Vibrational dynamics and structure of liquid Te

Journal of Non-Crystalline Solids 61 & 62 (1984) 373-378 North-Holland, Amsterdam

373

VIBRATIONAL DYNAMICS AND STRUCTURE OF LIQUID Te* J. R. MAGANA and J. S. LANNIN Department of Physics, The Pennsylvania State U n i v e r s i t y , U n i v e r s i t y Park, Pennsylvania 16802 USA Polarized and depolarized Raman spectra are reported for liquid Te near the melting point. The spectra d i f f e r significantly from liquid Se indicating that 2-fold coordinated chains are not present in substantial numbers. The three peaked nature and polarization properties of the Raman spectra are consistent with the Cabane-Friedel picture of As-like short range order. The Raman scattering and diffraction studies suggest a 3-fold coordinated, random network model. I.

INTRODUCTION Liquid(l-)

Te is an elemental disordered system whose s t r u c t u r e and physical

p r o p e r t i e s have been e x t e n s i v e l y studied e x p e r i m e n t a l l y .

I n t e r e s t in l-Te de-

r i v e s in part from i t s t r a n s i t i o n a l

l i e s between proto-

typical

character in which i t

l i q u i d semiconductors and l i q u i d m e t a l s . l , 2

of i n t e r e s t in the metal-nonmetal t r a n s i t i o n

process.

As such, i t s p r o p e r t i e s are Transport measurements

i n d i c a t e , f o r example, a more m e t a l - l i k e temperature dependence of the r e s i s t i v i t y above -650 C. 3 a first the f i r s t for. 4

In a d d i t i o n , s t r u c t u r a l

studies at high temperature i n d i c a t e

neighbor c o o r d i n a t i o n change with an increase In the number of atoms in coordination shell,

nl , i n d i c a t i v e of a trend toward m e t a l l l c behav-

These r a d i a l d i s t r i b u t i o n

functlon

increase r e l a t i v e to nl=2 of c r y s t a l l i n e

(rdf)

studles i n d i c a t e a s i g n i f i c a n t

and amorphous Te, with values near ~i=3

at the melting p o i n t . 5 Various s t r u c t u r a l

models f o r l-Te have been proposed, the e a r l i e s t suggest-

lng short chains whlch are coupled so as to y i e l d s e m i m e t a l l l c behavlor. 6 Cabane and Frledel 7 proposed an a l t e r n a t i v e model of a mixture of 2 - f o l d chain and 3 - f o l d pyramidal s i t e s based on the r a d i a l d i s t r i b u t i o n measurements

results. 4

Later

by Tourand below Tm suggest t h a t the number of 2 - f o l d s i t e s i s ,

however, a weaker f u n c t i o n of temperature, with predominant 3 - f o l d c o o r d i n a t l o n near and below Tm i m p l i e d . 5 questioned.

The model of Cabane and Friedel has, however, been

C u t l e r has, f o r example, proposed a model with an equal number of

3 - f o l d and l - f o l d

s i t e s t o account f o r t r a n s p o r t data. 1

More r e c e n t l y , Enderby

and Gay8 have noted t h a t a broad shoulder, r a t h e r than a d i s t i n c t at r ~ . 8 3 A ,

peak, occurs

I n d i c a t i n g t h a t the d i s o r d e r may exceed that implied by e a r l l e r

*Supported by NSF Grant DMR 8109033. 0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

rdf

J.R. Magaffa, J.S. Lannin / Vibrational dynamics and structure o f liquid Te

374

studies.

Enderby and Gay note that this may imply a bond number that differs

from the f i r s t coordination number. As such, this could suggest the p o s s i b i l i t y of strongly coupled chains.

Thus while the Cabane-Friedel model and Its modifi-

cation at high temperatures q u a l i t a t i v e l y accounts for the behavior of l-Te, its predominant As-like 3-fold coordination has not been confirmed.

The present

Raman scattering measurements address this question. Limited studies of the l a t t i c e dynamics of ]-Te have been performed to date. An early attempt to observe i n e l a s t i c , phonon-like excitations using neutron scattering exhibited a structureless background.9 This was interpreted as indicating weak covalent bonds that did not allow collective phonon excitations over a vibrational period.

This contrasted with similar measurements in l-Se,

which indicated narrow o p t i c - l i k e bands. Yashiro and Nishina have recently performed Raman scattering measurements of l-Te and c-TelO.

In contrast, they

observe Raman spectra in l-Te which are somewhat similar to that of Trigonal Te. They have interpreted t h e i r spectra to imply that coordination effect changes, as observed in rdf studies, are not manifest in the Raman spectra.

As w i l l be

discussed below, this interpretation is incorrectly based on a comparison of q=O excitations II in c-Te and the q u a l i t a t i v e l y different response associated with the peaks in l-Te related to the phonon density of states 12.

In addition,

the present more detailed Ramanmeasurements, in which the polarization analysis of the scattered l i g h t is studied, provide information relevant to various structural models of l-Te.

The results indicate a continuous random network

with predominant 3-fold coordination as suggested by Cabane and Friedel. 2.

EXPERIMENT Polarized (HH) and depolarized (VH) Raman spectra were obtained for l-Te

sealed in a heavy-walled, fused s i l i c a tube using 5145 A laser excitation and a Spex t h i r d monochromatorsystem. The pseudo backscattering geometry was employed on a furnace modified for this reflectance configuration.

The significant,

though monotonic, Ramanbackground contribution from the SiO2 cell near the melting point measuring temperature was subtracted to obtain the liquid spectra. 3.

RESULTS Figure 1 indicates the VH and HH Raman spectra of l-Te at T=460 C.

The

spectra i n d i c a t e several frequency bands t h a t are superimposed on a background t h a t is p r i m a r i l y associated with the SiO 2 c e l l , and Rayleigh t a i l

contributions.

but may also contain s t r a y - l i g h t

A s u b t r a c t i o n of the normalized c e l l back-

ground, which was obtained independently, y i e l d s the spectra e x h i b i t e d in Fig. 2. The p o l a r i z e d and depolarized spectra of Fig. 2 i n d i c a t e three d i s t i n c t bands at 85, 112, and 133cm- I .

The peak at ll2cm -I is found t o be rather polarized

J.R. Maga'~a, J.S. Lannin / Vibrational dynamics and structure o f liquid Te

I

I

I

I

I

VH~

6 I-

!

I

1

I

I

I

I

I

8

==

0::

<4-

] H

~

I

375

2

o

~2

"%

41 I

50

I

I I I0O oJ (cm-~)

I 150

50

I00 (cm-')

I

150

FIGURE 1 VH and HH Raman spectra of l-Te. having a depolarization ratio, dp = I(VH)/I(HH)=O.2. 133cm- l is r e l a t i v e l y depolarized, as dp = 0.6.

In contrast, the peak at

Figure 2 suggests the presence

of a pseudogap in the phonon spectrum at=lOOcm- l , although the possible contributions of stray l i g h t and a Rayleigh t a i l preclude an accurate estimate of its depth.

A comparison of the l-Te Raman spectra with those13 of l-Se indicate I

I

I

I

I

I

I

I

I

VH ~8

HH

~

I-

6

I

8

== ~4

I

50

I00 (~ (cm-')

150

I

50

I

I

tO0 ~(cm -I)

150

FIGURE 2 VH and HH Raman spectra of l-Te after background subtraction, striking differences in both the number of peaks as well as their p o l a r i z a b i l i t y . In the l a t t e r , only one polarized major peak, plus a weak shoulder to lower frequencies, occurs in the frequency range corresponding to ~ > lOOcm-l in l-Te. The results thus clearly indicate that substantial 2-fold coordination does not occur in l-Te.

ZR. Maga~a, J.S. Lannin / Vibrational dynamics and structure o f liquid Te

376

Recently, Raman spectra of l-Te were obtained above Tm under condltions in which the s c a t t e r e d l i g h t p o l a r i z a t i o n was not analyzed, lO the l-Te spectra were s i m i l a r to t h a t of c r y s t a l l i n e

I t was found t h a t

(c-) Te, which e x h i b i t s

three q=O modes, the middle of which is a p o l a r i z e d A mode. II

Based on t h i s re-

s u l t Yashiro and Nishina i n f e r t h a t evidence f o r the change from 2 - f o l d to 3 - f o l d c o o r d i n a t i o n upon m e l t i n g was not observed in the Raman spectra.

This r e s u l t i s ,

however, i n c o r r e c t as the three q=O, c-Te modes are a consequence of c r y s t a l momentum symmetry, whereas the l-Te spectra r e f l e c t the form of the phonon density of s t a t e s .

The overlap of the three c-Te peaks, which d i f f e r

s u b s t a n t i a l l y from

the phonon d e n s i t y of s t a t e s , 14 with the unanalyzed l-Te spectra is thus q u i t e accidental.

In c o n t r a s t , the three peaked s t r u c t u r e of l-Te may be i n t e r p r e t e d

in terms of a 3 - f o l d , pyramidal A s - l i k e short range order s i m l l a r to t h a t exh i b i t e d by a-As and a-P.

In the l a t t e r ,

however, the p o l a r i z e d peak correspond-

ing to that of l-Te is superimposed a broad background.15,16

The presence of a

three peaked s t r u c t u r e , with the center peak p o l a r i z e d , is also in accord with random network l a t t i c e a-P. 17

dynamical c a l c u l a t i o n s of Beeman and Alben f o r a-As and

While the a-As and a-P spectra i n d i c a t e a d d i t i o n a l weaker Raman features

t h a t do not have a p a r a l l e l d i s o r d e r in the l i q u i d .

in l - T e , these may be a consequence of a d d i t i o n a l

This is c o n s i s t e n t with the absence of a low angle d i f -

f r a c t i o n peak 8 at a wavevector k=IA -I f o r l - T e .

In a-As and a-P t h i s peak has

been i n t e r p r e t e d to i n d i c a t e i n t e r m e d i a t e range order between quasi-two dimensional units or layers in the bulk amorphous solids and ordered thin films.15,16 While the Raman spectra of Fig. 2 do not exhibit direct evidence f o r 2-fold coordination, i t is not possible to preclude the existence of a small number of such bonds. This is a consequence of the predicted positions of these peaks which would occur somewhat near the highest and lowest bands in l-Te.

The de-

polarization r a t i o indicates, however, that the number of ordered l-Se l i k e chains, i f present, must be rather small. 13 Alloy measurements are in progress in the l-Sel_xTex system that w i l l further address the changes in bonding known to occur with the addition of Se. The Raman results, in summary, provide strong evidence f o r the Cabane-Friedel model f o r 3-bonded, covalent interactions in l-Te.

I t is reasonable to assume,

that the semimetallic nature of l-Te is a consequence of the incursion of the shoulder between the f i r s t and second rdf peaks8 due to backbonding interactions.

The Raman scattering and d i f f r a c t i o n results thus suggest a continuous

random network model with 3-fold coordination.

The use of the random network

model constructed f o r a-As by Greaves et al. 18 would appear to be of value in t h i s regard.

Relaxation or reconstruction of this model to account f o r the

shoulder between the f i r s t and second rdf peaks would be of considerable value.

J R. Maga~a, J.S. Lannin / Vibrational dynamics and structure o f liquid Te

377

Acknowledgement We wish to thank Dr. G. Rosenblatt f o r assistance with furnace deslgn and Professor D. Beeman f o r communicating unpublished a-P c a l c u l a t i o n s . REFERENCES I) M. C u t l e r , Liquid Semlconductors, (Academic Press, New York, 1977). 2) N. F. Mort and E. A. Davis, E l e c t r o n i c Processes in Non-Crystalline Mater i a l s , (Clarendon Press, Oxford, 1979). 3) A. S. Epstein, H. Fritzsche, and K. L a r k - H o r o v i t z , Phys. Rev., I07 (1957) 412; J. C. Perron, Adv. Phys,, 16 (1967) 657. 4

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R. Buschert, I. G. Geib, and K. L a r k - H o r o v i t z , Phys. Rev., 98 (1953) I157.

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B. Cabane and J. F r i e d e l , J. Phys., 32 (1971) 73.

8

J. E. Enderby and M. Gay, J. Non-Cryst. S o l . , 35 & 36 (1980) pp. 1269-1275.

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14

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17

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