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THE ELECTRONIC AND OPTICAL PROPERTIES OF VANADIUM TELLURITE GLASSES
B. Flynn, A. Owen
To cite this version:
B. Flynn, A. Owen. THE ELECTRONIC AND OPTICAL PROPERTIES OF VANADIUM TELLURITE GLASSES. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-1005-C4-1008.
�10.1051/jphyscol:19814219�. �jpa-00220848�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppZdment au nOIO, Tome 42, octobre 1981 page C 4 - 1 0 0 5
THE ELECTRONIC
ANDOPTICAL PROPERTIES
OFVANADIUM TELLURITE GLASSES
B.W. F l y n n and A.E. Owen
Department of Electrical Engineering, University of Edinburgh, King's Bui Zdings, Edinburgh EH9 3JL, Scot Zand, U. K.
Abstract. Data a r e presented on the d.c. conductivity, thermopower and optical absorpti' n of vanadium t e l l u r i t e glasses, particularly as a function of the r a t i o (V9+/VtOtal). The r e s u l t s a r e interpreted in terms of polaron theory and comparisons made with other data. Differences and s i m i l a r i t i e s a r e noted.
Introduction. Preliminary data on the e l e c t r o n i c properties of V205-x.Te02 glasses have already been reported ( 1 ) and recently Isard
( 2 )has suggested t h a t the e l e c t - ronic behaviour
o fthese glasses corresponds more closely with simple polaron theory than any other t r a n s i t i o n metal oxide glass. This paper reports f u r t h e r measure- ments on the vanadium t e l l u r i t e glass system. Glasses were prepared a t i n t e r v a l s of 10 mole
%between 10% and 50% V205, by melting a t approximately 900°C and quench- ing by pouring the melts onto a metal p l a t e a t room temperature. To control the valence r a t i o
[(v4+/vTOtal)
=c ] , the glasses were e i t h e r melted in a i r a t s l i g h t l y d i f f e r e n t temperatures or elemental Te was added t o t h e melt. After annealing a t 250°C f o r 2 hours, samples were ground and lapped t o give d i s c s of about 1 cm i n diameter and 1-2
mnthick. Gold was deposited onto the f l a t faces of t h e d i s c s t o provide contacts f o r e l e c t r i c a l measurements. Analysis of the valence r a t i o ( c ) was carried out by chemical and colorimetric methods. Optical measurements were made with a Zeiss spectrophotometer, generally using thin glass samples prepared by breaking pieces from bubbles blown from molten g l a s s .
D.C.
Conductivity. Previous work ( 1 ) has shown t h a t above about 200
Kthe d.c.
conductivity of vanadium t e l l u t i t e glasses follows the usual activated form
with a constant activation energy,
U,while below 200
K Wdecreases continuously.
This behaviour can be explained i n general terms by the thermally activated hopping of small polarons in a disordered s t r u c t u r e . In the high temperature regime
(T > 200
K )the activation energy i s constant and probably a r i s e s from t h e hoppino
of c a r r i e r s plus a contribution (3 NO) from the s t r u c t u r a l disorder energy,
WD.A t temperatures below 200
Kthere i s a progressive reduction i n the a c t i v a t i o n energy a s expected from the continuous reduction in available phonon energy.
Conduction in transition-metal oxides may be described by the expression proposed by Mott ( 3 ) f o r conduction by hopping between localised s t a t e s ,
i . e .2 2
U =
vo(ne a
/kT)[c(1-c)] exp(-2aa)exp(-W/kT)
(2)where
v6i s a phonon frequency, n i s the number of t r a n s i t i o n metal ion s i t e s , a
i sArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814219
C4- 1006 JOURNAL DE PHYSIQUE
t h e s i t e s e p a r a t i o n and a i s a t u n n e l l i n g f a c t o r . From d e n s i t y and c o m p o s i t i o n d a t a n u m e r i c a l v a l u e s f o r t h e second and t h i r d terms i n e q u a t i o n 2 can be c a l c u l a t e d and knowing a. i n e q u a t i o n 1, v a l u e s o f v, exp(-Zaa), t h e number o f s i t e t r a n s i t i o n s which a c a r r i e r makes p e r second, may be determined. An e s t i m a t e o f a and v. can be made f r o m F i g u r e 1 i n which l o g [ v o e x p ( - 2 a a ) l i s p l o t t e d v e r s u s a, t h e s i t e spacing. From t h e s l o p e o f t h i s graph a
-
0.097 m-', i n cjood agreement w i t h d a t a g i v e n by A u s t i n and G a r b e t t ( 4 ) , and v,-
6 X 1013s-l.F i g . 1 C a r r i e r t r a n s i t i o n p r o b a b i l i t ! ~ F i g . 2 C o n d u c t i v i t y as a f u n c t i o n o f v e r s u s s i t e spacing. t h e v a l e n c e r a t i o (v4+/VTota1),
f o r d i f f e r e n t t o t a l V205 c o n t e n t s (mole %)
.
The b a r s i n F i g u r e 1 i n d i c a t e t h e spread caused by t h e d i f f e r e n t v a l e n c e r a t i o s , c, o f t h e d i f f e r e n t g l a s s e s . One f e a t u r e w o r t h y o f n o t e i s t h a t t h e t u n n e l l i n g f a c t o r (and s i t e s p a c i n g ) i s o b v i o u s l y v e r y s i g n i f i c a n t i n t h e t e l l u r i t e g l a s s e s , i n marked c o n t r a s t t o t h e phosphate g l a s s e s where no dependence o f c o n d u c t i v i t y on s i t e s p a c i n g i s observed ( 5 ) . T h i s a n a l y s i s assumes t h a t t h e phonon f r e q u e n c y i s independent o f s i t e s p a c i n g b u t Sayer and Mansingh ( 5 ) f o u n d t h a t i t decreases as t h e s i t e s p a c i n g i s reduced. I f t h i s i s a l s o t h e case f o r t e l l u r i t e g l a s s e s t h e e s t i a t e o f t h e phonon f r e q u e n c y would be l o w e r e d f r o m t h e r a t h e r h i g h v a l u e o f 6 x 1 0 y 3 s - l . The reasons why t e l l u r i t e g l a s s e s s h o u l d be d i f f e r e n t f r o m phosphate g l a s s e s i n t h i s r e s p e c t a r e n o t obvious. One p o s s i b i l i t y i s t h a t Te s i t e s can p a r t i c i p a t e i n t h e c o n d u c t i o n process t o a l i m i t e d e x t e n t . T e l l u r i u m may e x i s t i n t h e v a l e n c e s t a t e s ~ e and ~~ - e as w e l l as ~ e 4 + . I n t h e process o f m e l t i n g i t i s l i k e l y t h a t ~ + a s m a l l p r o p o r t i o n o f t h e Te02 i s o x i d i s e d e i t h e r by r e a c t i o n w i t h a t m o s p h e r i c oxy- gen a b o v e t h e m e l t o r b y r e a c t i o n w i t h V2O5. Under t h e s e c o n d i t i o n s charge t r a n s f e r cou:d a l s o o c c u r between Te i o n s o r between Te and V i o n s , e n a b l i n q a d d i t i o n a l charge t r a n s f e r processes.
The dependence o f t h e c o n d u c t i v i t y on t h e v a l e n c e r a t i o c i s shown i n F i g u r e 2 where t h e c o n d u c t i v i t y a t 300 K i s p l o t t e d as a f u n c t i o n o f c f o r d i f f e r e n t t o t a l vanadium c o n c e n t r a t i o n s . The c o n d u c t i v i t y i n c r e a s e s w i t h i n c r e a s i n g
v4+
c o n t e n t u n t i l c-
0 . 2 a f t e r w h i c h t h e c u r v e s l e v e l o u t and shown s i g n s o f a g r a d u a l decrease a t h i g h e rv4+
c o n c e n t r a t i o n s . T h e o r i e s o f hopping c o n d u c t i o n i n mixed v a l e n c e semiconductors p r e d i c t a c o n d u c t i v i t y maximum when t h e r e a r e equal numbers o foccupied and unoccupied s i t e s i . e . when c = 0.5. In t h e vanadium phosphate g l a s s system Linsley ( 6 ) found a well defined maximum i n c o n d u c t i v i t y a t c = 0.2 and t h i s discrepancy between theory and experiment has a l s o been observed by o t h e r work r s
( 7 , 8 , 9 ) . Three explanations have been advanced; ( i ) t h a t a p o r t i o n of t h e YE+
ions a r e bound i n s t r u c t u r a l complexes and a r e unable t o p a r t i c i p a t e i n t h e conduct- ion process ( 7 ) , ( i i ) Kinser and Wilson ( 8 ) have suggested t h a t t h e c o n d u c t i v i t y maximum a t
c
0.2 i s caused by t h e formation of a homogeneous g l a s s s t r u c t u r e around t h i s v a l u e , ( i i i b g l a s s - g l a s s phase s e p a r a t i o n occurs with one phase containing a l l of t h e V + ions while t h e o t h e r phase has only VS+ ions ( 9 ) . Information on t h e s t r u c t u r a l p r o p e r t i e s of V205-
Te02 g l a s s e s i s s c a n t y , but t h e formation of l a r g e vanado-tel l u r i t e complexes seems l e s s l i k e l y than t h e formation of vanadophosphate complexes i n V205-
P205 g l a s s e s .Thermopower. The thermopower ( S ) was measured and i n t h e range 290 K t o 400 K i t i s ,
v i r t u a l l y independent of temperature. Generally t h e thermopower becomes s m a l l e r on reducing t h e V205 c o n t e n t of t h e g l a s s , probably i n p a r t because t h e r a t i o c
i n c r e a s e s a s t h e vanadium content of t h e g l a s s decreases. The r e s u l t s of t h e room temperature measurements of thermopower f o r g l a s s e s of d i f f e r e n t compositions a r e p l o t t e d i n Figure 3 a s a f u n c t i o n of t h e r a t i o c . Also shown i n t h i s graph i s a d o t t e d l i n e corresponding t o t h e equation proposed by Heikes and Ure (10) f o r t h e thermopower i n narrow-band semiconductors, i . e .
b u t assuming t h a t t h e entropy term (AS/k) i s n e g l i g i b l y small. Heikes and Ure (10) do not d e f i n e t h e term (AS/k) but Austin and Mott (11) suggest t h a t i t has a value between 0.1 and 0.2 and consequently can be neglected. The experimental r e s u l t s do not agree well with values of S c a l c u l a t e d from equation 3 but note t h a t S = 0 when c
-
0.2 which i s t h e same r a t i o a t which t h e c o n d u c t i v i t y goes through a maximum. S i m i l a r arguments a r e a p p l i c a b l e t o t h e v a r i a t i o n of t h e thermopower with valence r a t i o a s were a p p l i e d t o t h e c o n d u c t i v i t y .Vanadium valence ratio Lc)
Fig. 3 Thermopower a s a f u n c t i o n of Fig. 4 Optical absorption versus photon
.p
valence r a t i o (v4+/vTOta, )
.
energy f o r two g l a s s e s containing 60 mole % V205, but d i f f e r i n g degrees of r e d u c t i o n , i . e . d i f f e r e n t (v4+/VTotal ) r a t i o s .C4- 1008 JOURNAL DE PHYSIQUE
O p t i c a l Absorption. O p t i c a l a b s o r p t i o n measurements were made as a f u n c t i o n o f photon energy a t room temperature (a 300 K) and 77 K: and a steep a b s o r p t i o n edge i s observed f o r each g l a s s a t about 2.2 t o 2.4 eV, a t room temperature, w i t h a small s h i f t o f about 0.05 eV t o h i g h e r energies on l o w e r i n g t h e temperature t o 77 K. The p o s i t i o n o f t h e edge a t 2.2 eV was found t o be independent of t o t a l vanadium content and t h e percentage o f reduced vanadium, s u p p o r t i n g t h e conclusion t h a t t h e 5undament- a1 edge i s t h e r e s u l t o f d i r e c t f o r b i d d e n t r a n s i t i o n s between the oxygen 2p bands and t h e vanadium 3d band o f t h e V205. F u r t h e r support f o r t h i s view i s s u p p l i e d by t h e o p t i c a l a b s o r p t i o n o f c r y s t a l l i n e V2O5. Kenny e t a1 (12) measured the absorption c o e f f i c i e n t i n V205, along t h e d i f f e r e n t c r y s t a l axes, and found, f o r each, a fundamental a b s o r p t i o n edge i n c r e a s i n g from 2.2 eV, which they a t t r i b u t e t o d i r e c t forbidden t r a n s i t i o n s .
The a b s o r p t i o n o f glasses o f t h e composition (V205)40(Te02)60 was s t u d i e d a t low values o f aqpt i n t h e r e g i o n o f t h e a b s o r p t i o n t a i l i n order t o observe t h e effects o f v a r y i n g t h e r a t i o c and t y p i c a l r e s u l t s are shown i n F i g u r e 4. A t t h e lower end o f the a b s o r p t i o n curve a shoulder appears which increases i n i n t e n s i t y as t h e vanadium i s p r o g r e s s i v e l y reduced. T h i s shoulder i s i n t e r p r e t e d as being the combination o f a broad a b s o r p t i o n band and t h e t a i l o f t h e a b s o r p t i o n edge.
Small p o l a r o n theory p r e d i c t s (11) a s e r i e s o f a b s o r p t i o n l i n e s a t m u l t i p l e s o f t w i c e t h e polaron b i n d i n g energy (Wp), which a r e broadened by thermal motion o f the l a t t i c e , r e s u l t i n g i n a s e r i e s o f Gausslan a b s o r p t i o n peaks. T h i s a b s o r p t i o n a r i s e s from t h e o p t i c a l e x c i t a t i o n o f a c a r r i e r from i t s p o l a r i s a t i o n w e l l and subsequent t r a n s f e r t o another l a t t i c e s i t e , corresponding, i n a vanadate glass, t o t h e t r a n s f e r o f an e l e c t r o n from a ~ 4 + s i t e t o a VS+ s i t e . I n t h e vanadate glasses i t i s o n l y possible, i n p r a c t i c e , t o observe t h e f i r s t a b s o r p t i o n peak s i n c e t h e fundamental a b s o r p t i o n edge a r i s i n g from t h e 2p and 3d d i r e c t f o r b i d d e n t r a n s i t i o n s of t h e V205 masks t h e h i g h e r o r d e r peaks.
The p o s i t i o n o f t h e polaron a b s o r p t i o n peak can be r e l a t e d t o t h e d.c.
c o n d u c t i v i t y and t h e thermopower data through hv = Eopt = 2 Wp where Eopt i s t h e i n c i d e n t photon energy and Hp i s t h e polaron b i n d i n g energy. The p o l a r o n b i n d i n g energy i s r e l a t e d t o t h e hopping energy \*!H by (11) UH = $ Wp. Therefore,
hv = EoPt = 4 WH. But WH = (W
- :
WO) where G! i s t h e conduction a c t i v a t i o n energy and WD i s t h e d i s o r d e r energy. T y p i c a l l y f o r a 40 mole % V205 glass W i s 0.28 eV and t h e thermopower data gives a value o f 0.02 eV f o r WD ( 1 ) . Thus, Eopt = 1.1 eV.This agrees w e l l w i t h t h e p o s i t i o n o f the shoulder i n t h e a b s o r p t i o n edge i n d i c a t i n g a polaron b i n d i n g energy o f 0.6
CV.
I t i s reasonable, t h e r e f o r e , t o i n t e r p r e t t h e shoulder i n terms o f polaron hopping b u t t h e present evidence i s n o t s u f f i c i e n t t o discount t h e p o s s i b i l i t y o f i t a r i s i n g from an i n t e r n a l d-d e x c i t a t i o n o f e l e c t r o n s i n t h e vanadium atoms.References
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