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ESR and microwave conductivity studies in DEM(TCNQ) above room temperature Cabañas, Francisco Xavier 1981

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ESR  AND MICROWAVE CONDUCTIVITY STUDIES I N DEM(TCNQ)  ABOVE ROOM TEMPERATURE  by  FRANCISCO XAVIER CABANAS B.Sc.  The 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 , 1979  A THESIS SUBMITTED I N PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of P h y s i c s )  We a c c e p t  t h i s t h e s i s as conforming  to the r e q u i r e d  standard  THE UNIVERSITY OF B R I T I S H COLUMBIA November 1981 © F r a n c i s c o X a v i e r C a b a n a s , 1981  2  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 o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t freely  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 .  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  I further  copying of t h i s  thesis  f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . understood that copying or p u b l i c a t i o n of t h i s for f i n a n c i a l  gain  PVv^StCj  The 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 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date  thesis  s h a l l n o t be a l l o w e d w i t h o u t my  permission.  Department o f  It is written  ABSTRACT  Two p h a s e t r a n s i t i o n s h a v e 400(3) K  been  found  in  DEM(TCNQ)  a n d a t 4 4 2 ( 6 ) K t o 4 5 3 ( 6 ) K u s i n g ESR a n d  2  at  measurements  of t h e 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  are less  than  the  obtained  from  values  temperature ESR  lines,  g  2  g  I  and  and  453(6) K  415 K  A,  II in  o n l y one l i n e  angular  dependence  = g c o s e + g^sine 2  2  = 2.002730(15)  conductivity energy  E  and  483 K  previously  dependent G u i n i e r measurements.  stacks, B  The  of  0  was  are  DEM(TCNQ) .  400(3) K  t o t h e two and  of  the  and  g  value  values  of  effect  conductivity  below  of  a  fitted  to  = 2.003551( 14)  and  298 K a n d 4 4 2 ( 6 ) K  the  g  x  was  semiconductor  The  -4k  F  phase  w i t h an e x i t a t i o n transition  is  a t 4 4 7 ( 9 ) K. The p h a s e t r a n s i t i o n a t 4 0 0 ( 3 ) K i s due  t o a t r a n s f e r o f s p i n d e n s i t y f r o m s t a c k A t o s t a c k B, no  two  r e m a i n s w i t h t h e same g v a l u e a s l i n e I .  were o b t a i n e d . Between that  400(3) K  corresponding  Above  2  = 0.385(52) e.v.  postulated  observed  Below  on  the  total  spin  susceptibility  t o w i t h i n the experimental  error.  or  and  has  on  the  TABLE OF CONTENTS  Abstract  ii  Table of Contents List  of Tables  List  of Figures  i i i . .v vi  Acknowlegements CHAPTER  1  ....vii  INTRODUCTION  1  1.1 S i g n i f i c a n c e o f DEM(TCNQ)  2  and R e l a t e d  1.2 C h a r a c t e r i s t i c s o f DEM(TCNQ)  Compounds  — 1 4  2  1 . 3 Outline CHAPTER 2  6  DIELECTRIC  CONSTANT AND CONDUCTIVITY THEORY AND  MEASUREMENTS  8  2.1 T h e o r y o f 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  Constant  Measuring  2.3 D i e l e c t r i c  Constant  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 ..15  CHAPTER 3  DIELECTRIC  Apparatus  12  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 3.2 D i s c u s s i o n  of  the  17 Dielectric  Constant  and  c o n d u c t i v i t y Measurements  22  (a) Q u o t e d E x p e r i m e n t a l E r r o r s  22  (b) Shape o f t h e S a m p l e s a s a S o u r c e o f E r r o r .......23 CHAPTER  4  ESR APPARATUS, MEASUREMENTS AND  SUSCEPTIBILITY  CALCULATIONS  24  4.1 ESR A p p a r a t u s  24  4.2 The ESR M e a s u r e m e n t s 4.3 C o n v o l u t e d  ....26  Gaussian-Lorentzian 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 CHAPTER 5  30  ESR EXPERIMENTAL RESULTS  31  5. 1 ESR g V a l u e s  31  5.2 A n g u l a r  34  Dependence of t h e g V a l u e s  5.3 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 S u s c e p t i b i l i t y  37  5.4 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 Peak t o Peak W i d t h s o f t h e ESR l i n e s CHAPTER 6  41  DISSCUSION OF.THE RESULTS  6.1 P h a s e T r a n s i t i o n s i n DEM(TCNQ) 6.2 C o m p a r i s o n HEM(TCNQ) CHAPTER 7  of  2  CONCLUSIONS  DEM(TCNQ)  2  43 43  2  to  MEM(TCNQ)  2  • AND FURTHER POSSIBLE EXPERIMENTS  and 4  5  ..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  Bibilography  ...49  V  L I S T OF TABLES  Table  2.1  Physical  D a t a o f t h e N y l o n T e s t Sample  Table  3.1  Physical  D a t a o f . DEM (.TCNQ )  Table  3.2  Experimental Results  Table  5.1  ESR g V a l u e s  Table  5.2  g„and g  x  f o r DEM (TCNQ)  2  S a m p l e s ... .  16 ...20 ...20  ... 2  .32 ......34  L I S T OF FIGURES  Figure  1.1  The MEM, DEM a n d HEM M o l e c u l e s  2  Figure  1.2  The TCNQ M o l e c u l e  2  Figure  1.3  Shape  o f a T y p i c a l DEM(TCNQ)  the R e l a t i v e D i r e c t i o n s  of  Stacks  2  A  Crystal and  B  Showing and  the  M a g n e t i c F i e l d H i n t h e ESR M e a s u r e m e n t s F i g u r e 2.1  Cavity  and  5  Sample 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 and C o n d u c t i v i t y Measurements F i g u r e 2.2  Resonance  Dielectric F i g u r e 2.3  of  the  Cavity  11 used  for  the  C o n s t a n t and C o n d u c t i v i t y Measurements Block  Diagram  of t h e D i e l e c t r i c  13  C o n s t a n t and  C o n d u c t i v i t y Measurements F i g u r e 3.1  Microwave  14  Conductivity  of  DEM(TCNQ)  2  vs  Temperature F i g u r e 3.2  Natural  18 Logarithim  C o n d u c t i v i t y o f DEM(TCNQ)  2  of  the  Microwave  vs Inverse Temperature  19  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  25  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  27  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 a n d b e l o w t h e  T r a n s i t i o n a t 400(3) K  Phase 28  Figure  5.1  DEM(TCNQ) Figure  5.2  Angular 2  Dependence  of  the  5.3  Temperature  Dependence  Temperature  Susceptibility Figure  5.4  of  of L i n e  5.5  Temperature  H a l f w i d t h of l i n e s Figure  5.6  Halfwidth  of  Angular of  line  of  the  Spin 36  2  o f t h e Sum o f t h e  I a n d I I o f DEM(TCNQ  Dependence  Spin 35  I I o f DEM (TCNQ)  T e m p e r a t u r e Dependence  the  2  Dependence  S u s c e p t i b i l i t i e s of L i n e s Figure  Value  a b o v e t h e P h a s e T r a n s i t i o n a t 4 0 0 ( 3 ) K ...... 33  S u s c e p t i b i l i t y o f L i n e I o f DEM (TCNQ) Figure  g  of  the  Spin 38  2  Peak t o Peak  I a n d I I o f DEM (TCNQ)  2  Dependence  Peak  to  above  t h e Phase  I  T r a n s i t i o n a t 400(3) K  of  of  the  DEM(TCNQ)  2  39 Peak  40  vi i i  ACKNOWLEGEMENTS  I w i s h 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 , D r . C. Schwerdtfeger,  F.  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  ideas both d u r i n g the performance  of  the  experiments  and t h e  preparation of the t h e s i s . I thesis  would  also like  t o t h a n k D r . J . F. C a r o l a n who r e a d t h e  f o r h i s s u g g e s t i o n s a n d comments.  I am a l s o Bodegom  of  DEM(TCNQ)  i n d e b t e d t o D r . G. A. S a w a t z k y  the University  of  Groningen  and  B.  f o r providing  van the  samples.  2  A post g r a d u a t e s c h o l a r s h i p from t h e N a t u r a l Engineering acknowleged. Sciences  Dr.  Research  Council  of  Canada  Sciences is  gratefully  R e s e a r c h f o r t h i s t h e s i s was f u n d e d by t h e  and  Engineering  grants to.Dr. Schwerdtfeger.  Research  .Council  and  Natural  o f Canada t h r o u g h  1  CHAPTER  1 INTRODUCTION  1 .1 S I G N I F I C A N C E OF DEM(TCNQ)  AND RELATED COMPOUNDS  2  tetracyanoquidimethane,  N—ethyl^N—ethyl—morpholinium DEM(TCNQ) ,  is  2  the morpholinium figure have  1.1  a member o f a f a m i l y  g r o u p c h a n g e s . When t h e r a d i c a l  i s r e p l a c e d by C H 2  MEM(TCNQ)  o f r e l a t e d compounds where  2  we h a v e HEM(TCNQ)  we h a v e  5  DEM(TCNQ) ,  (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 ) , 2  (HEM = N — e t h y l — m o r p h o l i n i u m ) .  o f t h e s e compounds i s t h e  behaviour  the  electrical  example of a p h y s i c a l  system  The  quasi  conductivity.  by R i n  by  2  characteristic of  denoted  CH  we  3  a n d by H  significant  one—dimensional This  where o n e — d i m e n s i o n a l  p r o v i d e s an t h e o r y c a n be  t e s t e d . The H u b b a r d H a m i l t o n i a n  (1.1)  and  t h e p r e d i c t i o n of 2k  F  and 4k  v a l u e o f U / t h a s been c r u c i a l p.2)  F  i n s t a b i l i t i e s depending  to this  theory  (see H u i z i n g a  on t h e 1980  2  CH  /  CH  2  2  \/  0  *N  \  /\  CH —CH 2  2  F i g u r e 1.1 The MEM, DEM and R = C H : DEM ; R = H : HEM 2  5  C?Hc  R HEM  Molecules R = CH  3  : MEM ;  3  Measurements HEM(TCNQ) .  The  2  been  have  reported  been  crystal by  performed  structure  Bosh  and  van  specific  heat  of  MEM(TCNQ)  o f MEM(TCNQ) Bodegom  t r a n s i t i o n s h a v e been f o u n d i n MEM(TCNQ) The  on  MEM(TCNQ)  2  a t 113 K h a s  2  (1977).  Two  phase  a t 18 K a n d 340 K. has  2  and  2  been  r e p o r t e d by  S a w a t z k y e t a l . ( 1 9 8 0 ) . They r e p o r t a peak i n t h e s p e c i f i c at  heat  19 K w i t h an e n t r o p y g a i n o f 1.4J—mole/K, a n d a n o t h e r peak a t  335 K  with  an e n t r o p y g a i n o f 1 4 J — m o l e / K , c o r r e s p o n d i n g t o t h e  two p h a s e t r a n s i t i o n s Huizinga, Spin—Peierls conductivity (1980),  i n MEM(TCNQ) . 2  Kommandeur, phase  Sawatzky,  transition  o f MEM(TCNQ)  2  at  and  Thole  18 K.  The h i g h  h a s been m e a s u r e d by  where a r e v e r s i b l e  report  temperature  Morrow  s e m i c o n d u c t o r — m e t a l phase  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  t r a n s i t i o n s a t 18 K a n d 340 K i n MEM(TCNQ)  et a l .  transition the  w i t h the 2k  2  a  F  phase  and 4k  F  instabilities. The  crystal  structure  o f t h e r e l a t e d compound HEM(TCNQ)  2  h a s been m e a s u r e d by v a n Bodegom a n d v a n de B o e r ( 1 9 8 1 ) . A p h a s e t r a n s i t i o n a t 450 K u s i n g a (DSC) at  technique  Differential  Scanning  and a sharp d e c l i n e i n t h e s p i n  425 K, i n d i c a t i n g a p h a s e t r a n s i t i o n , h a s  Huizinga  (1980  p.108)  a t t r i b u t e d by H u i z i n g a predicted HEM(TCNQ)  from 2  Calorimeter  susceptibility  been  reported  by  i n HEM(TCNQ) . T h i s phase t r a n s i t i o n i s 2  t o be  the theory,  t h a n f o r MEM(TCNQ)  related  to  the  implying that U/t 2  (see Huizinga  2k 0  F  instability  i s much l e s s f o r  1980 p . 1 0 1 ) .  4  1.2 CHARACTERISTICS OF DEM(TCNQ) The c r y s t a l Morrsink  and  2  s t r u c t u r e o f DEM(TCNQ)  v a n Bodegom ( 1 9 8 1 ) . They  has  2  been  measured  r e p o r t two t y p e s o f TCNQ  s h e e t s , A a n d B, t h a t a r e a t an a n g l e o f 60° ( s e e f i g . 1 . 3 ) . makes DEM(TCNQ)  fundamentally  2  by  different  from  This  MEM(TCNQ)  or  2  HEM(TCNQ) . 2  Sheet B behaves i n a o n e — d i m e n s i o n a l to  MEM(TCNQ)  in  2  i t s behavior  S p i n — P e i e r l s phase t r a n s i t i o n DEM(TCNQ)2 compares  at to  MEM(TCNQ) . 2  2k  F  room  h a s been o b s e r v e d  temperature. in  sheet  B  A of  23 K by S c h w e r d t f e g e r , O o s t r a a n d S a w a t z k y . T h i s  the Spin—Peierls This  instability  below  f a s h i o n and i s s i m i l a r  phase  phase  transition  at  18 K  in  t r a n s i t i o n probably corresponds t o the  i n t h e s t a c k o f TCNQ m o l e c u l e s  i n sheet  B  of  DEM(TCNQ) . 2  Unlike  t h e s t a c k s i n s h e e t B, t h e s t a c k s i n s h e e t A do n o t  •undergo a S p i n — P e i e r l s p h a s e t r a n s i t i o n temperature. 1.5 K. W h i l e manner  The  latter  stack  s t a c k B h a s been  between  remains  shown  two—dimensional  stack system  f o r sheet  A  and  paramagnetic  t o - behave  t o t h e s i n g l e s t a c k o f TCNQ m o l e c u l e s  corresponding  1.5 K  behaves  in a  room  down t o similar  i n MEM(TCNQ) ; t h e 2  more  like  (Schwerdtfeger, Oostra and Sawatzky).  a  5  F i g u r e 1.3 Shape o f a T y p i c a l DEM(TCNQ) C r y s t a l . The A r r o w s L a b e l l e d A and B I n d i c a t e t h e 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 t h e A r r o w L a b e l l e d H I n d i c a t e s t h e D i r e c t i o n o f t h e S t a t i c M a g n e t i c F i e l d f o r t h e ESR M e a s u r e m e n t s . 2  6  At  high  temperatures  M o r r s i n k a n d v a n Bodegom r e p o r t  r e v e r s i b l e p h a s e t r a n s i t i o n s a t 415 K a n d 483 K The  ESR  lines  (1980).  by  of  I and I I t h a t  follow  198  spectra  the  DEM(TCNQ)  originate  notation  The  of  2  DEM(TCNQ) . 2  b e t w e e n 23 K a n d 298 K h a s two  i n s h e e t s B and A r e s p e c t i v e l y .  Schwerdtfeger,  Wagner  and  a n g u l a r dependence of t h e v a l u e s of g  K, a n d o f g  spin—susceptibility  and  of temperature  (1980) ( s e e t a b l e  halfwidth has  been  of  lines  measured  I by  We  Sawatzky  a t 77 K a n d  a t 1.14 K, 4.2 K, 77 K, a n d 298 K a r e  S c h w e r d t f e g e r Wagner a n d S a w a t z k y  function  in  two  and  reported 5 . 2 ) . The II  as a  Schwerdtfeger,  O o s t r a and Sawatzky. A  qualitative similarity  o f DEM(TCNQ) to  300 K  a n d MEM(TCNQ)  2  is  reported  by  2  between t h e b u l k  i n t h e t e m p e r a t u r e range  a  from stacks  fundamental d i f f e r e n c e  from  4 K  K u i n d e r s m a , S a w a t z k y a n d Kommandeur  (1975). There a r e i n d i c a t i o n s that susceptibility  susceptibilities  the contribution  to the  bulk  A a n d B a r e s i m i l a r , however t h e r e i s i n t h e nature of t h e 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 conductivity experimental  2 we p r e s e n t t h e  theory  behind  measurements and a d e s c r i p t i o n procedure  used  to  the  microwave  o f t h e a p p a r a t u s and  measure  the  microwave  conductivity. The  microwave  conductivity  data  as  a  function  of  7  temperature at  between  290 K a n d 480 K, s h o w i n g a p h a s e  442(6) K t o g e t h e r , w i t h  the  c o n s t a n t a t room t e m p e r a t u r e In  real  part  of  transition  the  dielectric  i s p r e s e n t e d i n c h a p t e r 3.  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 o f t h e ESR a p p a r a t u s  and t h e e x p e r i m e n t a l methods u s e d  procedure,  together  with  the  numerical  i n t h e a n a l y s i s o f t h e ESR d a t a .  Chapter  5 c o n t a i n s t h e ESR r e s u l t s .  These r e s u l t s  include:  The s p i n s u s c e p t i b i l i t i e s o f l i n e s  I a n d I I , t h e sum o f t h e 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 I , and  halfwidth  of  lines  phase t r a n s i t i o n 453(6) K  in  dependence of  I  and  the  peak  to  peak  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 , t h e  t e m p e r a t u r e s t h a t were f o u n d  at  400(3) K  and  t h e 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 , t h e a n g u l a r  o f t h e g v a l u e a n d h a l f w i d t h , m e a s u r e d a t T=400(3) K,  t h e o n l y l i n e above t h e p h a s e t r a n s i t i o n a t 4 0 0 ( 3 ) K, a n d t h e  g v a l u e s , m e a s u r e d a t © = 60° ( s e e f i g . 1 . 3 ) , o f b o t h l i n e b e l o w the  phase t r a n s i t i o n a t  400(3) K  and  the  single  line  above  4 0 0 ( 3 ) K. Chapter DEM(TCNQ)  with  temperatures transitions  2  the a  in  a  comparison  results  DEM(TCNQ) 2  2  2  of t h e d i f f e r e n t with  the  a b o v e room  conclusions  between  f o r MEM(TCNQ)  correlation  i n DEM(TCNQ)  General DEM(TCNQ)  contains  and  2  together  6  and  two  the results f o r and  HEM(TCNQ)  phase  transition  postulated  2  phase  temperature.  some  a r e p r e s e n t e d i n c h a p t e r 7.  further  experiments  on  8  CHAPTER 2 DIELECTRIC  CONSTANT AND CONDUCTIVITY THEORY AND MEASUREMENTS  2.1 THEORY OF CAVITY PERTURBATION If a d i e l e c t r i c resonant for  cavity  s a m p l e o f v o l u m e V, i s  o f volume V  the s h i f t  0  introduced  i n t h e complex  into  a  frequency  small perturbations i s  [(E,-DO-EQ-D,)-(H,-Bo-Ho-B,)]dV =  (2.1) \ [E 'D -H .B ]dV V " - 0  0  0  0  J  0  where  n  0  = 2tri/ ( 1 + i / 2 Q )  unperturbed  and  0  0  and  perturbed  n,  = 2nv , (1 +i/2Q, )  complex  frequencies  (Waldron,1969.pp.87-93). I n t h e experiments c a v i t y w i t h t h e sample h o l d e r a l o n e , n , w  the  cavity  measured.  with  the  sample  holder  I f we assume t h a t t h e f i e l d  the unperturbed  field,  and  the  respectively  the frequency of the  and  the  frequency  and t h e sample, n , £  of were  d i s p l a c e d by t h e s a m p l e i s  approximating  d e n o m i n a t o r o f ( 2 . 1 ) we o b t a i n  are  n  0  by  n  H  i n the  9  [ ( E . D - E o ' D ) - ( H . B - H o ' B )]dV 5  n„-n-  r J  s  0  s  s  0  :  .  (2.2)  [E .D -H .B ]dV 0  where  E + E , 0  0  0  D + D , H + H , and B + B , a r e the e l e c t r i c and  s  0  magnetic f i e l d s sample.  0  s  0  s  0  s  i n the sample, and  For non magnetic  i s the volume  the  samples H = B = 0. I f the sample i s 0  0  ellipsoidal,  the dimensions of the sample are much l e s s  wavelength,  and the skindepth  smallest  of  than  a  i s l a r g e r than or c l o s e t o the  dimension of the sample, then the f i e l d  i n the sample  is  E +E 0  s  E  =  0  (2.3)  1+n(c-1)  where  n  i s the d e p o l a r i z i n g  dielectric equation  f a c t o r and e=€'+ie" i s the complex  constant of the sample  ( 2 . 3 ) i s substituted  (Buranov et a l , 1 9 7 1 , p . 5 2 8 ) . I f  i n t o equation  (2.2),  r e a l and imaginary p a r t s and assuming that E the  volume of the sample, we o b t a i n  a cylindrical 1/1 _ ( _ - _  2VQ  0  then equating  i s constant  over  f o r a sample on the a x i s of  cavity  Q/  =  oe"  (2.4)  U l + n U ' - l ) ] + (n ") } 2  e  2  and  :{(€'-D[l+n( '-l)+n(e") ]} 2  e  {1+n(€ *-1)} + (ne") 2  — 2  (2.5)  10  where  o  (2.6)  =  {E .D -H .B }dV 0  E  O  M  T  0  0  i s the magnitude of the e l e c t r i c  c a v i t y , and e  i s the p e r m i t i v i t y  0  a l , l 9 7 1 p.528). For the TM , 0  (E -D -H .B )dV 0  where  0  0  0  free  space  (Buranov e t  mode  (2.7)  =  0  J,(ka) = 0.51915  0  of  f i e l d on the a x i s of the  (Waldron,1970  pp.303-305).  o  i s then  1.8552V /V . The s o l u t i o n of (2.4) and (2.5) f o r e'-1 and e" i s 0  A  a  (2.8)  n2 2  and  -  n  A  fa  1T  t'-1  6  - - 6| \ n  2  (2.9)  - - 6 J  where  6 =  The q u a n t i t i e s  and  A =  - - -  (Buranov e t a l , l 9 7 l  p.528)  6, A, N and c can be measured or c a l c u l a t e d ,  hence e' and c" can be determined.  and  11  SAMPLE  5 F i g u r e 2.1 Cavity and Constant and C o n d u c t i v i t y  Sample Holder Measurements.  used  for  Dielectric  12  2.2 D I E L E C T R I C CONSTANT MEASURING APPARATUS The s a m p l e was mounted on t h e a x i s o f a c y l i n d r i c a l operating  in  microwave  source  HP86250B  RF  the  TM , 0  was  mode  0  an  at  HP8620C  power  sweep  the  the  reflected  i s shown  To o b t a i n inside  t a p e . The f i n e resistors  oscillator  with  next  glass  of  the  resonant  frequency  2.3.  temperature cylinder  temperature to  HP5245L  were d e t e r m i n e d . A b l o c k d i a g r a m o f  in figure  the coarse a  an  from t h e 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  Q of the resonance  apparatus  placed  ( s e e f i g . 2.1) t h e  frequency c o n v e r t e r . Scans  were o b t a i n e d ( s e e f i g . 2 . 2 ) f r o m w h i c h and  GHz  p l u g i n . The f r e q u e n c y was m e a s u r e d w i t h an  f r e q u e n c y c o u n t e r w i t h an HP5255A the  9.2  cavity  was  the  sample  cavity  was  t h a t was w r a p p e d w i t h h e a t i n g  obtained  by  using  two  power  t h e c a v i t y . The c a v i t y a n d h e a t i n g e l e m e n t s  were t h e n 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 o f 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 o f  bridge  u s i n g a t h e r m i s t o r as the temperature  This provided a temperature scan.  stability  o f 0.1°C  a  resistance  sensitive  element.  d u r i n g each  data  13  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 Constant and C o n d u c t i v i t y Measurements w i t h and w i t h o u t the Sample.  14  Frequency Counter  Variable Short Microwa ve Sweep Oscillator  X  Magic Tee  X  Variable Attenuator 0-40db  Matched Loaa  X-Y  Recorder  Scope  F i g u r e 2.3 Conductivity  Null Voltmeter  Variable Attenua tor 0-AOdb Programable Power Supply  1  "  Screw Tuner  Coarse Heater Supply  Isolator  D.V.M. (Thermocouple | Voltage)  Magic Tee  Isolator  Fine Heater Thermistor  i  H  Isolator  Cavity  Crystal Detector  Block Diagram Apparatus.  of  the Dielectric  Constant  and  15  2.3  D I E L E C T R I C CONSTANT AND CONDUCTIVITY MEASUREMENTS The  sample  resonant f r e q u e n c i e s of the c a v i t y , holder,  with  and  without  the  measured a s a f u n c t i o n of t e m p e r a t u r e . least of  squares f i t t o the a s t r a i g h t  the. resonant  perturbed  sample  In  by t h e  i n p l a c e were  the  latter  case  a  l i n e was made a n d t h e v a l u e s  f r e q u e n c y a n d Q o f t h e c a v i t y , were  calculated  f o r t h e t e 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 t h e s c a n s where t h e c a v i t y was  p e r t u r b e d by t h e sample  manner  i t -was  possible  holder to  and  the  sample.  In  this  determine a t each temperature t h e  r e s o n a n t f r e q u e n c y and Q of t h e c a v i t y owing  t o t h e empty  sample  holder. In order  to  determine  the  depolarizing  factor  of t h e  s a m p l e s , t h e l i n e a r d i m e n s i o n s o f t h e s a m p l e s were m e a s u r e d on a t r a v e l l i n g m i c r o s c o p e and t h e d e p o l a r i z i n g by i n t e r p o l a t i o n General  equation same  from a p l o t o f t h e D e m a g n e t i z i n g F a c t o r s o f t h e  Ellipsoid  t h e sample  (Osborn,l945  i sellipsoidal  ( 2 . 3 ) . I n making  volume  f a c t o r s were o b t a i n e d  and  p . 3 5 5 ) . The a p p r o x i m a t i o n t h a t  has t o  be  made  in  order  to  apply  t h i s approximation e l l i p s o i d s with the  l i n e a r d i m e n s i o n a l r a t i o s as t h e samples  were  used. The v o l u m e s samples 1.253  and  mg mm" , 3  measured  by  of t h e samples using  the  (Morrsink this  were d e t e r m i n e d by w e i g h i n g  known et  density  a l , 1981  of  DEM(TCNQ) , 2  p.107).  method a g r e e d w i t h t h e v o l u m e s  the l i n e a r d i m e n s i o n s of  the  samples  to  within  The  the d = M  volumes  o b t a i n e d from experimental  error. In  equation  (2.2) t h e  electric  field  i n t h e immediate  16  vicinity that  location  is valid of  o f t h e s a m p l e i s assumed  t o be e q u a l  of the c a v i t y without  to  the  field  t h e sample. T h i s  in  assumption  because t h e volume of t h e sample as w e l l as t h e  volume  t h e s a m p l e h o l d e r a r e n e g l i g i b l e when c o m p a r e d t o t h e v o l u m e  of t h e c a v i t y . TABLE 2 . 1  PHYSICAL  Sample  a (mm)  #1  4.66(2)  DATA OF THE NYLON TEST SAMPLE  c (mm)  b(mm)  0.33(2)  0.47(2)  source  of e r r o r  h o l d e r . T h i s was c h e c k e d by m e a s u r i n g of  Depolarizing Factor  0.71(5)  0.015(3)  3  Another p o s s i b l e  and  Volume (mm )  nylon  the  thread. A piece of nylon  v a l u e s of e ' = 2 . 9 4 T h i s compares w e l l €"=0.032-0.039  i s t h e shape o f t h e s a m p l e  and e " = 0 . 0 3 2  dielectric  constant  t h r e a d was m e a s u r e d a n d  were o b t a i n e d  with the published values  (Von H i p p i e , p p . 3 1 0 - 3 1 1 , 3 2 3 ) .  (see  table  2.1).  of t ' = 2 . 8 4 — 3 . 0 3 and  17  CHAPTER 3 D I E L E C T R I C CONSTANT AND CONDUCTIVITY RESULTS  3.1 EXPERIMENTAL RESULTS Three  crystals  of  DEM(TCNQ)  2  were u s e d  i n the 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 . The p h y s i c a l d a t a o f t h e 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, a n d c a r e principal  axes  the  approximating  calculating  the d e p o l a r i z i n g  f a c t o r s . These  the  of  d i m e n s i o n s of t h e samples  was l o w enough s o t h a t  of t h e s k i n d e p t h was g r e a t e r t h a n the  smallest  table  3.2.  It  temperature because measurements increases.  1.8 mm.  The  measured  the lowest value  This i s  larger  than  d i m e n s i o n o f t h e s a m p l e ( s e e t a b l e 3 . 2 ) . The room  temperature d i e l e c t r i c in  approximate  of the c r y s t a l s as a f u n c t i o n of  i s p l o t t e d i n f i g 3.1 a n d i n f i g 3.2. conductivity  in  field.  The m i c r o w a v e c o n d u c t i v i t y  microwave  values  used  a l o n g t h e d i r e c t i o n s p a r a l l e l and  perpendicular t o the e l e c t r i c  temperature  ellipsoid  the  of  e'  c o n s t a n t and c o n d u c t i v i t y a r e  was  only  the large to  be  possible  to  depolarizing unreliable  obtain factor  when  summarized e' a t room causes  the  the conductivity  18  to  d  m  d  d  (,-WO-U) F i g u r e 3.1 Temperature S o l i d Curve  ro  d  <M  d  —  d  AllAllOnQNOD  Microwave Conductivity of DEM(TCNQ) A : S a m p l e #1 Q : S a m p l e #2 Sample #3. i s E s t i m a t e d from t h e P o i n t s .  2  vs The  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 of t h e M i c r o w a v e C o n d u c t i v i t y DEM(TCNQ) vs Inverse Temperature. Symbols are defined 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 . 2  of in  .20  TABLE 3.1  PHYSICAL DATA OF DEM(TCNQ)  SAMPLES  2  Volume (mm )  c (mm)  Depolarizing Factor  Sample  a (mm)  b(mm)  #1  1 .36(2).  0.62(2)  0.16(2)  0. 1 1 4 0 ( 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. 1 6 5 ( 1 5 )  TABLE 3.2  3  EXPERIMENTAL  RESULTS  Sample  (Roomtemp)  e" (Roomtemp)  #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)  0.389(52)  3.7(9)  average  9.0(23)  3.5(10)  E (e.v.) 0  lntf o ln{(n-cm)- } 1  21  The  conductivity  b e h a v i o r below  was  370 K. The  found to f o l l o w a semiconductor  d a t a was  analysed  in  terms  like  of  the  equation  <y  where E  0  =  * exp(-E /2kT) 0  (3.1)  0  i s t h e band gap.  The  v a l u e s of E  t h e c r y s t a l s a r e shown i n t a b l e 3.2. an  ordinary  semiconductor  t e m p e r a t u r e of 442(6) observed,  indicating  behavior  phase  t r a n s i t i o n an a b r u p t d e c r e a s e The  were  heated  between  450 K  t e m p e r a t u r e c o n d u c t i v i t y was been  heated  up  to  K.  obtained for  0  are  observed  i n the  transition.  and  Above  and  25% o f t h e i r 480  sharply  K, and  at  conductivity  is  the  mass  is  phase  owing  when  the subsequent  l o w e r when t h e  a  observed.  f o u n d t o be n o n — r e v e r s i b l e ,  lost  from  sample  480 K. T h i s l o s s o f mass t o g e t h e r w i t h  d e c r e a s e of the c o n d u c t i v i t y s a m p l e a b o v e 450  ln<r  i n the c o n d u c t i v i t y  h e a t i n g o f t h e s a m p l e was  t o the f a c t t h a t the samples  and  Above 370 K d e v i a t i o n s  K a discontinuity a  0  i n d i c a t e s the d e c o m p o s i t i o n of  they room had the the  ,22  3.2  DISCUSSION  OF  THE  DIELECTRIC  CONSTANT  AND CONDUCTIVITY  MEASUREMENTS  (a) Q u o t e d E x p e r i m e n t a l The using  errors  sample's  measurements  f o r s a m p l e #3  longest  electric that  i n t h e d i m e n s i o n s o f t h e s a m p l e s were  repeated  measurements  field  instead  field  instead of  the  have  travelling larger  dimensions  and p e r p e n d i c u l a r  quoted e r r o r  estimated  errors  does  include  not  a  m i c r o s c o p e . The  error  of along the e l e c t r i c  of the l e n g t h ,  The  on  estimated  because  the  d i m e n s i o n was a t an a n g l e o f a b o u t 30° t o t h e  estimates  electric  Errors  width,  of  the  and t h i c k n e s s  This  sample  to the e l e c t r i c  i n the d e p o l a r i z i n g  in  field.  meant  along the  field  were u s e d  of t h e sample.  f a c t o r i s due  to the  the dimensions.of the samples. This  any  additional  errors  introduced  error  by t h e  a p p r o x i m a t i o n o f t h e shape o f t h e s a m p l e s a s e l l i p s o i d s . The  quoted  standard errors  errors  =  -E /2kT+ln 0  where t h e p a r a m e t e r s E incorporates  sample,  i t does  v a l u e s of E determined three  0  a n d ln<y  0  0  f o r each sample a r e t h e  from t h e f i t o f t h e c o n d u c t i v i t y d a t a t o  In*  error  for E  0  and  (3.2)  t f o  ln<jr  0  a r e d e t e r m i n e d by t h e f i t . T h i s  the s t a t i s t i c a l not  and l n c r  0  incorporate  described  as  from a f i t t o e q u a t i o n  s a m p l e s . The e r r o r s  inE  0  error any  i n the  systematic  average  in  (3.2) using  and lho-  0  data  obtained  f o r each e r r o r s . The  table  3.2 a r e  the data in  from t h e  this  case  23  are  the  include  standard  errors  from t h e f i t . These l a s t  some o f t h e s y s t e m a t i c  systematic  to  one  error  sample,  but  for  from d e p o l a r i z i n g  not  systematic this  to kind  a l l of  is the  error  f a c t o r . The e r r o r s q u o t e d f o r c' a n d e"  t h e a v e r a g e r e s u l t a r e t h e s t a n d a r d e r r o r o f t h e mean o f t h e  three  (b)  will  i n as f a r as t h i s e r r o r  m e a s u r e m e n t s . The l a r g e s t c o n t r i b u t i o n t o comes  errors  measurements.  Shape o f t h e S a m p l e s a s a S o u r c e o f E r r o r , The  factor  determination  o f e'  is  of t h e samples. A small  limited  by  depolarizing  factor  i s desirable  the e l e c t r i c  sample p e r m i t t i n g  a measurement o f e'. A n o t h e r d i s a d v a n t a g e o f a  depolarizing  determined only depolarizing  factor  i s that  factor  are  approximation  of  The very  long  selected  sample  significant errors optimal  factor  shape  inside  by  factor i s  contain  the  (2.8) and (2.9)  together an  the  with  ellipsoid  the would  into the f i n a l r e s u l t s . factor i s a  a n d t h i n c r y s t a l where a>>b>c. The c r y s t a l s u s e d were  were  depolarizing  f a r from  the  factor;  however  optimal  shape. T h i s  a c c u r a c y of t h e experiment and p r e c l u d e d a b o v e room  that  shape t o m i n i m i z e t h e d e p o l a r i z i n g  f o r low  crystals  terms  dominant i n e q u a t i o n s  i n the depolarizing the  penetrate  the d e p o l a r i z i n g  approximately. I f the  then t h e e r r o r s  introduce  to  depolarizing  because i t a l l o w s  large  field  the  temperature.  a  the  available  l i m i t e d the  measurement  of  e'  24  CHAPTER  4 ESR APPARATUS, MEASUREMENTS AND S U S C E P T I B I L I T Y CALCULATIONS  4.1 ESR APPARATUS Electron  s p i n r e s o n a n c e , ESR, was p e r f o r m e d a t X — b a n d . The  s a m p l e was p l a c e d  i n a rectangular cavity operating  mode. The c a v i t y was p l a c e d through 100  the  field  difference the  ESR r e s o n a n c e . The m a g n e t i c f i e l d  KHz by means o f c o i l s  static  center  was  in field of  the  cavity  HP5245L f r e q u e n c y  counter.  The  by  means  microwave source  counter  of  an  frequency  was. swept  was m o d u l a t e d a t exterior. NMR  The  p r o b e . The probe  and  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 frequencies  being  measured  was a V a r i a n VA—297 k l y s t r o n  9.2 GHz t h a t was p h a s e — l o c k e d microwave  cavity  that  b e t w e e n t h e p o s i t i o n o f t h e NMR  t h e NMR  at  p l a c e d on. t h e  measured  sample of L i F : L i ,  The  i n a magnetic f i e l d  i n t h e TE102  was  t o t h e sample c a v i t y m e a s u r e d by an HP5245L  w i t h an HP5255A f r e q u e n c y  converter.  on  an  operating frequency. frequency  Scope  Klystron Supply  I ,  I N.M.R. Oscillator  Klystron  Frequency  A.F.C.  Isolator  Counter  1  10db Directional Coupler  Variable Attenuator 0-40db  Crystal Bias  Mini Computer  I D.V.M. tThermocouptt Voltage)  Variable Attenuator 0-SOdb  1  Sample Heater Supply  Circulator  Pre Amplifier  Lock-in Amplifier  I  Tuned Crystal Detector  Recorder  Audio Amplifier  Magnetic Field Sweep  X-Y  N.M.R. Probe  WOKHz Modulation* Coils  D.C. Amplifier  Magnetic Field Sfctee/} Coils Magnet Pole Face Figure 4.1  Block  Nitrogen Gas Input  D i a g r a m o f t h e ESR A p p a r a t u s .  26  Absorption  of  the  sample  was  detected  with  a  lockin  a m p l i f i e r whose o u t p u t g a v e t h e d e r i v a t i v e o f t h e ESR s i g n a l a n d after  a m p l i f i c a t i o n was r e c o r d e d  digitally tape.  on an X—Y r e c o r d e r , o r s a m p l e d  w i t h a NOVA 2 m i c r o c o m p u t e r a n d p u n c h e d o u t  The  digitized  data  could  then  computing system f o r f u r t h e r p r o c e s s i n g .  be  read  on  paper  i n t o t h e UBC  A block diagram of the  a p p a r a t u s i s shown i n f i g . 4.1. The (fig.  sample  was  heated  by  means o f a V a r i a n  4.2) u s i n g n i t r o g e n g a s a s t h e h e a t t r a n s f e r  sample  temperature  was  measured  t h e r m o c o u p l e w i t h an i c e b a t h  using  an  flow  system  medium.  The  iron—constantan  as r e f e r e n c e .  4.2 THE ESR MEASUREMENTS Electron on  spin resonance  DEM(TCNQ)  2  as  a  (ESR) m e a s u r e m e n t s  function  of  temperature  t e m p e r a t u r e a n d 4 5 3 ( 6 ) K. The g v a l u e s were m e a s u r e d a s a f u n c t i o n transition the a n g l e The  t h e -g v a l u e  of  of l i n e  were  between  room  o f t h e two o b s e r v e d  lines  temperature.  Above  the  phase  I was m e a s u r e d a s a f u n c t i o n o f  between t h e c r y s t a l a x i s and t h e magnetic  field.  s p i n s u s c e p t i b i l i t i e s a n d peak t o peak w i d t h s  d e t e r m i n e d by f i t t i n g  the  curve  Gaussian-Lorentzian  with convoluted  performed  integral  of  the  output  functions.  were t h e n derivative  THERMOCOUPLE  SAMPLE TUBE A/ OUT L  .. K  GLASS DEWER  CAVITY  WAVEGUIDE  SAMPLE  HEATER  100 KHz MODULATION COILS  Figure  4.2  ESR C a v i t y  and V a r i a n  Flow  System.  Figure 4.3 T r a n s i t i o n at  Typical ESR 4 0 0 ( 3 ) K.  Spectra  above  and  below  the  Phase  29  The s p i n s u s c e p t i b i l i t i e s a n d peak then  be o b t a i n e d  ESR  spectra  to  peak  from the G a u s s i a n - L o r e n t z i a n  below  a n d above t h e o b s e r v e d  widths  could  functions. Typical  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 F I T  (a) F i t The d i g i t i z e d ESR o u t p u t and  then  fitted  s i g n a l was  integrated  numerically  with convoluted Gaussian-Lorentzian  functions.  The f i t was p e r f o r m e d by m i n i m i z i n g t h e sum o f s q u a r e d e v i a t i o n s b e t w e e n t h e d a t a p o i n t s and t h e 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 o f e a c h determined obtained widths  by  intergrating  from the f i t . were  the  The  from  the  functions obtained  p r o v i d e d t h e s u s c e p t i b i l i t i e s a n d peak each  individual  phase  line  transition.  susceptibilities,  in  and  peak  the to  were  peak  to  derivative  from t h e f i t . to  the convoluted  Above  curve  Gaussian-Lorentzian  corresponding  obtained  Gaussian-Lorentzian  individual  peak  functions peak  half  of  the  This  method  halfwidths  of  ESR s p e c t r a b e l o w t h e  phase  transition  peak h a l f w i d t h s were  from a f i t of a s i n g l e c o n v o l u t e d  then  Gaussian-Lorentzian  the  determined function  to the i n t e g r a t e d data. The  most  calculation arises  significant from  the  error  in  determination  the of  susceptibility the  baseline;  30  however found an  i n ' . 12%  of t h e d a t a  t h a t were n o t due incorrect  asymmetry  the  susceptibilities not  significant  distortions  to integrating a d e r i v a t i v e  baseline.  in  scans  These  derivative  curves.  i n these cases  i n c o r p o r a t e d i n the  distortions The  curve  were  due  meaning  i s s u s p e c t , and  were with  to of  an the  these scans  were  results.  (b) B a s e l i n e D e t e r m i n a t i o n The  problem  limitation initial  of  to the f i n a l  The  integrated  where t h e  increased,  this  decreased,  this  The  susceptibilities.  determined  data  were  before due  the  by a v e r a g i n g  The the  t o have  distortions  i s due were  peaks  decreased  T h e r e were before  it  t o r e m o v i n g t o o l a r g e a b a s e l i n e , and  where t h e t a i l  this  a l s o found  to  after  the peaks i n c r e a s e d b e f o r e  removing  minimized  b a s e l i n e o f t h e o r d e r o f 0.5%  computer. from  is  distortions  initial  the  to problems i n b a s e l i n e removal.  tail  t h e r e were s c a n s  These  of  significant  scan.  t h a t were a t t r i b u t e d  it  accuracy  e s t i m a t e o f t h e b a s e l i n e was  data a c r o s s the  scans  d e t e r m i n i n g the b a s e l i n e i s a  too  by  small  making  i n the  s o u r c e c o u l d be e s t i m a t e d t o be a b o u t  baseline.  changes  interactively  c o n t r i b u t i o n to the e r r o r  a  on  to the  the UBC  susceptibilities 10%.  31  CHAPTER 5 ESR EXPERIMENTAL RESULTS  5.1 ESR q VALUES A t room t e m p e r a t u r e two l i n e s  I and I I  are  found  i n the  s p e c t r u m o f D E M ( T C N Q ) , c o r r e s p o n d i n g t o t h e two s t a c k s B a n d A. 2  We  f o l l o w t h e n o t a t i o n o f S c h w e r d t f e g e r e t a l . 1980. The  DEM(TCNQ) 290-K  g  values  of  lines  c  and  = 400  average  a n d I I o f t h e ESR s p e c t r u m o f  were d e t e r m i n e d a s a f u n c t i o n o f t e m p e r a t u r e  2  401 K  K. of  temperatures  The the  between  a t an a n g l e 9 = 60° ( s e e f i g . 5 . 1 ) The g v a l u e  was f o u n d t o be i n d e p e n d e n t T  I  g g  values values  between  290 K  of  temperature given  measured  i n table  between  .290 K  I f o r T<T  f o r 9 = 60°  C  at  below.  above  the  phase  are the various  a n d 4 0 0 ( 3 ) K. The g v a l u e s o f b o t h  l i n e s below t h e phase t r a n s i t i o n and t h e g v a l u e f o r t h e line  and  transition  single  a r e summarized i n t h e t a b l e  32  TABLE 5. 1  ESR CJ VALUES  I  Temp.  T<T  C  T>T  C  II  2.002571(17)  2.003233(23) 2.003250(10)  The g v a l u e b e l o w t h e p h a s e t r a n s i t i o n change  apart  from e x p e r i m e n t a l t  at  different  temperatures  quoted f o r the g v a l u e above T mean o f two r e s u l t s  c  not  found  to  e r r o r . The e r r o r s q u o t e d f o r t h e  g v a l u e s below T a r e the s t a n d a r d e r r o r measurements  was  or  the below  are the standard  mean  of  T . The  errors  c  error  nine  of  a b o v e t h e p h a s e t r a n s i t i o n . The a b o v e  the  errors  do n o t i n c l u d e any s y s t e m a t i c e r r o r s f r o m t h e c a l i b r a t i o n . The c a l i b r a t i o n g  value  e t a l 1972  was p e r f o r m e d u s i n g a s a m p l e o f L i F : L i .  of L i i s 2.002317(2). ( P r e s s l e y p.345  and  p.336).  The  These  g value data  phase t r a n s i t i o n  imply that  i s line I.  e t a l 1963, a n d G o r d o n  calibration  s y s t e m a t i c e r r o r o f 0.001% t o t h e g v a l u e  The  introduces  a  measurements.  the remaining  l i n e above t h e  33  F i g u r e 5.1 A n g u l a r dependence of t h e g above the phase transition Q: c r y s t a l #5 X: repeat run of c r y s t a l a f i t t o e q u a t i o n 5.1  value  o f DEM(TCNQ) at 4.00(3) K #5. The s o l i d l i n e i s 2  34  5.2 ANGULAR DEPENDENCE OF THE CJ VALUES The a n g u l a r transition  dependence o f t h e  was  g  values  above  the  phase  m e a s u r e d by r o t a t i n g t h e s a m p l e i n t h e p l a n e o f  the s t a t i c magnetic The a n g u l a r  field.  This data  i s plotted  dependence o f t h e g v a l u e  i n f i g 5.1.  was  fitted  t o the  equation  g  The  =  2  g c o s e + g^sin © 2  resulting  values  2  values  quoted  (Schwerdtfeger  in  (5.1)  2  forg  the  |t  a n d g ^ a t 401 K t o g e t h e r w i t h t h e  literature  for  lower  temperatures  e t a l 1980) a r e s u m m a r i z e d i n t a b l e 5.2. AND . o _ FOR DEM(TCNQ)  TABLE 5.2  Lj  line  Temp. (K)  9„  401  2.002730(15)  2  I  line  2.003551(14)  II  9„  9x  -  -  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  1  r — — i  ID CVJ  c  -  0  i x(aiow/rw3)  A  i  n  i  a  i  F i g u r e 5.2 Temperature Dependence o f t h e S p i n S u s c e p t i b i l i t y of line I of DEM(TCNQ) • : c r y s t a l #4 0 : c r y s t a l #5 X : r e p e a t r u n o f c r y s t a l #5 #: c r y s t a l #6 The S o l i d C u r v e i s E s t i m a t e d from t h e P o i n t s 2  i  d  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 o f 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 I o f D E M ( T C N Q ) . S y m b o l s 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 t h e P o i n t s . 2  37  The  quoted  statistical  experimental  error  for g  e r r o r from t h e f i t of t h e data  i s the  x  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 b e t w e e n t h e v a l u e s at  q  and  )(  forg  ()  and  q  ±  298 K a n d 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 b e t w e e n  the  two  measurements.  This  c o n c l u s i o n i s reached because the  m e a s u r e m e n t s a t 298 K were c a l i b r a t e d u s i n g DPPH a n d no  difference  in  g  value  between  there  was  298 K a n d 401 K when b o t h  m e a s u r e m e n t s were p e r f o r m e d u s i n g t h e same  calibration.  5.3 TEMPERATURE DEPENDENCE OF THE S U S C E P T I B I L I T Y The s p i n s u s c e p t i b i l i t y b e l o w 270 K ( S c h w e r d t f e g e r of  lines  I  of  II  has  2  e t a l . 1 9 8 0 ) . The s p i n  a n d I I i s shown i n f i g s .  The sum o f t h e s u s c e p t i b i l i t i e s fig.  DEM(TCNQ)  been  susceptibility  5.2 a n d 5.3  of l i n e s  measured  respectively.  I and I I i s p l o t t e d  5.4. From t h i s d a t a we s e e t h a t t h e s u s c e p t i b i l i t y  g o e s t o z e r o a t 4 0 0 ( 3 ) K. Above 4 0 0 ( 3 ) K  observed,  indicating  susceptibility susceptibility  is  curve  i s continuous g  value  of  a c r o s s t h e phase line  data  I  before  (see s e c t i o n 5.1). This together  line  physical explanation for this  There  is  combined  transition. the  phase  line  above  with  i n d i c a t e s that there i s a transfer  f r o m s t a c k A t o s t a c k B a t 4 0 0 ( 3 ) K.  of l i n e  a t 4 0 0 ( 3 ) K. The s p i n  t h e same a s t h e g v a l u e o f t h e s i n g l e  the phase t r a n s i t i o n susceptibility  phase t r a n s i t i o n  one  of l i n e I i n c r e a s e s a t 400(3) K and t h e  We h a v e s e e n t h a t t h e transition  a  only  in  i s at  t r a n s f e r of s p i n .  the  of s p i n  present  no  38  F i g u r e 5.4 Temperature Dependence o f t h e Sum of the Spin S u s c e p t i b i l i t i e s o f l i n e s I and I I o f DEM(TCNQ) . Symbols a r e D e f i n e d i n F i g . 5.2 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 2  39  I  310  Figure 5.5 H a l f w i d t h of in F i g . 5.2  i  I  I  I  350 390 TEMPERATURE (K)  Temperature L i n e s I and II  I  I  430  Dependence of the Peak of DEM(TCNQ) . Symbols are 2  I  to Peak Defined  40  o  o  o  o( S S R Dd6 )  d  d  H V  F i g u r e 5.6 Angular Dependence o f t h e Peak t o Peak H a l f w i d t h o f l i n e I o f DEM(TCNQ) a b o v e t h e p h a s e t r a n s i t i o n a t 4 0 0 ( 3 ) K. The S o l i d L i n e i s a F i t t o H = H c o s e + H s i n e . 2  2  2  2  2  2  41  The  phase  transition  400(3)  at  reversible.  Sample #5 was t a k e n p a s t  401  then  K  and  measurements  to  the  was  found  phase  to  transition  be to  room t e m p e r a t u r e . The r e p e a t  were  no  different  in  the  or the g value.  tail  susceptibility,  and to  453(6)  zero  of  K  and  the does  combined not  susceptibility  follow  the  the Curie—Wiess law. This decrease  however, i s i r r e v e r s i b l e .  A  repeat  high i n the  measurement  t h e s a m p l e h a s been c o o l e d t o room t e m p e r a t u r e  produces  no  signal The s c a t t e r  of  #5  sharply  temperature  ESR  sample  415 K  Between  after  back  of  susceptibility  decreases  cooled  K  i n t h e data of t h e s u s c e p t i b i l i t y  temperature,  arises  from t h e e r r o r  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  as a f u n c t i o n  i n the d e t e r m i n a t i o n of  functions.  These  errors  arise  from t h e d e t e r m i n a t i o n of t h e 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 o f t h e ESR l i n e s  f u n c t i o n of temperature  was m e a s u r e d a s a  between 2 9 0 K a n d 4 5 3 ( 6 )  K. The d a t a a r e  plotted  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  numerical  intergration  of  each  Gaussian-Lorentzian  f u n c t i o n o b t a i n e d from t h e f i t . The  data  temperature  show  a broadening  as t h e phase t r a n s i t i o n  of both l i n e s temperature,  with • increasing 400(3)  K,  is  42  approached  from  below.  The  phase  transition  a t 400(3) K i s  a c c o m p a n i e d by a d i s c o n t i n o u s d r o p i n t h e peak t o peak h a l f w i d t h o f l i n e I , and t h e 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 a s a f u n c t i o n o f a n g l e B above  the  halfwidth The the  phase  transition  i n t h i s case  angular  is  i s similar  The  scans.  a minimum f o r  dependence of t h e g v a l u e s  a maximum ( s e e f i g 5 . 1 ) . T h i s r e s u l t  to that  exhibits reported  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 arises  f i g 5.6.  of the h a l f w i d t h reaches  same 9 where t h e a n g u l a r  curve  in  was m e a s u r e d d i r e c t l y f r o m t h e ESR  dependence  by S c h w e r d t f e g e r  plotted  f o r stack  i n t h e peak t o peak  halfwidth  vs  temperature  from t h e d e t e r m i n a t i o n of t h e G a u s s i a n - L o r e n t z i a n  fitting  f u n c t i o n . The most  errors  is  significant  contribution  to  these  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 t h e peak t o peak h a l f w i d t h a s 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  f r o m t h e ESR d a t a  scans.  43  CHAPTER 6 DISSCUSION OF THE  RESULTS  6. 1 PHASE TRANSITIONS .IN DEM (TCNQ)  2  The ESR m e a s u r e m e n t s 4 0 0 ( 3 ) K, At t h i s in  where t h e s u s c e p t i b i l i t y  temperature there  the  show a r e v e r s i b l e p h a s e t r a n s i t i o n  microwave  of l i n e  I I goes i n t o l i n e I .  i s no i n d i c a t i o n o f a p h a s e  conductivity  or  at  in  the  transition  total  spin  susceptibility. This density and  phase  transition  from s t a c k  spin  B to stack  susceptibility  transition  and t h a t  the  susceptibility  spin  to  the  Non—reversible 415 K  while  A. The are  the s p i n  at 400(3) K i m p l i e s that and  i s explained  by a t r a n s f e r o f  fact that  continuous  susceptibility  over  this  the c o n t r i b u t i o n from stack  temperature  to  the  A and s t a c k  of  transition  conductivity B are  equal.  are observed  above  the c r y s t a l  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  phase  equals the value  e f f e c t s i n the s u s c e p t i b i l i t y this  conductivity  i m m e d i a t e l y below the phase  susceptibility  below  the  spin  effects  behaves i n a  could  possibly  be due t o t h e o n s e t o f t h e d e c o m p o s i t i o n o f t h e s a m p l e . At  a  transition 420 K  and  temperature  of  442(6) K  a  i n the microwave c o n d u c t i v i t y 453(6) K  the  spin  non—reversible is  observed.  susceptibility  phase Between  decreases  44  irreversibly spin  to zero. This  susceptibility  to  i n d i c a t e s t h a t the zero  c o n d u c t i v i t y are d i f f e r e n t  and  the  decrease  discontinuity  manifestations  of  the  of  the  in  the  same  phase  K and  480  transition. The was  decrease  f o u n d t o be  other  hand  DEM(TCNQ) 483  K.  2  due  c o n d u c t i v i t y b e t w e e n 460  t o a d e c o m p o s i t i o n of  showed two et a l  postulate  4 5 3 ( 6 ) K and Similarly  the  temperature  (Morrsink  We  of  the  we  dependent  the  sample.  Guinier  the  that  the  Guinier  postulate  phase  transition  measurement t h a t the  at  be  heated  in  by  483  at 442(6) K K  the  temperature,  then  at  K obtained  i s -that  with  the  the  slowest  lowest  m e a s u r e m e n t s and  temperature. pattern  Although  rate being  this  i s seen i n the  with  also was the  transition  finally and  by  effects.  sample  phase  at  these  t e m p e r a t u r e s can  which  to  related.  structural  f a s t e s t r a t e of h e a t i n g  investigation, a similar below.  phase t r a n s i t i o n  t h e ESR  measurements w i t h the transition  the  v a r i o u s experiments, the  measurement  as n o t e d  are  p h a s e t r a n s i t i o n a t 415  the d i f f e r e n t r a t e s  conductivity  phase  and  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 ,  d i f f e r e n c e s i n the  explained  K  r e v e r s i b l e phase t r a n s i t i o n  e l e c t r o n i c e f f e c t s occurring before  These  of  1980).  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 phase  the  photographs  r e v e r s i b l e p h a s e t r a n s i t i o n s a t 415  400(3) K i s r e l a t e d t o the  two  On  K  the the  highest  requires other  Guinier  further  compounds  45  6.2 COMPARISON OF DEM(TCNQ) The  ESR o f HEM(TCNQ)  with a decrease differential HEM(TCNQ)  to  This  of  dependent  corresponds  to  are  MEM(TCNQ)  the  2  a t 425 K  susceptibility (DSC)  and  measurement  a t 450 K.  a of  (Huizinga  1980  G u i n i e r p h o t o g r a p h s o f HEM(TCNQ)  a t 456 R ( v a n Bodegom, 2k  1980 p . 1 0 8 ) . T h e s e  temperatures  spin  calorimeter  a phase t r a n s i t i o n  (Huizinga  the  AND  2  a phase t r a n s i t i o n  a phase t r a n s i t i o n  p.108).Temperature exhibit  exhibits  2  zero  scanning  exhibits  2  TO HEM(TCNQ)  2  phase  F  transition  differences  very s i m i l a r  1979,  in  t o those reported  p.73).  i n HEM(TCNQ)  phase  2  2  transition  f o r DEM(TCNQ)  2  above. In  MEM(TCNQ)  instabilities. Spin—Peierls  transition  The  phase  transition  we  2  2k  find F  both  the  instability  transition  at  h a s been a t t r i b u t e d a t 335 K ( H u i z i n g a  2k  and  F  h a s been a t t r i b u t e d  18 K  while  the  remains  2  to  MEM(TCNQ)2 at  relate be  to  the  answered.  phase  transitions  The S p i n — P e i e r l s  B  of  a b o v e room t e m p e r a t u r e i n DEM(TCNQ) transition  phase  corresponds  2  and  DEM(TCNQ)  2  phase t r a n s i t i o n i n phase the  2  phase  phase  f  i n MEM(TCNQ)  23 K i n s t a c k B o f DEM(TCNQ) . The n a t u r e o f  which  F  1980 ) .  a t 18 K i s c l o s e t o t h e S p i n - P e i e r l s  transitions  4k  4k  to the  t o the semiconductor—metal  The q u e s t i o n o f how t h e p h a s e t r a n s i t i o n s HEM(TCNQ)  the  two  should  2  t o the 4k  transition  F  phase  indicate  instability in  S t a c k B of DEM(TCNQ) . 2  The p h a s e t r a n s i t i o n There i s a t r a n s f e r there  is  no  a t 400(3) K i s  of spin d e n s i t y  effect  on  the  related  to  stack  A.  f r o m s t a c k A t o s t a c k B, b u t  total  bulk  s u s c e p t i b i l i t y or the  46  conductivity effects  on t h e s u s c e p t i b i l i t y  at 453(6) phase  t o w i t h i n the s c a t t e r  K and  442(6)  transition  K  may  latter  different MEM(TCNQ)  from 2  the  data.  The  radical  and t h e c o n d u c t i v i t y a r e o b s e r v e d  correspond  phase  the  respectively,  however i s n o t c l e a r b e c a u s e The  in  indicating  to the 4k  F  that  this  instability.  This  of t h e d e c o m p o s i t i o n of t h e sample.  transition  in  DEM(TCNQ)  semiconductor—metal  i n t h a t no m e t a l l i c  phase  conductivity  is  is  2  however  transition observed  in  above  t h i s p h a s e t r a n s i t i o n . T h i s h o w e v e r c a n be e x p l a i n e d b e c a u s e t h e phase  transition  temperature  MEM(TCNQ) , n a m e l y b e c a u s e 2  DEM(TCNQ)  2  sample.  the  2  i s h i g h e r than i n  of the probable decomposition of  i n DEM(TCNQ)  t o 450 K i n HEM(TCNQ)  susceptibility  to  2  between t h e  2  and t h e phase  transition  i n that  i n b o t h phase  transitions  31 K  zero  between  and  the  the  decrease  Guinier  the  in  both  i s p r o b a b l y t h e speed a t which b o t h k i n d s of measurements  were made electronic  or  a  and  difference the  in  structural  the  temperature  effects.  some o f t h e s i m i l a r i t i e s .  between  The p r o x i m i t y  t e m p e r a t u r e s o f t h e s e two p h a s e t r a n s i t i o n s c o u l d for  of  measurements  r e s p e c t i v e l y . The c a u s e o f t h e t e m p e r a t u r e d i s c r e p a n c y cases  the  decreases 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 o f 30 K a n d susceptibility  DEM(TCNQ)  T h e r e a r e a l s o some s i m i l a r i t i e s  h i g h e s t phase t r a n s i t i o n a t 425  in  also  the  i n the account  47  CHAPTER 7  7.1  CONCLUSIONS AND FURTHER POSSIBLE  EXPERIMENTS  CONCLUSIONS The ESR a n d m i c r o w a v e c o n d u c t i v i t y  temperature  showed  phase t r a n s i t i o n density value  of  the  A t o stack  single  identical  t o that  observed  in  of l i n e  discontinuity  phase  was a t r a n s f e r o f  I.  This  this  phase  phase  spin  of the g  transition  transition  was  was not  of t h e t o t a l s p i n s u s c e p t i b i l i t y nor phase  4 4 2 ( 6 ) K a n d 4 5 3 ( 6 ) K. T h i s  transition  that  was  manifested  isa  by  a  i n t h e microwave c o n d u c t i v i t y and t h e d e c r e a s e t o  to  instability  temperatures  conductivity  photograph r e s u l t s  s u s c e p t i b i l i t y . This  t h e 4k  The p h a s e t r a n s i t i o n microwave  above  room  a reversible  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 . The s e c o n d  zero of the t o t a l spin due  there  Firstly  above  2  B. The a n g u l a r d e p e n d e n c e  was o b s e r v e d b e t w e e n  non—reversible  possibly  line  a measurement  i n a measurement transition  two p h a s e t r a n s i t i o n s .  a t 4 0 0 ( 3 ) K where  from stack  o f DEM(TCNQ)  methods  f o r both phase  phase t r a n s i t i o n  i n stack  determined were  lower  transitions.  is  B o f DEM(TCNQ) . 2  by than  the  ESR  and  the Guinier  48  7.2  FURTHER DIRECTIONS There  related  are  TCNQ  experiments (a)  many  compounds;  i n DEM(TCNQ)  A study  temperatures  of  on  i n v o l v e d would sample  possibilities  be  the  the  2  however we  for  further  will  only consider further  t h a t a r e s u g g e s t e d by t h i s dependence  of  from 6—hours  to  1—2  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  ESR  and microwave  (b) as  The  phase  conductivity  days  for  transition times  heating  480 K. T h i s w o u l d  and  the  the  Guinier  r e a l p a r t o f t h e d i e l e c t r i c c o n s t a n t c a n be  measured  measurements.  a f u n c t i o n of t e m p e r a t u r e p a s t t h e s e phase t r a n s i t i o n s .  experiment  i s c o n t i n g e n t on  DEM(TCNQ)  which are very d i f f i c u l t  2  the  clarify  t e m p e r a t u r e s between  measurements  on  work:  r a t e o f h e a t i n g of. t h e c r y s t a l . The  f r o m room t e m p e r a t u r e t o a b o u t  photograph  the  research  obtaining  thin  t o grow.  long  crystals  This of  49  BIBLIOGRAPHY  van Bodegom, (1981) B o s h , A.,  B.,  and van de B o e r , J . L., A c t a . C r y s t B37,  and v a n Bodegom, B.,  A c t a . C r y s t . B33,  Buranov, L. I . , and Shchegolev, I . E x p e r i m e n t a l T e c h n i q u e s 14, 528 (1971)  F.,  3013  1195  (1977)  Instruments  and  G o r d o n , A. J . , and F o r d , R. A., The C h e m i c a l C o m p a n i o n ; A Handbook o f P r a c t i c a l . D a t a T e c h n i q u e s and R e f e r e n c e s (John W i l l e y a n d S o n s : New Y o r k , 1 9 7 2 T von Hippel, A., T a b l e s o f 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 T e c h n o l o g y ) 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  H u i z i n g a , S., Kommandeur, J . , S a w a t z k y , G. A., P h y s Rev B19, 4723 (1979) K u i n d e r s m a , P. I . , S a w a t z k y , G. A., and P h y s i c s C ( S o l i d S t a t e ) -8^ 3005 ( 1975) .  (1980)  and T h o l e , B.  Kommandeur,  J.,  T., J.  M o r r o w , M., H a r d y , 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., V e r w e y , A., and S a w a t z k y , G. A., Can. J_j_ o f P h y s . 58 334 (1980) M o r r s i n k , H.,  a n d van Bodegom, B.,  O s b o r n , J . A.,  P h y s . Rev.  Pressley, (1963)  J.,  R.  60 351  Acta Cryst.  37, 107  (1981)  (1945)  and B e r k , H. L., B u l l .  Am.  P h y s . S o c . 8,  345  50  S a w a t z k y , G. A., H u i z i n g a , of the International C o n d u c t o r s , S e p t e m b e r 1978 i n P h y s i c s . p . 34, {"Berlin:  S., a n d Kommandeur, J . , P r o c e e d i n g s C o n f e r e n c e on Q u a s i — o n e — d i m e n s i o n a l Dubrobnik Y u g o s l a v i a , Lecture Notes S p r i n g e r V e r l a g , 1978)  Schwerdtfeger, C. F., O o s t r a , S., a n d S a w a t z k y , 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 . , a n d S a w a t z k y , G. A., S t a t e Comm. 3 5 , 7 (1980) Waldron, R. A., T h e o r y o f G u i d e d E l e c t r o m a g n e t i c Waves v a n N o s t r a n d R e i n h o l d Company, 19701 ~ W a l d r o n , R. A., The T h e o r y o f W a v e g u i d e s a n d C a v i t i e s G o r d o n a n d B r e a c h S c i e n c e P u b l i s h e r s , 1969)  Solid  (London:  (New Y o r k :  

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