PAIR ORDERING IN DISORDERED CHAIN MOLECULES OF Se0.25-Te0.75 LIQUID BY PULSED NEUTRON TOTAL SCATTERING

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PAIR ORDERING IN DISORDERED CHAIN

MOLECULES OF Se0.25-Te0.75 LIQUID BY PULSED

NEUTRON TOTAL SCATTERING

M. Misawa, K. Suzuki

To cite this version:

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JOURNAL DE PHYSIQUE CoZZoque C 8 , suppZdment au n o 8 , Tome 41, aoct 1980, page C8-203

PAIR ORDERING IN DISORDERED CHAIN MOLECULES OF Se0.z5-Te0.75 LIQUID BY

PULSED NEUTRON TOTAL SCATTER1 NG

M . Misawa and K . Suzuki

The Researeh I n s t i t u t e for Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan.

INTRODUCTION

Selenium-tellurium liquid undergoes metal-nonmetal transition over a certain temperature [l] and pressure [2] range as a function of the liquid composition. With increasing temperature, the density[3] of SeO. ~ 5 - ~ ~ 0 . 7 5 liquid decreases to reach the minimum at about 400 OC and then increases gradually up to the maximum near 650 "C, beyond which the density again decreases, as shown in Fig. 1. Such an anomalous density change of SeOez5-Te _0.75 liquid has been known to be accompanied by drastic variations in the electrical conductivity[l] and magnetic susceptibil- ity[4] with temperature.

The purpose of this study is to examine the modification in the atomic scale structure of Se0.25-Te 0.75 liquid through the metal-nonmetal transition by the measurement of the high resolution radial distribution function using apulsed neutron total scattering technique.

EXPE-RIMENTAL

A Se0.25-Te0,75 sample was prepared from 9 9 . 9 9 9 9 % purity Te and 9 9 . 9 9 9 % Se which were melted at 650 O C for 7 hours to

complete alloying reaction. The sample was sealed in vacua in a silica tube with 10.0 mm in inner-diameter, 0.4 mm in wall-thick- ness and 70 mm in length. The structure factor S ( Q ) of SeO. 25-Te0. 75 liquid was measured at two different temperatures, i.e. 414 and 541 OC, using a pulsedneutron total scattering spectrometer installed at 300 MeV electron LINAC, Tohoku University. As shown in Fig. 1, Se0.25-Te0.75 liquid has the minimum density and exists in low electrical conductivity state at 414 O C ,

while the liquid at 541 OC is in the high electrical conductivity state with negative thermal expansion coefficient. Details of the spectrometer and the procedures of data processing from observed T-0-F spectrum to S(Q) have been fully described in previous papers [5,61.

Se.25Te.75 liquid

Fig. 1 Temperature dependences of e l e c t r i c a l conductivity (0)

,

magnetic s u s c e p t i b i l i t y ( x ) and d e n s i t y o f Se0.25-Te0.75 l i q u i d [ 1 , 3 , 4 1 . Arrows show the temperatures a t which S ( Q ) have been measured by pulsed neutron t o t a l s c a t t e r i n g i n t h i s study.

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JOURNAI. DE PHYSIQUE r l n m

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Fig. 2 E x p e r i m e n t a l s t r u c t u r e f a c t o r s S (Q) o f Te, S e 0.25-Te0.75 and S e l i q u i d s . RESULTS

The S(9) of Se0-25-Te0.75 liquid measured at 414 and 541 OC are shown in Fig.2 together with those of pure Se and Te liq- uids just above their melting points for comparison. The first peak in S(Q) of the liquid at 541 OC is higher than the second peak, while the both peaks in S(Q) have nearly same height at 414 OC. The Fourier transform of S(9) truncated at high scattering vector provides the well-resolved profile of the first peak in the radial distribution function RDF.

Fig. 3(a) shows that the atomic distribution of Se0,25-Te0.15 liquid increases around the first minimum in RDF and decreases around r=0.26 nm with increasing temperature. The characteristic variation in the atomic arrangement of Se0.25-Te0.75 liquid between the both temperatures can be clearly found on the differential profile ARDF=RDF (541°C) -RDF (414OC) as shown in Fig. 3 (b)

.

Fig. 3 (a) T o t a l RDFs o f Se0.25-Te0. 75 l i q u i d measured a t 414 and 541 OC. (b) D i f f e r e n c e between

the two RDFs a s shown i n ( a ) .

DISCUSSION

The first peak in RDF is constructed from three different contributions, i.e. Se-Se, Se-Te and Te-Te partial pair distri- butions in Se0.25-Te0.75 liquid. If it is assumed that the three partial pair distri- butions have a gaussian form and inter- atomic distances in Se-Se and Te-Te pairs in the liquid have the same values as in their pure states, i.e. r Se-Se -0.238 nm[71 and rTe-Te=0.292 nm[8] respectively and that of Se-Te pair is rSe-Te=0.265 nmwhich is the average of the two pure states, the first peak in RDF can be resolved into the three gaussian curves as shown in Fig. 4

by the least mean squares method.

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where bse(bTe) i s t h e n e u t r o n s c a t t e r i n g l e n g t h o f S e ( T e ) n u c l e u s , < b > = c b _{Se Se}

+

cTebTer c S e ( c T e ) means t h e a t o m i c f r a c t i o n o f S e ( T e ) atom and n _Se-Se_{( " ~ e - ~ e}

I " ~ e - ~ e ) i s t h e f r a c t i o n o f n e a r e s t n e i g h b o r Se-Se (Se-Te, Te-Te) p a i r i n t h e l i q u i d . F o r t h e s i m p l i c i t y , w e c o n s i d e r t h a t a l l o f S e atoms a n d p a r t o f Te atoms a r e accommodated i n v e r y l o n g d i s o r d e r e d c h a i n s [9,101 w i t h 2 - c o o r d i n a t i o n s , b u t t h e o t h e r p a r t o f Te atoms e x i s t i n t h e c o n f i g u r a - t i o n w i t h 3 - c o o r d i n a t i o n s . The v a l u e o f n Se-Se c a n v a r y from 0 t o 2cSe, where n Se-Se=O means t h e f o r m a t i o n o f o n l y u n l i k e p a i r s , n Se-Se=2cSe o n l y l i k e p a i r s and

Fig'. 4 The f i r s t peaks i n R D F r e s o l v e d i n t o three gaussian form p a r t i a l p a i r d i s t r i b u t i o n s . Dot-dashed, s o l i d and d o t t e d curves mean Se-Se, Se-Te and Te-Te p a r t i a l p a i r d i s t r i b u t i o n s .

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n Se-Se-2cie c o r r e s p o n d s t o s t a t i s t i c a l random mixing. The v a l u e s o f n

Se-Te a n d n a r e w r i t t e n a s a f u n c t i o n o f n Te-Te Se-Se a s f o l l o w s n = 2cSe

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n Se-Te Se-Se ' where x i s t h e f r a c t i o n o f Te atoms h a v i n g 2 - c o o r d i n a t i o n s . F i g . 5 shows t h e t h r e e terms i n t h e r i g h t hand s i d e o f eq. (1) a s a f u n c t i o n o f n Se-Se u s i n g e q s . ( 2 ) a n d ( 3 ) . The Te-Te(2) and Te-Te(3) l i n e s - i n F i g . 5 i n d i c a t e Te atoms o c c u p y i n g t h e 2 - c o o r d i - n a t i o n s and 3 - c o o r d i n a t i o n s s t a t e s i n t h e l i q u i d r e s p e c t i v e l y . The e x p e r i m e n t a l v a l u e s o f t h e a r e a u n d e r t h e g a u s s i a n c u r v e s f o r Se-Se, Se-Te a n d Te-Te p a r t i a l p a i r d i s t r i b u t i o n s a s shown i n F i g . 4 a r e p l o t t e d o n t h e c o r r e s p o n d i n g l i n e s i n F i g . 5 by c l o s e d c i r c l e s f o r t h e l i q u i d a t 414

OC a n d by open c i r c l e s f o r t h e l i q u i d a t 541 OC.

The v a l u e s o f t h e a r e a f o r Se-Se and Se-Te p a r t i a l p a i r d i s t r i b u t i o n s a r e s i t u a t e d a t t h e n e a r l y i d e n t i c a l p o s i t i o n s on t h e n Se-Se a b s c i s s a o n t h e c o r r e s p o n d - i n g l i n e s i n F i g . 5. T h i s means t h a t Se atoms i n t h e l i q u i d s t a y i n t h e 2 - c o o r d i - n a t i o n s s t a t e a t t h e b o t h t e m p e r a t u r e s . However, we c a n n o t f i n d t h e v a l u e s o f t h e a r e a f o r Te-Te p a r t i a l p a i r d i s t r i b u t i o n s on e i t h e r Te-Te(2) o r Te-Te(3) l i n e , i f t h e p o s i t i o n s o n t h e n Se-Se a b s c i s s a d e t e r m i n e d from Se-Se a n d Se-Te p a r t i a l p a i r d i s t r i b u t i o n s a r e a c c e p t e d . F i g . 5 i m p l i e s t h a t a b o u t 8 5 % o f t o t a l Te

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Te p a i r s occupy t h e 2 - c o o r d i n a t i o n s s t a t e o f t h e d i s o r d e r e d c h a i n i n S e 0.25- TeO. 75 l i q u i d a t 414 OC. I n c o n t r a s t t o i t , more t h a n 75% o f t o t a l Te-Te p a i r s a r e t r a n s f e r e d t o s t a y i n t h e 3 - c o o r d i n a t i o n s s t a t e a n d o n l y 25% o f t o t a l Te-Te p a i r s r e m a i n i n t h e 2 - c o o r d i n a t i o n s s t a t e i n t h e l i q u i d a t 541 OC. The v a h e o f nseSse p l o t t e d i n F i g . 5 i s s m a l l e r t h a n 2c

S e in

c a s e o f t h e l i q u i d a t 414 OC and i s l a r g e r t h a n 2cze i n c a s e o f t h e l i q u i d ' a t 541 O C ,

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JOURNAL DE PHYSIQUE

CONCLUSIVE REMARKS

The high resolution radial distribution functions of Se0.25-Te0.75 liquid have

0

541 "C

been experimentally observed by the pulsed neutron total scattering technique using

-

, O r

1

electron LINAC. Based on the observstions, the structure variation from the low density state to the high density state in Se0.25-Te0.75 liquid caused by the nonmetal +metal transition from the low conductivity state to the high conductivity state with increasing temperature is concluded to correspond to an atomic scale structure modification that Se-Te unlike atom pairs are decomposed into Se-Se and Te-Te like atom pairs in the disordered chain with 2- coordinations and the majority of Te-Te like atom pairs are transfered from the 2- coordinations state to the 3-coordinations state through the transition.

ACKNOWLEDGEMENT

This study is partly supported by The Mitsubishi Foundation. One of the authors

(ME!) gratefully acknowledges a support

2 C ~ e

from the 1977 RCA grant program.

fraction of ( S e

-Se)pair

REFESIENCES

unlike pair random Like pair _{[11 J.C.Perron:Advances in Physics,}

₁₆

(Se-Te) mixing (Se-Se ,Te-Te) ₍₁₉₆₇₎_657.

Fig. 5 Weighted p a r t i a l n e a r e s t neighbors bib.ni- j/<b>2 i n eq. ( 1 ) versus f r a c t i o n o f Se-Se pai2 nse-se. The i - j means the p a i r between i and j a t o m s ( i , j = S e , T e ) . Experimental values are p l o t t e d by c l o s e d c i r c l e s ( 4 1 4 OC) and open c i r c l e s

(541 O C ) . Hatched regions correspond to t h e nse-Se

values expected i n Se 0.25-Te0. 75 l i q u i d a t t h e both temperatures.

scattered. This means that Se and Te atoms in Se0.25-Te0.75 liquid tend to form Se-Te unlike atom pairs at 414 OC, while Se-Se and Te-Te like atom pairs are most likely

formed at 541 OC. The occurrence of such

a drastic concentration separation in Se0.25-Te0.75 liquid may be thermodynamic- ally appreciated by the endothermic DTA curverl21 during heating up through the transition temperature.

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[I21 M-Fischer and H.Krebs:Glastech. Ber., 47(1974)42.