 180 F i g u r e 21 - T e m p e r a t u r e - d e p e n d e n c e o f t h e i n t e g r a t e d i n t e n s i t y o f t h e 155, 164 and 172 bands 200 43 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 F i g u r e 23 - T e m p e r a t u r e - d e p e n d e n c e of t h e i n t e g r a t e d i n t e n s i t y o f t h e 96, 113, 125 and 135cm" 1 bands 45 4.8 Evidence Of Hydrogen Bond Stretching One of the most interesting features of the spectrum of (TMA)(I)(TCNQ) i s the very strong absorption band observed at 1l3cm _ 1. In t h i s section we w i l l give i t a preliminary assignment. (TMA)(I)(TCNQ) i s a hydrogen bonded compound. The structure of the bond i s shown schematically in figure 24. Figure-24 - Hydrogen bond - - C - C The nitrogen atom of the Trimethylammonium ion forms four covalent bonds of the type (SP) 3; Three with the methyl groups and one with the hydrogen atom. The nitrogen atom being more electro-negative than the hydrogen, the pairs of electrons forming the covalent bond w i l l s h i f t toward the former. We therefore have a bare proton s i t t i n g at the end of the bond. On the other hand, the nitrogen atom on the TCNQ molecule, being 46 more electro-negative than the carbon, w i l l be s l i g h t l y negatively charged. This results in the formation of a weak bound state between the nitrogen atom on the TCNQ molecule and the hydrogen atom on the TMA ion. The binding energy i s equivalent to a strong van der Waals inte r a c t i o n (0,4-40 Kj/mole). The inter-molecular modes of vibration between the TCNQ molecule and. the TMA ion w i l l include stretching, bending and torsional motions. The stretching motion normally has the highest frequency (50-200cm-1). Because of the large masses involve and the weakness of the bond, one expects the stretching motion to be very large and substantially anharmonic. We also expect i t to stongly couple with electromagnetic radiation, since i t involves a large vibrating dipole. The end resu l t i s the appearance of a strong and broad absorption band in the spectrum of hydrogen bonded compound. In TABLE 1- (21) we l i s t the observed frequencies of hydrogen-bond stretching modes for various phenol-amines complexes. Table I - Observed frequencies of hydrogen bond stretching modes (21) cm"' 134 130 123 120 143 141 rUO H) Phenol in CCU solution ISO PhenoW, in CO* solution 143 Phenol-pyridine, C«H,OH NC,Hj PhenoWi-pyridine, C*H,OD • N C , H 9 Phenol-trie thylamine PhenoW,-trictbyl»mine Pbenol-Hrimethylamine in Cd» solution PhenoUi-trimethylamine in CCU solution 47 In TABLE 2- (21) we l i s t the structural parameters and force contants for phenol-trimethylamines complexes. Table II - Force constants for phenol-trimethylamines complexes (21) Force constant of i - , , mdyne, A Simplified normal 1:1 Complex coordinate O - H N PhOH NR PhOH-trimethylamine 0.27 fl09< 0.44 PhOp-trimethylamine 0.27 0.43 PhOH-triethylamine 0 24 , 0 44 PhOD-triethylamine 0.22 0.42 PhOH-pyridine 0.23 0 8 < 0.45 PhOD-pyridine 0.23 043 We note that the force constant of the hydrogen bond for a l l the complexes is about 0.3 mdyne/A. Using this value and setting a l l the other force constants to inf i n i t y , we can estimate the frequency of this mode of vibration in (TMA)(I)(TCNQ). The calculation gives 103cm"1. The 113cm"1 band therefore bears a l l the insigna (strong intensity, very broad and value of i t s frequency agreeing with the simplified normal coordinate calculation) of an hydrogen bond stretching mode. The strong temperature-dependence of this mode at the transition temperature is however surprising. On the other hand, one expect that this type of lattice mode wil l be stongly affected by a structural distortion. 48 4.9 Detection Of An Activation Energy Of 365cm'1 The o p t i c a l measurments made by Hoffman et a l . (22) indicates the presence of an energy gap of 0.1eV at a l l temperatures. The measurments of the temperature-dependence of the D.C. conductivity made by Delhaes (6) also show a gap at a l l temperatures. The temperatue-dependence of the D.C. conductivity of some of the c r y s t a l s studied by Abkowitz et a l . (7) had a metallic behavior above 240K, which was explained in terms of a strongly temperature-dependent mobility of excited c a r r i e r s accross a band gap (section 2.3). A l l these measurments agree that a gap i s 'present at a l l temperatures. Whether a c r y s t a l exhibits a metallic behavior above 240K or not could be due to the degree of disorder of the iodine chains of the c r y s t a l s studied (section 2.1). The high value of spin (section 2.6) i s an indication that e-e correlations are strong and i t i s therefore plausible that these interactions are responsible for the gap observed at room temperature. When the temperature i s lowered below 150K, we interpret our results as indicating that a s t r u c t u r a l Peierls d i s t o r t i o n takes place. This 2Kp d i s t o r t i o n would reduce the e-e interaction, and would explain the temperature-dependence of *f spin. The gap at low temperatures would therefore be a Pe i e r l s gap. From the temperature-dependence of our infrared spectra, we have attempted to extract the value of this low temperatures P e i e r l s gap. Figure 25 shows spectra obtained at 20,100 and 180K, between 20 and 340 cm"1. We note the strong variation of the 49 background intensity (B) with temperatures. The background intensity i s usually a measure of the contribution of the free c a r r i e r s to the t o t a l absorption (w), and should therefore be proportional to the D.C. conductivity. Figure 26 shows a plot of InB as a function of 1/T. The values of B were measured at 200cm"1. The curve can be divided into two regions with activations energies equal to 0.05eV (400cm"1) between 150 and 100K, and equal to O.OleV (100cm"1) for temperatures between 100 and 40K. This f i r s t value correlates well with the one found in figure 15 (365cm" 1). In figure 27 we show a lower resolution spectrum (16cm"1) up to 1000cm"1, recorded at room temperature, which confirms the existence of an a c t i v a t i o n energy of 365cm"1. We note that these results are in agreement with the frequency dependent conductivity obtained by Tanner et a l . (15) (figure 10) from their room temperature r e f l e c t i v i t y measurements. They observed a rapidly increasing conductivity which peaks near 2000cm"1, but interpreted t h e i r results as indicating the presence of a 0.1-0.14eV (1000cm"1) gap. In our case the temperature-dependence of the background intensity (figure 26) c l e a r l y indicates that a 365cm"1 a c t i v a t i o n energy i s responsible for the abrupt increase observed in the infrared absorption. We also note that while our measurments were made on powders, the sharp r i s e in the infrared absorption indicates that the conducting d i r e c t i o n dominates the absorption process. The results of Tanner et a l . (15) (figure 10) shows that the absorption along the a and c directions i s weakly frequency dependent and does not exhibit such a sharp r i s e with frequency. 50 This value of a c t i v a t i o n energy (365cm" 1) i s much smaller than the values found by Delhaes et a l . (6) and Abkowitz et a l . (7) from t h e i r D.C. c o n d u c t i v i t y measurments, which should detect the smallest gap present i n the system. We e x p l a i n the discrepency i n the f o l l o w i n g manner: - The i n f r a r e d background absorption measured i s not caused by free c a r r i e r s i n the conduction band but by e l e c t r o n i c t r a n s i t i o n s between l o c a l i s e d s t a t e s i n the band gap. Such t r a n s i t i o n s would lead to i n f r a r e d absorption but would not c o n t r i b u t e to the D.C. c o n d u c t i v i t y . These l o c a l i s e d s t a t e s could be introduced by chemical i m p u r i t i e s or by some randomness i n the p o t e n t i a l at each TCNQ s i t e . The gaps measured would represent the energy needed f o r an e l e c t r o n to hop from one l o c a l i s e d s t a t e to the next. to i r ~ i i i i i i — i — i — i — i — i — i — i — i — i — i — r ID 0 8 06 O o Q4 0.2 resolution 2 cm 1 180 K° J 120 K' 20 K 0 0 J L X J I I L J L J I I I I I I L 2 0 6 0 100 140 180 220 260 3 0 0 340 380 Figure 25 - Temperature-dependence of the Background I n t e n s i t y B 1 5 0 100 4 0 KELVIN 4 8 12 16 20 24 2 8 30 (T^)-F i g u r e 26 - P l o t o f l n ( B ) a s a f u n c t i o n o f 1/T o H o o 53 i—i—r T=300 K resolution =16 crrf1 O Figure 27 -200 400 600 800 1000 cm'1 Infrared Spectrum of (TMA)(I)(TCNQ) Between 20 and 1000cm"1 54 4.10 S m a l l - p a r t i c l e S c a t t e r i n g S i n c e t h e measurments were made on powdered s a m p l e s , s m a l l -p a r t i c l e s c a t t e r i n g must be c o n s i d e r e d . In t h e l i m i t where t h e d i a m e t e r o f t h e p a r t i c l e s d i s much s m a l l e r t h a n t h e i n c i d e n t w a v e l e n g h t , t h e s c a t t e r i n g c a n be d e s c r i b e d by ( 1 7 ) : where I 0 i s t h e i n t e n s i t y o f t h e i n c i d e n t beam, I(©) t h e i n t e n s i t y s c a t t e r e d a t an a n g l e measured f r o m I e and n i s t h e r e f r a c t i v e i n d e x . d and X a r e t h e p a r t i c l e s d i a m e t e r and w a v e l e n g h t of t h e i n c i d e n t beam r e s p e c t i v e l y . The t e r m 1/A»VI) i s n o t s t r o n g l y f r e q u e n c y - d e p e n d e n t . The r e s u l t s of T a n n e r e t a l (15) shows t h a t t h i s t e r m s l o w l y v a r y f r o m 0.3 t o 0.5, between 400 and 1 0 0 0 c m - 1 . In t h e c a s e where s m a l l - p a r t i c l e s c a t t e r i n g d o m i n a t e s , t r a n s m i s s i o n measurments s h o u l d have a X ^ d e p e n d e n c e . In f i g u r e 28 we have p l o t t e d a s a f u n c t i o n o f \ec^X , u s i n g t h e d a t a shown i n f i g u r e 27. Between 400 and 800cm" 1, t h e r e l a t i o n s h i p i s l i n e a r w i t h a s l o p e e q u a l t o 2 + 0 . 5 . Above 800cm" 1, i n c r e a s e s a t a much f a s t e r r a t e and t h e t a n g e n t t o t h e s l o p e a t 900 cm" 1 i s e q u a l t o 4 ± 0 . 5 . We c o n c l u d e t h a t above 800cm" 1 s c a t t e r i n g may be s i g n i f i c a n t . But t h i s i s n o t t h e c a s e below 400cm" 1 and t h e s h a r p r i s e o b s e r v e a t 365 cm" 1 c a n be s a f e l y a t t r i b u t e d t o an i n c r e a s e d i n i n f r a r e d a b s o r p t i o n . 55 56 4.11 C o n c l u s i o n s We have o b s e r v e d t h e a c t i v a t i o n o f s e v e r a l i n f r a r e d a b s o r p t i o n bands a s t h e t e m p e r a t u r e i s r e d u c e d below 150K i n ( T M A ) ( I ) ( T C N Q ) . We have t e n t a t i v e l y a s s i g n e d two o f them t o t h e t o t a l l y s y m m e t r i c modes AgV/9 (313cm" 1) and Ag|/^° ( 2 2 6 c m " 1 ) . We have i n t e r p r e t e d t h e s e r e s u l t s a s i n d i c a t i n g t h a t a p e r i o d i c l a t t i c e d i s t o r t i o n t a k e s p l a c e n e a r 150K. We have i n t e r p r e t e d t h e s p l i t t i n g o f t h e 169cm" 1 band (room t e m p e r a t u r e v a l u e ) i n t e r m o f a B r i l l o u i n zone f o l d i n g e f f e c t d r i v e n by t h e p e r i o d i c l a t t i c e d i s t o r t i o n , s u g g e s t i n g t h a t t h e 150K d i s t o r t i o n i s a P e i e r l s d i s t o r t i o n . The a c t i v a t i o n o f A g / 9 and Ag/" 1 0 w o u l d be due t o a v i b r o n i c e f f e c t . We a l s o have e v i d e n c e f o r t h e e x i s t e n c e o f a s e c o n d t r a n s i t i o n n e a r 100K. Our r e s u l t s i n d i c a t e t h a t t h e TCNQ s t a c k s do n o t p l a y a m a j o r r o l e i n t h i s t r a n s i t i o n . T h i s r e s u l t i s i n a greement w i t h t h e i n t e r p r e t a t i o n g i v e n by B r i l l e t a l . (8) t h a t t h e t r a n s i t i o n i n v o l v e s t h e " f r e e z e o u t " o f t h e m e t h y l r o t a t i o n g r o u p s . M o r e o v e r , t h i s i n t e r p r e t a t i o n i s s u p p o r t e d by t h e r e s u l t s of t h e D.C. c o n d u c t i v i t y , t h e r m o e l e c t r i c power and s p e c i f i c h e a t measurments, w h i c h do not e x h i b i t any anomaly below 150K. A s e c o n d d i s t o r t i o n of t h e TCNQ s t a c k s would c e r t a i n l y a f f e c t t h e s e p h y s i c a l p r o p e r t i e s . We have d e t e c t e d two a b s o r p t i o n e dges a t 90 and 365cm" 1, w h i c h we have a t t r i b u t e d t o t r a n s i t i o n s between l o c a l i s e d s t a t e s i n t h e band gap. We have a s s i g n e d t h e s t o n g mode a t 113cm" 1 t o t h e h y d r o g e n bond s t r e t c h i n g mode. 57 BIBLIOGRAPHY L. Broomhead, S c i e n c e e t A v e n i r , #422, p.64, ( 1 9 8 2 ) . P h y s i c s Today, 17, F e b r u a r y ( 1 9 8 1 ) . H.K. Ng, T. Timusk, J.M. D e l r i e u ,D. Jerome, K. B e c h g a a r d and J.M. F a b r e , t o be p u b l i s h e d i n J o u r n a l de P h y s i q u e . L o w - D i m e n s i o n a l C o o p e r a t i v e Phenomena; The P o s s i b i l i t y of'  H i g h T e m p e r a t u r e S u p e r c o n d u c t i v i t y , e d i t e d by H.J. K e l l e r , Plenum P r e s s , M.J. R i c e , p.23, ( 1 9 7 4 ) . S y n t h e s i s and P r o p e r t i e s o f L o w - d i m e n s i o n a l M a t e r i a l s , e d i t e d by J . S . M i l l e r and A . J . E p s t e i n , p u b l i s h e d by t h e New York Academy o f S c i e n c e , M.A. A b k o w i t z , J.W. B r i l l , P.M. C h a i k i n , A . J . E p s t e i n , M.F. F r o i x , C . H . G r i f f i t h s , W. G u n n i n g , A . J . He e g e r , W.A. L i t t l e , J . S . M i l l e r , M. N o v a t n y . D.B. T a n n e r and M.L. S l a d e , p.459, ( 1 9 7 8 ) . P. D e l h a e s , A. C o u g r a n d , A. F l a n d r o i s , S. C h a s s e a u , D. G a u l t i e r , J . Hauw and C. D u p u i s , L e c t u r e N o t e s i n P h y s i c s , B e r l i n : S p r i n g e r , 65 , 494, ( 1 9 7 7 ) . A. A b k o w i t z , A. E p s t e i n , A*.J. G r i f f i t h s a n d C H . M i l l e r , J . Am. Chem. S o c , 99 , 5304, ( 1 9 7 7 ) . J.W. B r i l l , A . J . E p s t e i n and J . S . M i l l e r , P h y s . Rev. B. 20 , 681, ( 1 9 7 9 ) . A. C o u g r a n d , S. F l a n d r o i s and P. D e l h a e s , M o l . C r y s . L i q . C r y s . , 32 , 165, ( 1 9 7 6 ) . A. F i l h o l , M. R o v i r a , C. Hauw. J . G a u t h i e r , D. C h a u s s e a u and P. D u p u i s , A c t a C r y s t . , B35 ,1652, ( 1 9 7 9 ) . E x t e n d e d L i n e a r C h a i n Compounds , J_ , e d i t e d by J . S . M i l l e r , Plenum P r e s s P h i l i p Coppens, p.345, ( 1 9 8 2 ) . C. C o u l o n , S. F l a n d r o i s a nd P. D e l h a e s , P h y s . Rev. B, 23 , 6, 2850, ( 1 9 8 1 ) . A . J . E p s t e i n , E.M. C o n w e l l , D .J. Sandman and J . S . M i l l e r , S o l . S t a t e Comm., 23 , 355, ( 1 9 7 7 ) . H i g h l y C o n d u c t i n g O n e - D i m e n s i o n a l S o l i d s e d i t e d by J . T . D e v r e e s e , R.P. E v r a r d and V.E. van D o r e n , Plenum P r e s s , T.D. S c h u l t z and R.A. C r a v e n , p.161, ( 1 9 7 9 ) . D. B.' T a n n e r , J . E . D e i s , A . J . E p s t e i n and J . S . M i l l e r , S o l . S t a t e Comm., 3_1_ , 671, ( 1 9 7 9 ) . 58 L e c t u r e N o t e s i n P h y s i c s : Q u a s i O n e - D i m e n s i o n a l C o n d u c t o r s I_I_ , 96 , E d i t e d by S. B a r i s i c , A. B j e l i s , J.R. Cooper and B. L e o n t i c , S p r i n g e r - V e r l a g , S. F l a n d r o i s , C. C o u l o n , J . A m i e l , P. D e l h a e s and P. D u p u i s , p. 1 8 8 , ( 1 9 7 8 ) . G.C. P a p a v a s s i l i o u , P r o g . S o l . S t a t e Chem., J_2 , 185, (1979) . L e c t u r e N o t e s i n P h y s i c s : Q u a s i O n e - d i m e n s i o n a l c o n d u c t o r s J_ , 9_5 , e d i t e d by S. B a u s i c , A. B j e l i s , J.R. Cooper and B. L e o n t i c , S p r i n g - V e r l a g , M.J. R i c e , p. 2 3 0 , ( 1 9 7 9 ) . R. B o z i o and C. P e c i l e , S o l . S t a t e Comm., 3J7 , 1 9 3 , ( 1 9 8 1 ) . J . E . E l d r i d g e and F r a n c e s E. B a t e s , Phys Rev.B 26 , 1 5 9 0 , ( 1 9 8 2 ) . F a r - I n f r a r e d S p e c t r o s c o p y , W i l l e y - I n t e r s c i e n c e , K.D. M o l l e r and W.G. R o t h s c h i l d , c h a p t e r 6 . D.M. Hoffman, J . E . D e i s , D.B. T a n n e r , A . J . E p s t e i n and J . S . M i l l e r , Chemica S c r i p t a , j_7 , 1 1 1 , ( 1 9 8 1 ) . 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            data-media="{[{embed.selectedMedia}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.831.1-0085031/manifest

Comment

Related Items