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ESR and microwave conductivity studies in DEM(TCNQ) above room temperature 1981

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ESR AND MICROWAVE CONDUCTIVITY STUDIES IN DEM(TCNQ) 2 ABOVE ROOM TEMPERATURE by FRANCISCO XAVIER CABANAS B.Sc. The U n i v e r s i t y of B r i t i s h C olumbia, 1979 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of P h y s i c s ) We ac c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA November 1981 © F r a n c i s c o X a v i e r Cabanas, 1981 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 requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, 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 reference and study. I f u r t h e r agree t h a t permission f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted 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 understood th a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of P V v ^ S t C j The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date ABSTRACT Two phase t r a n s i t i o n s have been found i n DEM(TCNQ) 2 a t 400(3) K and a t 442(6) K t o 453(6) K u s i n g ESR and measurements of the microwave c o n d u c t i v i t y . These t e m p e r a t u r e s a re l e s s than the v a l u e s of 415 K and 483 K p r e v i o u s l y o b t a i n e d from temperature dependent G u i n i e r measurements. Below 400(3) K two ESR l i n e s , I and I I a r e observed c o r r e s p o n d i n g t o the two s t a c k s , B and A, i n DEM(TCNQ) 2. Above 400(3) K and below 453(6) K o n l y one l i n e remains w i t h the same g v a l u e as l i n e I . The a n g u l a r dependence of the g v a l u e was f i t t e d t o g 2 = g 2 c o s 2 e + g ^ s i n e and v a l u e s of g x = 2.003551( 14) and g = 2.002730(15) were o b t a i n e d . Between 298 K and 442(6) K the c o n d u c t i v i t y was t h a t of a semiconductor w i t h an e x i t a t i o n energy E 0 = 0.385(52) e.v. The -4k F phase t r a n s i t i o n i s p o s t u l a t e d a t 447(9) K. The phase t r a n s i t i o n a t 400(3) K i s due t o a t r a n s f e r of s p i n d e n s i t y from s t a c k A t o s t a c k B, and has no e f f e c t on the t o t a l s p i n s u s c e p t i b i l i t y or on the c o n d u c t i v i t y t o w i t h i n the e x p e r i m e n t a l e r r o r . TABLE OF CONTENTS A b s t r a c t i i Ta b l e of C o n t e n t s i i i L i s t of T a b l e s . .v L i s t of F i g u r e s v i Acknowlegements . . . . v i i CHAPTER 1 INTRODUCTION 1 1.1 S i g n i f i c a n c e of DEM(TCNQ) 2 and R e l a t e d Compounds — 1 1.2 C h a r a c t e r i s t i c s of DEM(TCNQ) 2 4 1 . 3 O u t l i n e 6 CHAPTER 2 DIELECTRIC CONSTANT AND CONDUCTIVITY THEORY AND MEASUREMENTS 8 2.1 Theory of C a v i t y P e r t u r b a t i o n 8 2.2 D i e l e c t r i c C o n s tant Measuring Apparatus 12 2.3 D i e l e c t r i c C o n s t a n t and C o n d u c t i v i t y Measurements ..15 CHAPTER 3 DIELECTRIC CONSTANT AND CONDUCTIVITY RESULTS ..17 3.1 E x p e r i m e n t a l R e s u l t s 17 3.2 D i s c u s s i o n of the D i e l e c t r i c C onstant and c o n d u c t i v i t y Measurements 22 (a) Quoted E x p e r i m e n t a l E r r o r s 22 (b) Shape of the Samples as a Source of E r r o r .......23 CHAPTER 4 ESR APPARATUS, MEASUREMENTS AND SUSCEPTIBILITY CALCULATIONS 24 4.1 ESR Apparatus 24 4.2 The ESR Measurements ....26 4.3 C o n v o l u t e d G a u s s i a n - L o r e n t z i a n F i t ......29 (a) F i t .,29 (b) B a s e l i n e D t e r m i n a t i o n 30 CHAPTER 5 ESR EXPERIMENTAL RESULTS 31 5. 1 ESR g V a l u e s 31 5.2 A n g u l a r Dependence of the g V a l u e s 34 5.3 Temperature Dependence of the S u s c e p t i b i l i t y 37 5.4 Temperature Dependence of the Peak t o Peak Widths of the ESR l i n e s 41 CHAPTER 6 DISSCUSION OF.THE RESULTS 43 6.1 Phase T r a n s i t i o n s i n DEM(TCNQ) 2 43 6.2 Comparison of DEM(TCNQ) 2 t o MEM(TCNQ) 2 and HEM(TCNQ) 2 • 4 5 CHAPTER 7 CONCLUSIONS AND FURTHER POSSIBLE EXPERIMENTS ..47 7.1 C o n c l u s i o n s 47 7.2 F u r t h e r D i r e c t i o n s 48 B i b i l o g r a p h y ...49 V LIST OF TABLES T a b l e 2.1 P h y s i c a l Data of the Nylon T e s t Sample 16 T a b l e 3.1 P h y s i c a l Data of . DEM (.TCNQ ) 2 Samples ... . ...20 T a b l e 3.2 E x p e r i m e n t a l R e s u l t s ...20 T a b l e 5.1 ESR g V a l u e s ... .32 T a b l e 5.2 g„and g x f o r DEM (TCNQ) 2 ......34 LIST OF FIGURES F i g u r e 1.1 The MEM, DEM and HEM M o l e c u l e s 2 F i g u r e 1.2 The TCNQ M o l e c u l e 2 F i g u r e 1.3 Shape of a T y p i c a l DEM(TCNQ) 2 C r y s t a l Showing the R e l a t i v e D i r e c t i o n s of S t a c k s A and B and the Magnetic F i e l d H i n the ESR Measurements 5 F i g u r e 2.1 C a v i t y and Sample H o l d e r used f o r D i e l e c t r i c C o n s t ant and C o n d u c t i v i t y Measurements 11 F i g u r e 2.2 Resonance of the C a v i t y used f o r the D i e l e c t r i c C onstant and C o n d u c t i v i t y Measurements 13 F i g u r e 2.3 B l o c k Diagram of the D i e l e c t r i c C o n s t a n t and C o n d u c t i v i t y Measurements 14 F i g u r e 3.1 Microwave C o n d u c t i v i t y of DEM(TCNQ) 2 vs Temperature 18 F i g u r e 3.2 N a t u r a l L o g a r i t h i m of the Microwave C o n d u c t i v i t y of DEM(TCNQ) 2 vs I n v e r s e Temperature 19 F i g u r e 4.1 B l o c k Diagram of the ESR Apparatus 25 F i g u r e 4.2 ESR C a v i t y and V a r i a n Flow System 27 F i g u r e 4.3 T y p i c a l ESR S p e c t r a above and below the Phase T r a n s i t i o n a t 400(3) K 28 F i g u r e 5.1 Angul a r Dependence of the g Va l u e of DEM(TCNQ) 2 above the Phase T r a n s i t i o n a t 400(3) K ...... 33 F i g u r e 5.2 Temperature Dependence of t h e S p i n S u s c e p t i b i l i t y of L i n e I of DEM (TCNQ) 2 35 F i g u r e 5.3 Temperature Dependence of the S p i n S u s c e p t i b i l i t y of L i n e I I of DEM (TCNQ) 2 36 F i g u r e 5.4 Temperature Dependence of the Sum of the S p i n S u s c e p t i b i l i t i e s of L i n e s I and I I of DEM(TCNQ 2 38 F i g u r e 5.5 Temperature Dependence of the Peak t o Peak H a l f w i d t h of l i n e s I and I I of DEM (TCNQ) 2 39 F i g u r e 5.6 A n g u l a r Dependence of the Peak t o Peak H a l f w i d t h of l i n e I of DEM(TCNQ) 2 above the Phase T r a n s i t i o n a t 400(3) K 40 v i i i ACKNOWLEGEMENTS I wish t o e x p r e s s my g r a t i t u d e t o my s u p e r v i s o r , Dr. C. F. S c h w e r d t f e g e r , f o r h i s s u p e r v i s i o n , a s s i s t a n c e , and exchange of i d e a s both d u r i n g the performance of the e x p e r i m e n t s and the p r e p a r a t i o n of the t h e s i s . I would a l s o l i k e t o thank Dr. J . F. C a r o l a n who read the t h e s i s f o r h i s s u g g e s t i o n s and comments. I am a l s o i n d e b t e d t o Dr. G. A. Sawatzky and Dr. B. van Bodegom of the U n i v e r s i t y of Groningen f o r p r o v i d i n g the DEM(TCNQ) 2 samples. A post g r a d u a t e s c h o l a r s h i p from the N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l of Canada i s g r a t e f u l l y acknowleged. R e s e a r c h f o r t h i s t h e s i s was funded by the N a t u r a l S c i e n c e s and E n g i n e e r i n g Research . C o u n c i l of Canada through g r a n t s t o . D r . S c h w e r d t f e g e r . 1 CHAPTER 1 INTRODUCTION 1 .1 SIGNIFICANCE OF DEM(TCNQ) 2 AND RELATED COMPOUNDS N — e t h y l ^ N — e t h y l — m o r p h o l i n i u m t e t r a c y a n o q u i d i m e t h a n e , DEM(TCNQ) 2, i s a member of a f a m i l y of r e l a t e d compounds where the m o r p h o l i n i u m group changes. When the r a d i c a l denoted by R i n f i g u r e 1.1 i s r e p l a c e d by C 2 H 5 we have DEM(TCNQ) 2, by CH 3 we have MEM(TCNQ) 2 (MEM = N - m e t h y l — N - e t h y l - m o r p h o l i n i u m ) , and by H we have HEM(TCNQ) 2 (HEM = N — e t h y l — m o r p h o l i n i u m ) . The s i g n i f i c a n t c h a r a c t e r i s t i c of the s e compounds i s the q u a s i o n e — d i m e n s i o n a l b e h a v i o u r of the e l e c t r i c a l c o n d u c t i v i t y . T h i s p r o v i d e s an example of a p h y s i c a l system where o n e — d i m e n s i o n a l t h e o r y can be t e s t e d . The Hubbard H a m i l t o n i a n and the p r e d i c t i o n of 2k F and 4k F i n s t a b i l i t i e s depending on the v a l u e of U/t has been c r u c i a l t o t h i s t h e o r y (see H u i z i n g a 1980 p.2) (1.1) 2 CH2 CH2 C?Hc / \ / 0 *N \ / \ C H 2 — C H 2 R Figure 1.1 The MEM, DEM and HEM Molecules R = CH3 : MEM ; R = C 2H 5 : DEM ; R = H : HEM 3 Measurements have been performed on MEM(TCNQ) 2 and HEM(TCNQ) 2. The c r y s t a l s t r u c t u r e of MEM(TCNQ) 2 a t 113 K has been r e p o r t e d by Bosh and van Bodegom (1977). Two phase t r a n s i t i o n s have been found i n MEM(TCNQ) 2 a t 18 K and 340 K. The s p e c i f i c heat of MEM(TCNQ) 2 has been r e p o r t e d by Sawatzky e t a l . (1980). They r e p o r t a peak i n the s p e c i f i c heat a t 19 K w i t h an e n t r o p y g a i n of 1.4J—mole/K, and a n o t h e r peak a t 335 K w i t h an e n t r o p y g a i n of 14J—mole/K, c o r r e s p o n d i n g t o the two phase t r a n s i t i o n s i n MEM(TCNQ) 2. H u i z i n g a , Kommandeur, Sawatzky, and Thole r e p o r t a S p i n — P e i e r l s phase t r a n s i t i o n a t 18 K. The h i g h temperature c o n d u c t i v i t y of MEM(TCNQ) 2 has been measured by Morrow e t a l . (1980), where a r e v e r s i b l e s e m i c o n d u c t o r — m e t a l phase t r a n s i t i o n i s r e p o r t e d a t 340 K. H u i z i n g a (1980) has c o r r e l a t e d the phase t r a n s i t i o n s a t 18 K and 340 K i n MEM(TCNQ) 2 w i t h the 2k F and 4k F i n s t a b i l i t i e s . The c r y s t a l s t r u c t u r e of the r e l a t e d compound HEM(TCNQ) 2 has been measured by van Bodegom and van de Boer (1981). A phase t r a n s i t i o n a t 450 K u s i n g a D i f f e r e n t i a l S c a n n i n g C a l o r i m e t e r (DSC) t e c h n i q u e and a sharp d e c l i n e i n the s p i n s u s c e p t i b i l i t y at 425 K, i n d i c a t i n g a phase t r a n s i t i o n , has been r e p o r t e d by H u i z i n g a (1980 p.108) i n HEM(TCNQ) 2. T h i s phase t r a n s i t i o n i s a t t r i b u t e d by H u i z i n g a t o be r e l a t e d t o t h e 2k F i n s t a b i l i t y p r e d i c t e d from the t h e o r y , i m p l y i n g t h a t U / t 0 i s much l e s s f o r HEM(TCNQ) 2 than f o r MEM(TCNQ) 2 (see H u i z i n g a 1980 p.101). 4 1.2 CHARACTERISTICS OF DEM(TCNQ) 2 The c r y s t a l s t r u c t u r e of DEM(TCNQ) 2 has been measured by M o r r s i n k and van Bodegom (1981). They r e p o r t two t y p e s of TCNQ s h e e t s , A and B, t h a t a r e a t an a n g l e of 60° (see f i g . 1 . 3 ) . T h i s makes DEM(TCNQ) 2 f u n d a m e n t a l l y d i f f e r e n t from MEM(TCNQ) 2 or HEM(TCNQ) 2. Sheet B behaves i n a o n e — d i m e n s i o n a l f a s h i o n and i s s i m i l a r t o MEM(TCNQ) 2 i n i t s b e h a v i o r below room t e m p e r a t u r e . A S p i n — P e i e r l s phase t r a n s i t i o n has been observed i n sheet B of DEM(TCNQ)2 a t 23 K by S c h w e r d t f e g e r , O o s t r a and Sawatzky. T h i s compares t o the S p i n — P e i e r l s phase t r a n s i t i o n a t 18 K i n MEM(TCNQ) 2. T h i s phase t r a n s i t i o n p r o b a b l y c o r r e s p o n d s t o the 2k F i n s t a b i l i t y i n the s t a c k of TCNQ m o l e c u l e s i n sheet B of DEM(TCNQ) 2. U n l i k e the s t a c k s i n sheet B, t h e s t a c k s i n sheet A do not •undergo a S p i n — P e i e r l s phase t r a n s i t i o n between 1.5 K and room te m p e r a t u r e . The l a t t e r s t a c k remains paramagnetic down t o 1.5 K. W h i l e s t a c k B has been shown t o - behave i n a s i m i l a r manner t o the s i n g l e s t a c k of TCNQ m o l e c u l e s i n MEM(TCNQ) 2; the c o r r e s p o n d i n g s t a c k f o r sheet A behaves more l i k e a t w o — d i m e n s i o n a l system ( S c h w e r d t f e g e r , O o s t r a and Sa w a t z k y ) . 5 F i g u r e 1.3 Shape of a T y p i c a l DEM(TCNQ) 2 C r y s t a l . The Arrows L a b e l l e d A and B I n d i c a t e the D i r e c t i o n s of S t a c k s A and. B R e s p e c t i v e l y and the Arrow L a b e l l e d H I n d i c a t e s the D i r e c t i o n of the S t a t i c M a gnetic F i e l d f o r the ESR Measurements. 6 At h i g h temperatures M o r r s i n k and van Bodegom r e p o r t two r e v e r s i b l e phase t r a n s i t i o n s a t 415 K and 483 K i n DEM(TCNQ) 2. The ESR s p e c t r a of DEM(TCNQ) 2 between 23 K and 298 K has two l i n e s I and I I t h a t o r i g i n a t e i n s h e e t s B and A r e s p e c t i v e l y . We f o l l o w the n o t a t i o n of S c h w e r d t f e g e r , Wagner and Sawatzky (1980). The a n g u l a r dependence of the v a l u e s of g a t 77 K and 198 K, and of g a t 1.14 K, 4.2 K, 77 K, and 298 K are r e p o r t e d by S c h w e r d t f e g e r Wagner and Sawatzky (1980) (see t a b l e 5.2). The s p i n — s u s c e p t i b i l i t y and h a l f w i d t h of l i n e s I and I I as a f u n c t i o n of temperature has been measured by S c h w e r d t f e g e r , O o s t r a and Sawatzky. A q u a l i t a t i v e s i m i l a r i t y between the b u l k s u s c e p t i b i l i t i e s of DEM(TCNQ) 2 and MEM(TCNQ) 2 i n the temperature range from 4 K t o 300 K i s r e p o r t e d by Kuindersma, Sawatzky and Kommandeur (1975). There a r e i n d i c a t i o n s t h a t the c o n t r i b u t i o n t o the b u l k s u s c e p t i b i l i t y from s t a c k s A and B a r e s i m i l a r , however t h e r e i s a fundamental d i f f e r e n c e i n the n a t u r e of the s t a c k s A and B i n DEM(TCNQ) 2. 1.3 OUTLINE In c h a p t e r 2 we p r e s e n t the t h e o r y b e h i n d the microwave c o n d u c t i v i t y measurements and a d e s c r i p t i o n of the a p p a r a t u s and e x p e r i m e n t a l procedure used t o measure the microwave c o n d u c t i v i t y . The microwave c o n d u c t i v i t y d a t a as a f u n c t i o n of 7 temperature between 290 K and 480 K, showing a phase t r a n s i t i o n a t 442(6) K t o g e t h e r , w i t h the r e a l p a r t of the d i e l e c t r i c c o n s t a n t a t room temperature i s p r e s e n t e d i n c h a p t e r 3. In c h a p t e r 4 we p r e s e n t a d e s c r i p t i o n of the ESR a p p a r a t u s and the e x p e r i m e n t a l p r o c e d u r e , t o g e t h e r w i t h the n u m e r i c a l methods used i n the a n a l y s i s of the ESR d a t a . Chapter 5 c o n t a i n s the ESR r e s u l t s . These r e s u l t s i n c l u d e : The s p i n s u s c e p t i b i l i t i e s of l i n e s I and I I , the sum of the s p i n s u s c e p t i b i l i t i e s of l i n e s I and I I , and the peak t o peak h a l f w i d t h of l i n e s I and I I as a f u n c t i o n of t e m p e r a t u r e , the phase t r a n s i t i o n t e m p e r a t u r e s t h a t were found a t 400(3) K and 453(6) K i n the s u s c e p t i b i l i t y and h a l f w i d t h d a t a , the a n g u l a r dependence of the g v a l u e and h a l f w i d t h , measured a t T=400(3) K, of the o n l y l i n e above the phase t r a n s i t i o n a t 400(3) K, and the g v a l u e s , measured a t © = 60° (see f i g . 1.3), of b o t h l i n e below the phase t r a n s i t i o n a t 400(3) K and the s i n g l e l i n e above 400(3) K. Chapter 6 c o n t a i n s a comparison between the r e s u l t s f o r DEM(TCNQ) 2 and the r e s u l t s f o r MEM(TCNQ) 2 and HEM(TCNQ) 2 t o g e t h e r w i t h a c o r r e l a t i o n of the d i f f e r e n t phase t r a n s i t i o n t e m p e r a t u r e s i n DEM(TCNQ) 2 w i t h the two p o s t u l a t e d phase t r a n s i t i o n s i n DEM(TCNQ) 2 above room t e m p e r a t u r e . G e n e r a l c o n c l u s i o n s and some f u r t h e r e x p e r i m e n t s on DEM(TCNQ) 2 a r e p r e s e n t e d i n c h a p t e r 7. 8 CHAPTER 2 DIELECTRIC CONSTANT AND CONDUCTIVITY THEORY AND MEASUREMENTS 2.1 THEORY OF CAVITY PERTURBATION I f a d i e l e c t r i c sample of volume V, i s i n t r o d u c e d i n t o a re s o n a n t c a v i t y of volume V 0 the s h i f t i n the complex f r e q u e n c y f o r s m a l l p e r t u r b a t i o n s i s [ ( E , - D O - E Q - D , ) - ( H , - B o - H o - B , ) ] d V = (2.1) \ [ E 0 ' D 0 - H 0 . B 0 ] d V J V 0 " - - - where n 0 = 2tri/ 0 (1+i/2Q 0 ) and n, = 2nv , (1 +i/2Q, ) a r e the u n p e r t u r b e d and p e r t u r b e d complex f r e q u e n c i e s r e s p e c t i v e l y (Waldron,1969.pp.87-93). In the e x p e r i m e n t s the f r e q u e n c y of the c a v i t y w i t h the sample h o l d e r a l o n e , nw, and the frequ e n c y of the c a v i t y w i t h the sample h o l d e r and the sample, n £, were measured. I f we assume t h a t the f i e l d d i s p l a c e d by the sample i s the u n p e r t u r b e d f i e l d , and a p p r o x i m a t i n g n 0 by nH i n the denominator of (2.1) we o b t a i n 9 n„-n-s [(E 5.D 0-Eo'D s)-(H s.B 0-Ho'B )]dV r J . : ( 2 . 2 ) [E 0.D 0-H 0.B 0]dV where E 0+ E s , D0+ Ds, H 0 + H s, and B0+ Bs, are the e l e c t r i c and magnetic f i e l d s in the sample, and is the volume of the sample. For non magnetic samples H0= B0= 0. If the sample is e l l i p s o i d a l , the dimensions of the sample are much less than a wavelength, and the skindepth i s larger than or close to the smallest dimension of the sample, then the f i e l d in the sample i s E 0 E 0+E s = ( 2 . 3 ) 1+n(c -1) where n i s the depolarizing factor and e=€'+ie" i s the complex d i e l e c t r i c constant of the sample (Buranov et a l , 1 9 7 1 , p . 5 2 8 ) . If equation ( 2 . 3 ) i s substituted into equation ( 2 . 2 ) , then equating real and imaginary parts and assuming that E 0 i s constant over the volume of the sample, we obtain for a sample on the axis of a c y l i n d r i c a l cavity 1 / 1 oe" _ ( _ - _ = ( 2 . 4 ) 2 V Q Q/ U l+nU ' - l ) ] 2 + (ne")2} and :{(€'-D[l+n(e'-l)+n(e")2]} — ( 2 . 5 ) {1+n(€ *-1)}2 + (ne")2 10 where o = {E 0.D 0-H 0.B 0}dV (2.6) E O M T i s the magnitude of the e l e c t r i c f i e l d on the axis of the cavity, and e 0 i s the permitivity of free space (Buranov et al,l971 p.528). For the TM 0, 0 mode (E 0-D 0-H 0.B 0)dV = (2.7) where J,(ka) = 0.51915 (Waldron,1970 pp.303-305). o i s then 1.8552V /V 0. The solution of (2.4) and (2.5) for e'-1 and e" i s a A n 22 (2.8) and t'-1 1 T fa - 6 - - 6| n \ n A 2 - - 6 J (2.9) where 6 = and A = - - - (Buranov et a l , l 9 7 l p.528) The quantities 6, A, N and c can be measured or calculated, and hence e' and c" can be determined. 11 SAMPLE 5 F i g u r e 2.1 C a v i t y a n d S a m p l e H o l d e r u s e d f o r D i e l e c t r i c C o n s t a n t a n d C o n d u c t i v i t y M e a s u r e m e n t s . 12 2.2 DIELECTRIC CONSTANT MEASURING APPARATUS The sample was mounted on the a x i s of a c y l i n d r i c a l c a v i t y o p e r a t i n g i n the TM 0, 0 mode a t 9.2 GHz (see f i g . 2.1) the microwave s o u r c e was an HP8620C sweep o s c i l l a t o r w i t h an HP86250B RF p l u g i n . The f r e q u e n c y was measured w i t h an HP5245L fr e q u e n c y c o u n t e r w i t h an HP5255A fre q u e n c y c o n v e r t e r . Scans of the power r e f l e c t e d from the c a v i t y as a f u n c t i o n of f r e q u e n c y were o b t a i n e d (see f i g . 2 . 2 ) from which the resonant f r e q u e n c y and the Q of the resonance were d e t e r m i n e d . A b l o c k diagram of the a p p a r a t u s i s shown i n f i g u r e 2.3. To o b t a i n the c o a r s e temperature the sample c a v i t y was p l a c e d i n s i d e a g l a s s c y l i n d e r t h a t was wrapped w i t h h e a t i n g t a p e . The f i n e temperature was o b t a i n e d by u s i n g two power r e s i s t o r s next t o the c a v i t y . The c a v i t y and h e a t i n g elements were then p l a c e d i n s i d e a g l a s s dewer f o r i n s u l a t i o n . C o n t r o l of the f i n e t e m p e r a t u r e was a c c o m p l i s h e d by means of a r e s i s t a n c e b r i d g e u s i n g a t h e r m i s t o r as the temperature s e n s i t i v e element. T h i s p r o v i d e d a temperature s t a b i l i t y of 0.1°C d u r i n g each data scan. 13 Figure 2.2 Resonance of the C a v i t y used for the D i e l e c t r i c Constant and C o n d u c t i v i t y Measurements with and without the Sample. 14 Microwa ve Sweep Oscillator Matched Loaa X-Y Recorder X H Scope Variable Short Magic Tee Variable Attenuator 0-40db Screw Tuner Isolator Magic Tee Isolator i Crystal Detector X Isolator Null Voltmeter Frequency Counter Variable Attenua tor 0-AOdb Programable Power Supply 1 " Coarse Heater Supply D.V.M. (Thermocouple | Voltage) Fine Heater Cavity Thermistor F i g u r e 2.3 Block Diagram of the D i e l e c t r i c Constant and C o n d u c t i v i t y A p p a r a t u s . 15 2.3 DIELECTRIC CONSTANT AND CONDUCTIVITY MEASUREMENTS The r e s o n a n t f r e q u e n c i e s of the c a v i t y , p e r t u r b e d by the sample h o l d e r , w i t h and w i t h o u t the sample i n p l a c e were measured as a f u n c t i o n of t e m p e r a t u r e . In the l a t t e r case a l e a s t squares f i t t o the a s t r a i g h t l i n e was made and the v a l u e s of t he. re s o n a n t f r e q u e n c y and Q of the c a v i t y , were c a l c u l a t e d f o r the te m p e r a t u r e s c o r r e s p o n d i n g t o the scans where the c a v i t y was p e r t u r b e d by the sample h o l d e r and the sample. In t h i s manner i t -was p o s s i b l e t o determine a t each temperature the resonant f r e q u e n c y and Q of the c a v i t y owing t o the empty sample h o l d e r . In o r d e r t o determine the d e p o l a r i z i n g f a c t o r of the samples, the l i n e a r d i m e n s i o n s of the samples were measured on a t r a v e l l i n g m i c r o s c o p e and the d e p o l a r i z i n g f a c t o r s were o b t a i n e d by i n t e r p o l a t i o n from a p l o t of the Demagne t i z i n g F a c t o r s of the G e n e r a l E l l i p s o i d (Osborn,l945 p.355). The a p p r o x i m a t i o n t h a t the sample i s e l l i p s o i d a l has t o be made i n o r d e r t o a p p l y e q u a t i o n ( 2 . 3 ) . I n making t h i s a p p r o x i m a t i o n e l l i p s o i d s w i t h the same volume and l i n e a r d i m e n s i o n a l r a t i o s as the samples were used. The volumes of the samples were d e t e r m i n e d by w e i g h i n g the samples and u s i n g the known d e n s i t y of DEM(TCNQ) 2, d M= 1.253 mg mm"3, ( M o r r s i n k e t a l , 1981 p.107). The volumes measured by t h i s method agreed w i t h the volumes o b t a i n e d from the l i n e a r d i m e n s i o n s of the samples t o w i t h i n e x p e r i m e n t a l e r r o r . In e q u a t i o n (2.2) the e l e c t r i c f i e l d i n the immediate 16 v i c i n i t y of the sample i s assumed t o be e q u a l t o the f i e l d i n t h a t l o c a t i o n of the c a v i t y w i t h o u t the sample. T h i s assumption i s v a l i d because the volume of the sample as w e l l as the volume of the sample h o l d e r a r e n e g l i g i b l e when compared t o the volume of the c a v i t y . TABLE 2 . 1 PHYSICAL DATA OF THE NYLON TEST SAMPLE Sample a (mm) b(mm) c (mm) Volume (mm3) D e p o l a r - i z i n g F a c t o r #1 4 . 6 6 ( 2 ) 0 . 4 7 ( 2 ) 0 . 3 3 ( 2 ) 0 . 7 1 ( 5 ) 0 . 0 1 5 ( 3 ) Another p o s s i b l e source of e r r o r i s the shape of the sample h o l d e r . T h i s was checked by measuring the d i e l e c t r i c c o n s t a n t and of n y l o n t h r e a d . A p i e c e of n y l o n t h r e a d was measured and v a l u e s of e ' = 2 . 9 4 and e " = 0 . 0 3 2 were o b t a i n e d (see t a b l e 2 . 1 ) . T h i s compares w e l l w i t h the p u b l i s h e d v a l u e s of t ' = 2 . 8 4 — 3 . 0 3 and € " = 0 . 0 3 2 - 0 . 0 3 9 (Von H i p p i e , p p . 3 1 0 - 3 1 1 , 3 2 3 ) . 17 CHAPTER 3 DIELECTRIC CONSTANT AND CONDUCTIVITY RESULTS 3.1 EXPERIMENTAL RESULTS Three c r y s t a l s of DEM(TCNQ) 2 were used i n the d i e l e c t r i c c o n s t a n t and c o n d u c t i v i t y measurements. The p h y s i c a l d ata of the c r y s t a l s a r e shown i n t a b l e 3.1. The v a l u e s a, b, and c are the p r i n c i p a l axes of the a p p r o x i m a t i n g e l l i p s o i d used i n c a l c u l a t i n g the d e p o l a r i z i n g f a c t o r s . These v a l u e s approximate the d i m e n s i o n s of the samples a l o n g the d i r e c t i o n s p a r a l l e l and p e r p e n d i c u l a r t o the e l e c t r i c f i e l d . The microwave c o n d u c t i v i t y of the c r y s t a l s as a f u n c t i o n of temperature i s p l o t t e d i n f i g 3.1 and i n f i g 3.2. The measured microwave c o n d u c t i v i t y was low enough so t h a t the low e s t v a l u e of the s k i n depth was g r e a t e r than 1.8 mm. T h i s i s l a r g e r than the s m a l l e s t d i m e n s i o n of the sample (see t a b l e 3.2). The room temperature d i e l e c t r i c c o n s t a n t and c o n d u c t i v i t y a r e summarized i n t a b l e 3.2. I t was o n l y p o s s i b l e t o o b t a i n e' a t room temperature because the l a r g e d e p o l a r i z i n g f a c t o r causes the measurements of e' t o be u n r e l i a b l e when the c o n d u c t i v i t y i n c r e a s e s . 18 to m ro <M — d d d d d d (,-WO-U) AllAllOnQNOD F i g u r e 3.1 Microwave C o n d u c t i v i t y of DEM(TCNQ) 2 vs Temperature A : Sample #1 Q:Sample #2 Sample #3. The S o l i d Curve i s E s t i m a t e d from the P o i n t s . I 0 2 / T F i g u r e 3.2 N a t u r a l L o g a r i t h m o f t h e M i c r o w a v e C o n d u c t i v i t y o f D E M ( T C N Q ) 2 v s I n v e r s e T e m p e r a t u r e . S y m b o l s a r e d e f i n e d i n f i g . 3 . 1 . The S o l i d C u r v e i s E s t i m a t e d f r o m t h e P o i n t s . .20 TABLE 3.1 PHYSICAL DATA OF DEM(TCNQ) 2 SAMPLES Sample a (mm) b(mm) c (mm) Volume (mm3) D e p o l a r - i z i n g F a c t o r #1 1 .36(2). 0.62(2) 0.16(2) 0. 1 140(3) 0.055(5) #2 0.92(2) 0.69(2) 0.15(2) 0.0651(3) 0.095(5) #3 0.81(5) 0.75(5) 0.25(2) 0.1416(3) 0. 165(15) TABLE 3.2 EXPERIMENTAL RESULTS Sample (Room- temp) e" (Room- temp) E 0 (e.v.) lntf o ln{(n-cm)- 1} #1 11.0 5.5 0.328(26) 2.9(5) #2 4.3 2.7 0.403(43) 3.8(7) #3 11.6 2.4 0.43(12) 4.3(21) average 9.0(23) 3.5(10) 0.389(52) 3.7(9) 21 The c o n d u c t i v i t y was found t o f o l l o w a semiconductor l i k e b e h a v i o r below 370 K. The d a t a was a n a l y s e d i n terms of the e q u a t i o n <y = * 0 e x p ( - E 0 / 2 k T ) (3.1) where E 0 i s the band gap. The v a l u e s of E 0 and ln<r 0 o b t a i n e d f o r the c r y s t a l s a r e shown i n t a b l e 3.2. Above 370 K d e v i a t i o n s from an o r d i n a r y semiconductor b e h a v i o r are observed and a t a temperature of 442(6) K a d i s c o n t i n u i t y i n the c o n d u c t i v i t y i s o b s e r v e d , i n d i c a t i n g a phase t r a n s i t i o n . Above the phase t r a n s i t i o n an abrupt decrease i n the c o n d u c t i v i t y i s o b s e r v e d . The h e a t i n g of the sample was found t o be n o n — r e v e r s i b l e , owing t o the f a c t t h a t the samples l o s t 25% of t h e i r mass when they were heated between 450 K and 480 K, and the subsequent room temperature c o n d u c t i v i t y was s h a r p l y lower when the sample had been heated up t o 480 K. T h i s l o s s of mass t o g e t h e r w i t h the d e c r e a s e of the c o n d u c t i v i t y i n d i c a t e s the d e c o m p o s i t i o n of the sample above 450 K. ,22 3.2 DISCUSSION OF THE DIELECTRIC CONSTANT AND CONDUCTIVITY MEASUREMENTS (a) Quoted E x p e r i m e n t a l E r r o r s The e r r o r s i n the di m e n s i o n s of the samples were e s t i m a t e d u s i n g r e p e a t e d measurements on a t r a v e l l i n g m i c r o s c o p e . The measurements f o r sample #3 have l a r g e r e r r o r because the sample's l o n g e s t dimension was a t an a n g l e of about 30° t o the e l e c t r i c f i e l d i n s t e a d of a l o n g the e l e c t r i c f i e l d . T h i s meant t h a t e s t i m a t e s of the dime n s i o n s of the sample a l o n g the e l e c t r i c f i e l d and p e r p e n d i c u l a r t o the e l e c t r i c f i e l d were used i n s t e a d of the l e n g t h , w i d t h , and t h i c k n e s s of the sample. The quoted e r r o r i n the d e p o l a r i z i n g f a c t o r i s due t o the e s t i m a t e d e r r o r s i n the d i m e n s i o n s . o f the samples. T h i s e r r o r does not i n c l u d e any a d d i t i o n a l e r r o r s i n t r o d u c e d by the a p p r o x i m a t i o n of the shape of the samples as e l l i p s o i d s . The quoted e r r o r s f o r E 0 and ln<y0 f o r each sample a re the s t a n d a r d e r r o r s from the f i t of the c o n d u c t i v i t y d a t a t o I n * = - E 0 / 2 k T + l n t f o (3.2) where the parameters E 0 and l n < j r 0 a r e dete r m i n e d by the f i t . T h i s e r r o r i n c o r p o r a t e s the s t a t i s t i c a l e r r o r i n the d a t a f o r each sample, i t does not i n c o r p o r a t e any s y s t e m a t i c e r r o r s . The v a l u e s of E 0 and l n c r 0 d e s c r i b e d as average i n t a b l e 3.2 are de t e r m i n e d from a f i t t o e q u a t i o n (3.2) u s i n g the d a t a from the t h r e e samples. The e r r o r s i n E 0 and lho- 0 o b t a i n e d i n t h i s case 23 are the s t a n d a r d e r r o r s from the f i t . These l a s t e r r o r s w i l l i n c l u d e some of the s y s t e m a t i c e r r o r i n as f a r as t h i s e r r o r i s s y s t e m a t i c t o one sample, but not s y s t e m a t i c t o a l l the measurements. The l a r g e s t c o n t r i b u t i o n t o t h i s k i n d of e r r o r comes from d e p o l a r i z i n g f a c t o r . The e r r o r s quoted f o r c' and e" f o r t h e average r e s u l t a r e the s t a n d a r d e r r o r of t h e mean of t h e t h r e e measurements. (b) Shape of the Samples as a Source of E r r o r , The d e t e r m i n a t i o n of e' i s l i m i t e d by the d e p o l a r i z i n g f a c t o r of the samples. A s m a l l d e p o l a r i z i n g f a c t o r i s d e s i r a b l e because i t a l l o w s the e l e c t r i c f i e l d t o p e n e t r a t e i n s i d e the sample p e r m i t t i n g a measurement of e'. Another d i s a d v a n t a g e of a l a r g e d e p o l a r i z i n g f a c t o r i s t h a t the d e p o l a r i z i n g f a c t o r i s de t e r m i n e d o n l y a p p r o x i m a t e l y . I f the terms t h a t c o n t a i n the d e p o l a r i z i n g f a c t o r a re dominant i n e q u a t i o n s (2.8) and (2.9) then t h e e r r o r s i n the d e p o l a r i z i n g f a c t o r t o g e t h e r w i t h the a p p r o x i m a t i o n of the sample shape by an e l l i p s o i d would i n t r o d u c e s i g n i f i c a n t e r r o r s i n t o t h e f i n a l r e s u l t s . The o p t i m a l shape t o m i n i m i z e the d e p o l a r i z i n g f a c t o r i s a ve r y l o n g and t h i n c r y s t a l where a>>b>c. The c r y s t a l s used were s e l e c t e d f o r low d e p o l a r i z i n g f a c t o r ; however the a v a i l a b l e c r y s t a l s were f a r from the o p t i m a l shape. T h i s l i m i t e d the a c c u r a c y of the experiment and p r e c l u d e d a measurement of e' above room t e m p e r a t u r e . 24 CHAPTER 4 ESR APPARATUS, MEASUREMENTS AND SUSCEPTIBILITY CALCULATIONS 4.1 ESR APPARATUS E l e c t r o n s p i n resonance, ESR, was performed a t X—band. The sample was p l a c e d i n a r e c t a n g u l a r c a v i t y o p e r a t i n g i n the TE102 mode. The c a v i t y was p l a c e d i n a magnetic f i e l d t h a t was. swept thr o u g h the ESR resonance. The magnetic f i e l d was modulated a t 100 KHz by means of c o i l s p l a c e d on. the c a v i t y e x t e r i o r . The s t a t i c f i e l d was measured by means of an NMR probe. The d i f f e r e n c e i n f i e l d between t h e p o s i t i o n of t h e NMR probe and the c e n t e r of the c a v i t y was c a l i b r a t e d w i t h a s t a n d a r d ESR sample of L i F : L i , the NMR f r e q u e n c i e s b e i n g measured on an HP5245L fr e q u e n c y c o u n t e r . The microwave source was a V a r i a n VA—297 k l y s t r o n o p e r a t i n g a t 9.2 GHz t h a t was p h a s e — l o c k e d t o the sample c a v i t y f r e q u e n c y . The microwave freq u e n c y was measured by an HP5245L fr e q u e n c y c o u n t e r w i t h an HP5255A f r e q u e n c y c o n v e r t e r . Scope N.M.R. Oscillator Frequency Counter Variable Attenuator 0-40db D.V.M. tThermocouptt Voltage) Sample Heater Supply N.M.R. Probe WOKHz Modulation* Coils Magnetic Field Sfctee/} Coils I Klystron Klystron Supply I , A.F.C. Isolator 1 10db Directional Coupler Crys- tal Bias Variable Attenuator 0-SOdb 1 I Pre Amplifier Circulator I Tuned Crystal Detector Audio Amplifier Magnet Pole Face Nitrogen Gas Input Mini Computer Lock-in Amplifier X-Y Recorder Magnetic Field Sweep D.C. Amplifier F i g u r e 4 . 1 B l o c k D i a g r a m o f t h e ESR A p p a r a t u s . 26 A b s o r p t i o n of the sample was d e t e c t e d w i t h a l o c k i n a m p l i f i e r whose output gave the d e r i v a t i v e of the ESR s i g n a l and a f t e r a m p l i f i c a t i o n was r e c o r d e d on an X—Y r e c o r d e r , or sampled d i g i t a l l y w i t h a NOVA 2 microcomputer and punched out on paper ta p e . The d i g i t i z e d d a t a c o u l d then be read i n t o t he UBC computing system f o r f u r t h e r p r o c e s s i n g . A b l o c k diagram of the ap p a r a t u s i s shown i n f i g . 4.1. The sample was heated by means of a V a r i a n f l o w system ( f i g . 4.2) u s i n g n i t r o g e n gas as the heat t r a n s f e r medium. The sample temperature was measured u s i n g an i r o n — c o n s t a n t a n thermocouple w i t h an i c e b a t h as r e f e r e n c e . 4.2 THE ESR MEASUREMENTS E l e c t r o n s p i n resonance (ESR) measurements were performed on DEM(TCNQ) 2 as a f u n c t i o n of temperature between room temperature and 453(6) K. The g v a l u e s of the two o b s e r v e d l i n e s were measured as a f u n c t i o n of t e m p e r a t u r e . Above the phase t r a n s i t i o n the -g v a l u e of l i n e I was measured as a f u n c t i o n of the a n g l e between the c r y s t a l a x i s and the magnetic f i e l d . The s p i n s u s c e p t i b i l i t i e s and peak t o peak w i d t h s were then d e t e r m i n e d by f i t t i n g the i n t e g r a l of the output d e r i v a t i v e curve w i t h c o n v o l u t e d G a u s s i a n - L o r e n t z i a n f u n c t i o n s . SAMPLE TUBE WAVEGUIDE 100 KHz MODULATION COILS THERMOCOUPLE A/L OUT .. GLASS K DEWER CAVITY SAMPLE HEATER F i g u r e 4 . 2 ESR C a v i t y a n d V a r i a n F l o w S y s t e m . F i g u r e 4 . 3 T y p i c a l ESR S p e c t r a a b o v e T r a n s i t i o n a t 4 0 0 ( 3 ) K . a n d b e l o w t h e P h a s e 29 The s p i n s u s c e p t i b i l i t i e s and peak t o peak w i d t h s c o u l d then be o b t a i n e d from the G a u s s i a n - L o r e n t z i a n f u n c t i o n s . T y p i c a l ESR s p e c t r a below and above the obser v e d phase t r a n s i t i o n a r e shown i n f i g . 4 . 3 . 4 . 3 CONVOLUTED GAUSSIAN-LORENTZIAN FIT (a) F i t The d i g i t i z e d ESR output s i g n a l was i n t e g r a t e d n u m e r i c a l l y and then f i t t e d w i t h c o n v o l u t e d G a u s s i a n - L o r e n t z i a n f u n c t i o n s . The f i t was performed by m i n i m i z i n g the sum of square d e v i a t i o n s between the d a t a p o i n t s and the f u n c t i o n s . The s u s c e p t i b i l i t i e s of each i n d i v i d u a l c u r v e were then d e t e r m i n e d by i n t e r g r a t i n g the G a u s s i a n - L o r e n t z i a n f u n c t i o n s o b t a i n e d from the f i t . The c o r r e s p o n d i n g peak t o peak h a l f w i d t h s were o b t a i n e d from the d e r i v a t i v e of the G a u s s i a n - L o r e n t z i a n f u n c t i o n s o b t a i n e d from the f i t . T h i s method p r o v i d e d the s u s c e p t i b i l i t i e s and peak t o peak h a l f w i d t h s of each i n d i v i d u a l l i n e i n the c o n v o l u t e d ESR s p e c t r a below the phase t r a n s i t i o n . Above t h e phase t r a n s i t i o n the s u s c e p t i b i l i t i e s , and peak t o peak h a l f w i d t h s were d e t e r m i n e d from a f i t of a s i n g l e c o n v o l u t e d G a u s s i a n - L o r e n t z i a n f u n c t i o n t o the i n t e g r a t e d d a t a . The most s i g n i f i c a n t e r r o r i n the s u s c e p t i b i l i t y c a l c u l a t i o n a r i s e s from the d e t e r m i n a t i o n of the b a s e l i n e ; 30 however in'. 12% of the d a t a scans s i g n i f i c a n t d i s t o r t i o n s were found t h a t were not due t o i n t e g r a t i n g a d e r i v a t i v e curve w i t h an i n c o r r e c t b a s e l i n e . These d i s t o r t i o n s were due t o an asymmetry i n the d e r i v a t i v e c u r v e s . The meaning of the s u s c e p t i b i l i t i e s i n t h e s e c a s e s i s s u s p e c t , and these scans were not i n c o r p o r a t e d i n the r e s u l t s . (b) B a s e l i n e D e t e r m i n a t i o n The problem of d e t e r m i n i n g the b a s e l i n e i s a s i g n i f i c a n t l i m i t a t i o n t o the f i n a l a c c u r a c y of the s u s c e p t i b i l i t i e s . The i n i t i a l e s t i m a t e of the b a s e l i n e was d e t e r m i n e d by a v e r a g i n g the d a t a a c r o s s the s c a n . The i n t e g r a t e d d a t a were a l s o found t o have d i s t o r t i o n s t h a t were a t t r i b u t e d t o problems i n b a s e l i n e removal. There were scans where the t a i l b e f o r e the peaks d e c r e a s e d b e f o r e i t i n c r e a s e d , t h i s i s due t o removing too l a r g e a b a s e l i n e , and t h e r e were scans where the t a i l a f t e r the peaks i n c r e a s e d b e f o r e i t d e c r e a s e d , t h i s i s due t o removing too s m a l l a b a s e l i n e . These d i s t o r t i o n s were m i n i m i z e d by making changes t o the i n i t i a l b a s e l i n e of the o r d e r of 0.5% i n t e r a c t i v e l y on the UBC computer. The c o n t r i b u t i o n t o the e r r o r i n the s u s c e p t i b i l i t i e s from t h i s source c o u l d be e s t i m a t e d t o be about 10%. 31 CHAPTER 5 ESR EXPERIMENTAL RESULTS 5.1 ESR q VALUES At room temperature two l i n e s I and I I are found i n the spectrum of DEM(TCNQ) 2, c o r r e s p o n d i n g t o the two s t a c k s B and A. We f o l l o w the n o t a t i o n of S c h w e r d t f e g e r e t a l . 1980. The g v a l u e s of l i n e s I and I I of the ESR spectrum of DEM(TCNQ) 2 were determined as a f u n c t i o n of temperature between 290-K and 401 K a t an a n g l e 9 = 60° (see f i g . 5 . 1 ) The g v a l u e was found t o be independent of temperature between .290 K and T c = 400 K. The g v a l u e s g i v e n i n t a b l e I f o r T<T C a r e the average of the g v a l u e s measured f o r 9 = 60° a t v a r i o u s t e m p e r a t u r e s between 290 K and 400(3) K. The g v a l u e s of both l i n e s below the phase t r a n s i t i o n and the g v a l u e f o r the s i n g l e l i n e above the phase t r a n s i t i o n a re summarized i n the t a b l e below. 32 TABLE 5. 1 ESR CJ VALUES Temp. I I I T<TC T>TC 2.003233(23) 2.003250(10) 2.002571(17) The g v a l u e below the phase t r a n s i t i o n was not found t o change a p a r t from e x p e r i m e n t a l e r r o r . The e r r o r s quoted f o r the g v a l u e s below T t a r e the s t a n d a r d e r r o r or the mean of n i n e measurements a t d i f f e r e n t t e m p e r a t u r e s below T c . The e r r o r s quoted f o r the g v a l u e above T c are the s t a n d a r d e r r o r of the mean of two r e s u l t s above the phase t r a n s i t i o n . The above e r r o r s do not i n c l u d e any s y s t e m a t i c e r r o r s from the c a l i b r a t i o n . The c a l i b r a t i o n was performed u s i n g a sample of L i F : L i . The g v a l u e of L i i s 2.002317(2). ( P r e s s l e y e t a l 1963, and Gordon et a l 1972 p.345 and p.336). The c a l i b r a t i o n i n t r o d u c e s a s y s t e m a t i c e r r o r of 0.001% t o the g v a l u e measurements. These g v a l u e d a t a i m p l y t h a t the r e m a i n i n g l i n e above the phase t r a n s i t i o n i s l i n e I . 33 F i g u r e 5.1 A n g u l a r dependence of the g v a l u e of DEM(TCNQ) 2 above the phase t r a n s i t i o n a t 4.00(3) K Q: c r y s t a l #5 X: repeat run of c r y s t a l #5. The s o l i d l i n e i s a f i t t o e q u a t i o n 5.1 34 5.2 ANGULAR DEPENDENCE OF THE CJ VALUES The a n g u l a r dependence of the g v a l u e s above the phase t r a n s i t i o n was measured by r o t a t i n g the sample i n t h e pl a n e of the s t a t i c magnetic f i e l d . T h i s d a t a i s p l o t t e d i n f i g 5.1. The a n g u l a r dependence of the g v a l u e was f i t t e d t o the e q u a t i o n g 2 = g 2 c o s 2 e + g^sin 2© (5.1) The r e s u l t i n g v a l u e s f o r g | t and g^ a t 401 K t o g e t h e r w i t h the v a l u e s quoted i n the l i t e r a t u r e f o r lower t e m p e r a t u r e s ( S c h w e r d t f e g e r e t a l 1980) a r e summarized i n t a b l e 5.2. TABLE 5.2 AND . oLj_ FOR DEM(TCNQ) 2 Temp. (K) l i n e 9„ I l i n e 9„ I I 9x 401 2.002730(15) 2.003551(14) - - 298 2.00235 2.00325 2. 00230 2 .00335 77 2.00219 2.00231 2. 00223 2 .00315 4.2 - - 2. 00270 2 .00399 1.14 - - 2. 00147 2 .00399 35 r i r — — i 1 ID CVJ c - 0 i x ( a i o w / r w 3 ) A i n i a i i d 3 o s n s F i g u r e 5.2 Temperature Dependence of the S p i n S u s c e p t i b i l i t y of l i n e I of DEM(TCNQ) 2 • : c r y s t a l #4 0 : c r y s t a l #5 X : repeat run of c r y s t a l #5 #: c r y s t a l #6 The S o l i d Curve i s E s t i m a t e d from the P o i n t s 36 LU or < cr LU GL LU £.oi x (aiouj/nw3) A i n i a i i d 3 D s n s F i g u r e 5.3 Temperature Dependence of the S p i n S u s c e p t i b i l i t y of l i n e I I of DEM(TCNQ) 2. Symbols a r e D e f i n e d i n F i g . 5.2. The S o l i d Curve i s E s t i m a t e d from the P o i n t s . 37 The quoted e x p e r i m e n t a l e r r o r f o r g ) ( and qx i s the s t a t i s t i c a l e r r o r from the f i t of the da t a t o e q u a t i o n 5.1. The s y s t e m a t i c d i f f e r e n c e between the v a l u e s f o r g ( ) and q± a t 298 K and a t 420 K c o u l d be due t o a s y s t e m a t i c e r r o r between the two measurements. T h i s c o n c l u s i o n i s reached because the measurements a t 298 K were c a l i b r a t e d u s i n g DPPH and t h e r e was no d i f f e r e n c e i n g v a l u e between 298 K and 401 K when both measurements were performed u s i n g the same c a l i b r a t i o n . 5.3 TEMPERATURE DEPENDENCE OF THE SUSCEPTIBILITY The s p i n s u s c e p t i b i l i t y of DEM(TCNQ) 2 has been measured below 270 K (S c h w e r d t f e g e r e t a l . 1980). The s p i n s u s c e p t i b i l i t y of l i n e s I and I I i s shown i n f i g s . 5.2 and 5.3 r e s p e c t i v e l y . The sum of the s u s c e p t i b i l i t i e s of l i n e s I and I I i s p l o t t e d i n f i g . 5.4. From t h i s d a t a we see t h a t the s u s c e p t i b i l i t y of l i n e I I goes t o z e r o a t 400(3) K. Above 400(3) K o n l y one l i n e i s ob s e r v e d , i n d i c a t i n g a phase t r a n s i t i o n a t 400(3) K. The s p i n s u s c e p t i b i l i t y of l i n e I i n c r e a s e s a t 400(3) K and the combined s u s c e p t i b i l i t y c urve i s c o n t i n u o u s a c r o s s the phase t r a n s i t i o n . We have seen t h a t the g v a l u e of l i n e I b e f o r e the phase t r a n s i t i o n i s the same as the g v a l u e of the s i n g l e l i n e above the phase t r a n s i t i o n (see s e c t i o n 5.1). T h i s t o g e t h e r w i t h the s u s c e p t i b i l i t y d a t a i n d i c a t e s t h a t t h e r e i s a t r a n s f e r of s p i n from s t a c k A t o s t a c k B a t 400(3) K. There i s a t p r e s e n t no p h y s i c a l e x p l a n a t i o n f o r t h i s t r a n s f e r of s p i n . 38 F i g u r e 5.4 Temperature Dependence of the Sum of the S p i n S u s c e p t i b i l i t i e s of l i n e s I and I I of DEM(TCNQ) 2. Symbols a r e D e f i n e d i n F i g . 5.2 The S o l i d Curve i s E s t i m a t e d from the P o i n t s 39 I i I I I I I I 310 350 390 430 TEMPERATURE (K) F i g u r e 5 . 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 P e a k t o P e a k H a l f w i d t h o f L i n e s I a n d I I o f D E M ( T C N Q ) 2 . S y m b o l s a r e D e f i n e d i n F i g . 5 . 2 40 o o o o d d d ( S S R D 6 ) H V F i g u r e 5.6 A n g u l a r Dependence of t h e Peak t o Peak H a l f w i d t h of l i n e I of DEM(TCNQ) 2 above the phase t r a n s i t i o n a t 400(3) K. The S o l i d L i n e i s a F i t t o H 2 = H 2cos 2e + H 2 s i n 2 e . 41 The phase t r a n s i t i o n a t 4 0 0 ( 3 ) K was found t o be r e v e r s i b l e . Sample #5 was taken p a s t the phase t r a n s i t i o n t o 401 K and then c o o l e d back t o room t e m p e r a t u r e . The re p e a t measurements of sample #5 were no d i f f e r e n t i n the s u s c e p t i b i l i t y or the g v a l u e . Between 4 1 5 K and 4 5 3 ( 6 ) K the combined s u s c e p t i b i l i t y d e c r e a s e s s h a r p l y t o z e r o and does not f o l l o w the h i g h temperature t a i l of the C u r i e — W i e s s law. T h i s d e c r e a s e i n the s u s c e p t i b i l i t y , however, i s i r r e v e r s i b l e . A repeat measurement a f t e r the sample has been c o o l e d t o room temperature produces no ESR s i g n a l The s c a t t e r i n the da t a of the s u s c e p t i b i l i t y as a f u n c t i o n of t e m p e r a t u r e , a r i s e s from the e r r o r i n the d e t e r m i n a t i o n of the G a u s s i a n - L o r e n t z i a n f i t t i n g f u n c t i o n s . These e r r o r s a r i s e from the d e t e r m i n a t i o n of the b a s e l i n e . 5 . 4 TEMPERATURE DEPENDENCE OF THE PEAK TO PEAK WIDTHS OF THE ESR LINES The peak t o peak w i d t h of the ESR l i n e s was measured as a f u n c t i o n of temperature between 290 K and 4 5 3 ( 6 ) K. The da t a a r e p l o t t e d i n f i g 5 . 5 . The peak t o peak w i d t h s were c a l c u l a t e d by a double n u m e r i c a l i n t e r g r a t i o n of each G a u s s i a n - L o r e n t z i a n f u n c t i o n o b t a i n e d from the f i t . The d a t a show a broade n i n g of both l i n e s w i t h • i n c r e a s i n g t emperature as the phase t r a n s i t i o n t e m p e r a t u r e , 4 0 0 ( 3 ) K, i s 42 approached from below. The phase t r a n s i t i o n a t 400(3) K i s accompanied by a d i s c o n t i n o u s drop i n the peak t o peak h a l f w i d t h of l i n e I , and the d i s a p p e a r a n c e of l i n e I I . The peak t o peak h a l f w i d t h as a f u n c t i o n of a n g l e f o r s t a c k B above the phase t r a n s i t i o n i s p l o t t e d i n f i g 5.6. The h a l f w i d t h i n t h i s case was measured d i r e c t l y from the ESR sc a n s . The a n g u l a r dependence of the h a l f w i d t h r e a c h e s a minimum f o r the same 9 where the a n g u l a r dependence of the g v a l u e s e x h i b i t s a maximum (see f i g 5.1). T h i s r e s u l t i s s i m i l a r t o t h a t r e p o r t e d by S c h w e r d t f e g e r e t a l (1980) f o r s t a c k A a t 77 K The s c a t t e r i n the peak t o peak h a l f w i d t h vs temperature c u r v e a r i s e s from the d e t e r m i n a t i o n of the G a u s s i a n - L o r e n t z i a n f i t t i n g f u n c t i o n . The most s i g n i f i c a n t c o n t r i b u t i o n t o th e s e e r r o r s i s the d e t e r m i n a t i o n of the b a s e l i n e (see s e c t i o n 4.3). The e s t i m a t e d e r r o r s i n the peak t o peak h a l f w i d t h as a f u n c t i o n of a n g l e a r e a r e e s t i m a t e d from the ESR data s c a n s . 43 CHAPTER 6 DISSCUSION OF THE RESULTS 6. 1 PHASE TRANSITIONS .IN DEM (TCNQ) 2 The ESR measurements show a r e v e r s i b l e phase t r a n s i t i o n a t 400(3) K, where the s u s c e p t i b i l i t y of l i n e I I goes i n t o l i n e I . At t h i s t e mperature t h e r e i s no i n d i c a t i o n of a phase t r a n s i t i o n i n the microwave c o n d u c t i v i t y or i n the t o t a l s p i n s u s c e p t i b i l i t y . T h i s phase t r a n s i t i o n i s e x p l a i n e d by a t r a n s f e r of s p i n d e n s i t y from s t a c k B t o s t a c k A. The f a c t t h a t the c o n d u c t i v i t y and s p i n s u s c e p t i b i l i t y a r e c o n t i n u o u s over t h i s phase t r a n s i t i o n and t h a t the s p i n s u s c e p t i b i l i t y e q u a l s the v a l u e of the s p i n s u s c e p t i b i l i t y i m m e d i a t e l y below the phase t r a n s i t i o n a t 400(3) K i m p l i e s t h a t the c o n t r i b u t i o n t o the c o n d u c t i v i t y and t o the s u s c e p t i b i l i t y from s t a c k A and s t a c k B are e q u a l . N o n — r e v e r s i b l e e f f e c t s i n the s u s c e p t i b i l i t y a r e observed above 415 K w h i l e below t h i s t e mperature the c r y s t a l behaves i n a r e v e r s i b l e manner. These n o n — r e v e r s i b l e e f f e c t s c o u l d p o s s i b l y be due t o the onset of the d e c o m p o s i t i o n of the sample. At a temperature of 442(6) K a n o n — r e v e r s i b l e phase t r a n s i t i o n i n the microwave c o n d u c t i v i t y i s o b s e r v e d . Between 420 K and 453(6) K the s p i n s u s c e p t i b i l i t y d e c r e a s e s 44 i r r e v e r s i b l y t o z e r o . T h i s i n d i c a t e s t h a t the d e c rease of the s p i n s u s c e p t i b i l i t y t o z e r o and the d i s c o n t i n u i t y i n the c o n d u c t i v i t y a r e d i f f e r e n t m a n i f e s t a t i o n s of the same phase t r a n s i t i o n . The d e c r e a s e of the c o n d u c t i v i t y between 460 K and 480 K was found t o be due t o a d e c o m p o s i t i o n of the sample. On the o t h e r hand te m p e r a t u r e dependent G u i n i e r photographs of DEM(TCNQ) 2 showed two r e v e r s i b l e phase t r a n s i t i o n s a t 415 K and 483 K. ( M o r r s i n k e t a l 1980). We p o s t u l a t e t h a t the phase t r a n s i t i o n a t 442(6) K t o 453(6) K and the G u i n i e r measurement a t 483 K a r e r e l a t e d . S i m i l a r l y we p o s t u l a t e t h a t the r e v e r s i b l e phase t r a n s i t i o n a t 400(3) K i s r e l a t e d t o the phase t r a n s i t i o n a t 415 K o b t a i n e d by the G u i n i e r measurement. A p o s s i b l e e x p l a n a t i o n i s - t h a t t h e s e two phase t r a n s i t i o n s o c c u r over a range of t e m p e r a t u r e s , w i t h the e l e c t r o n i c e f f e c t s o c c u r r i n g b e f o r e the s t r u c t u r a l e f f e c t s . These d i f f e r e n c e s i n the phase t r a n s i t i o n t e m p e r a t u r e s can a l s o be e x p l a i n e d by the d i f f e r e n t r a t e s a t which the sample was heated i n t h e v a r i o u s e x p e r i m e n t s , th e s l o w e s t r a t e b e i n g the c o n d u c t i v i t y measurement w i t h the l o w e s t phase t r a n s i t i o n t e m p e r a t u r e , then the ESR measurements and f i n a l l y the G u i n i e r measurements w i t h the f a s t e s t r a t e of h e a t i n g and the h i g h e s t phase t r a n s i t i o n t e m p e r a t u r e . A l t h o u g h t h i s r e q u i r e s f u r t h e r i n v e s t i g a t i o n , a s i m i l a r p a t t e r n i s seen i n the o t h e r compounds as noted below. 45 6.2 COMPARISON OF DEM(TCNQ) 2 TO HEM(TCNQ) 2 AND MEM(TCNQ) 2 The ESR of HEM(TCNQ) 2 e x h i b i t s a phase t r a n s i t i o n a t 425 K w i t h a d e c r e a s e t o z e r o of the s p i n s u s c e p t i b i l i t y and a d i f f e r e n t i a l s c a n n i n g c a l o r i m e t e r (DSC) measurement of HEM(TCNQ) 2 e x h i b i t s a phase t r a n s i t i o n a t 450 K. ( H u i z i n g a 1980 p.108).Temperature dependent G u i n i e r photographs of HEM(TCNQ) 2 e x h i b i t a phase t r a n s i t i o n a t 456 R (van Bodegom, 1979, p.73). T h i s c o r r e s p o n d s t o the 2k F phase t r a n s i t i o n i n HEM(TCNQ) 2 ( H u i z i n g a 1980 p.108). These d i f f e r e n c e s i n phase t r a n s i t i o n t e m p e r a t u r e s a r e v e r y s i m i l a r t o tho s e r e p o r t e d f o r DEM(TCNQ) 2 above. In MEM(TCNQ) 2 we f i n d b o th the 2 k F and the 4 k F i n s t a b i l i t i e s . The 2 k F i n s t a b i l i t y has been a t t r i b u t e d t o the S p i n — P e i e r l s phase t r a n s i t i o n a t 18 K w h i l e the 4k f phase t r a n s i t i o n has been a t t r i b u t e d t o the s e m i c o n d u c t o r — m e t a l phase t r a n s i t i o n a t 335 K ( H u i z i n g a 1980 ). The q u e s t i o n of how the phase t r a n s i t i o n s i n MEM(TCNQ) 2 and HEM(TCNQ) 2 r e l a t e t o the phase t r a n s i t i o n s B of DEM(TCNQ) 2 remains t o be answered. The S p i n — P e i e r l s phase t r a n s i t i o n i n MEM(TCNQ)2 a t 18 K i s c l o s e t o the S p i n - P e i e r l s phase t r a n s i t i o n a t 23 K i n s t a c k B of DEM(TCNQ) 2. The n a t u r e of the two phase t r a n s i t i o n s above room temperature i n DEM(TCNQ) 2 s h o u l d i n d i c a t e which phase t r a n s i t i o n c o r r e s p o n d s t o the 4k F i n s t a b i l i t y i n Stack B of DEM(TCNQ) 2. The phase t r a n s i t i o n a t 400(3) K i s r e l a t e d t o s t a c k A. There i s a t r a n s f e r of s p i n d e n s i t y from s t a c k A t o s t a c k B, but t h e r e i s no e f f e c t on the t o t a l b u l k s u s c e p t i b i l i t y or the 46 c o n d u c t i v i t y t o w i t h i n the s c a t t e r i n the d a t a . The r a d i c a l e f f e c t s on the s u s c e p t i b i l i t y and the c o n d u c t i v i t y a r e observed at 453(6) K and 442(6) K r e s p e c t i v e l y , i n d i c a t i n g t h a t t h i s phase t r a n s i t i o n may c o r r e s p o n d t o t h e 4k F i n s t a b i l i t y . T h i s however i s not c l e a r because of the d e c o m p o s i t i o n of the sample. The l a t t e r phase t r a n s i t i o n i n DEM(TCNQ) 2 i s however d i f f e r e n t from the s e m i c o n d u c t o r — m e t a l phase t r a n s i t i o n i n MEM(TCNQ) 2 i n t h a t no m e t a l l i c c o n d u c t i v i t y i s o b s e r v e d above t h i s phase t r a n s i t i o n . T h i s however can be e x p l a i n e d because the phase t r a n s i t i o n temperature i n DEM(TCNQ) 2 i s h i g h e r than i n MEM(TCNQ) 2, namely because of the p r o b a b l e d e c o m p o s i t i o n of the DEM(TCNQ) 2 sample. There a r e a l s o some s i m i l a r i t i e s between the h i g h e s t phase t r a n s i t i o n i n DEM(TCNQ) 2 and the phase t r a n s i t i o n a t 425 t o 450 K i n HEM(TCNQ) 2 i n t h a t i n both phase t r a n s i t i o n s the s u s c e p t i b i l i t y d e c r e a s e s t o z e r o and t h e r e a r e temperature d i s c r e p a n c i e s of 30 K and 31 K between the decrease of the s u s c e p t i b i l i t y t o z e r o and the G u i n i e r measurements r e s p e c t i v e l y . The cause of the temp e r a t u r e d i s c r e p a n c y i n both c a s e s i s p r o b a b l y the speed a t which b o t h k i n d s of measurements were made or a d i f f e r e n c e i n the temperature between the e l e c t r o n i c and the s t r u c t u r a l e f f e c t s . The p r o x i m i t y i n the temper a t u r e s of the s e two phase t r a n s i t i o n s c o u l d a l s o account f o r some of the s i m i l a r i t i e s . 47 CHAPTER 7 CONCLUSIONS AND FURTHER POSSIBLE EXPERIMENTS 7.1 CONCLUSIONS The ESR and microwave c o n d u c t i v i t y of DEM(TCNQ) 2 above room temp e r a t u r e showed two phase t r a n s i t i o n s . F i r s t l y a r e v e r s i b l e phase t r a n s i t i o n a t 400(3) K where t h e r e was a t r a n s f e r of s p i n d e n s i t y from s t a c k A t o s t a c k B. The a n g u l a r dependence of the g v a l u e of the s i n g l e l i n e above t h i s phase t r a n s i t i o n was i d e n t i c a l t o t h a t of l i n e I . T h i s phase t r a n s i t i o n was not observ e d i n a measurement of the t o t a l s p i n s u s c e p t i b i l i t y nor i n a measurement of the microwave c o n d u c t i v i t y . The second phase t r a n s i t i o n was observed between 442(6) K and 453(6) K. T h i s i s a n o n — r e v e r s i b l e phase t r a n s i t i o n t h a t was m a n i f e s t e d by a d i s c o n t i n u i t y i n the microwave c o n d u c t i v i t y and the decrease t o z e r o of the t o t a l s p i n s u s c e p t i b i l i t y . T h i s phase t r a n s i t i o n i s p o s s i b l y due t o the 4k i n s t a b i l i t y i n s t a c k B of DEM(TCNQ) 2. The phase t r a n s i t i o n t e m p e r a t u r e s d e t e r m i n e d by the ESR and microwave c o n d u c t i v i t y methods were lower than the G u i n i e r photograph r e s u l t s f o r both phase t r a n s i t i o n s . 48 7.2 FURTHER DIRECTIONS There a r e many p o s s i b i l i t i e s f o r f u r t h e r r e s e a r c h on r e l a t e d TCNQ compounds; however we w i l l o n l y c o n s i d e r f u r t h e r e x p e r i m e n t s i n DEM(TCNQ) 2 t h a t a r e suggested by t h i s work: (a) A study of the dependence of the phase t r a n s i t i o n t e m p e r a t u r e s on the r a t e of h e a t i n g of. the c r y s t a l . The t i m e s i n v o l v e d would be from 6—hours t o 1—2 days f o r h e a t i n g the sample from room temperature t o about 480 K. T h i s would c l a r i f y the d i s c r e p a n c y i n the phase t r a n s i t i o n t e m p e r a t u r e s between the ESR and microwave c o n d u c t i v i t y measurements and the G u i n i e r photograph measurements. (b) The r e a l p a r t of the d i e l e c t r i c c o n s t a n t can be measured as a f u n c t i o n of temperature p a s t t h e s e phase t r a n s i t i o n s . T h i s experiment i s c o n t i n g e n t on o b t a i n i n g t h i n l o n g c r y s t a l s of DEM(TCNQ) 2 which a r e v e r y d i f f i c u l t t o grow. 49 BIBLIOGRAPHY van Bodegom, B., and van de Boer, J . L., A c t a . C r y s t B37, 1195 (1981) Bosh, A., and van Bodegom, B., A c t a . C r y s t . B33, 3013 (1977) Buranov, L. I . , and Shchegolev, I . F., I n s t r u m e n t s and E x p e r i m e n t a l Techniques 14, 528 (1971) Gordon, A. J . , and F o r d , R. A., The Chemical Companion; A Handbook of P r a c t i c a l . Data Techniques and R e f e r e n c e s (John W i l l e y and Sons: New York, 1972T von H i p p e l , A., T a b l e s of D i e l e c t r i c M a t e r i a l s ( L a b o r a t o r y f o r I n s u l a t i o n R e s e a r c h , M a s s a c h u s e t t s I n s t i t u t e of Technology) H u i z i n g a , S., PhD T h e s i s , U n i v e r s i t y of Groningen (1980) H u i z i n g a , S., Kommandeur, J . , Sawatzky, G. A., and T h o l e , B. T., Phys Rev B19, 4723 (1979) Kuindersma, P. I . , Sawatzky, G. A., and Kommandeur, J . , J . P h y s i c s C ( S o l i d S t a t e ) -8^ 3005 ( 1975) . Morrow, M., Hardy, W. N., C a r o l a n , J . F., B e r l i n s k y , A. J . , W e i l e r , L., G u j r a l , V. K., J a n o s s y , A., H o l c z e r , K., M i h l a y , G., G r i i n e r , G., H u i z i n g a , S., Verwey, A., and Sawatzky, G. A., Can. J_j_ of Phys. 58 334 (1980) M o r r s i n k , H., and van Bodegom, B., A c t a C r y s t . 37, 107 (1981) Osborn, J . A., Phys. Rev. 60 351 (1945) P r e s s l e y , R. J . , and Berk, H. L., B u l l . Am. Phys. Soc. 8, 345 (1963) 50 Sawatzky, G. A., H u i z i n g a , S., and Kommandeur, J . , P r o c e e d i n g s of the I n t e r n a t i o n a l Conference on 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 , September 1978 Dubrobnik Y u g o s l a v i a , L e c t u r e Notes i n P h y s i c s . p . 34, {"Berlin: S p r i n g e r V e r l a g , 1978) S c h w e r d t f e g e r , C. F., O o s t r a , S., and Sawatzky, G. A., Phys Rev (I n p r i n t 1981) S c h w e r d t f e g e r , C. F., Wagner, H. J . , and Sawatzky, G. A., S o l i d S t a t e Comm. 35, 7 (1980) Waldron, R. A., Theory of Guided E l e c t r o m a g n e t i c Waves (London: van N o s t r a n d R e i n h o l d Company, 19701 ~ Waldron, R. A., The Theory of Waveguides and C a v i t i e s (New York: Gordon and Bre a c h S c i e n c e P u b l i s h e r s , 1969)

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