Raman scattering and structural order in liquid GeSe2

Journal of Non-Crystalline Solids 59 & 60 (1983) 1055-1058 North-Holland PublishingCompany

1055

RAMAN SCATTERING AND STRUCTURAL ORDER IN LIQUID GeSe2 J. R. MAGANA and J. S. LANNIN Department of Physics, The Pennsylvanla State U n l v e r s i t y , U n i v e r s l t y Park, PA 16802 USA High temperature polarized and depolarlzed Raman spectra have been obtalned in llquld GeSe2 near the melting point of 770 C. The spectra indicate a number of s l m l a r l t l e s to amorphous GeSe2, including the presence of the "companion" line which has been associated with Intermediate range order in the amorphous state. The Raman spectra, as well as previous low angle diffraction studies, indicate that intermediate range structural order is also present in liquid GeSe2. I.

INTRODUCTION Considerable current i n t e r e s t e x l s t s regarding the s t r u c t u r e of amorphous

(a-) GeSe2,

A number of s t r u c t u r a l

models I-5 have been suggested, ranging from

a random network with short range chemical order to p a r a c r y s t a l l l n e r a f t or ribbon-liKe clusters. configurations.

Included in these models in a d d l t l o n , are extended rln9

Central to the s t r u c t u r a l

controversy has been the assign-

ment of the "companion" l i n e whose frequency is ~10% above t h a t of the A1 peak. The l a t t e r

15 associated with t e t r a h e d r a l

models f o r the s t r u c t u r a l

A l l of the above

o r l g i n of the companlon mode requlre the presence

of i n t e r m e d i a t e range order (IRO), i . e . , spheres of Ge and Se.

breathing modes.

order beyond the f i r s t

Low angle neutron d i f f r a c t l o n

coordlnatlon

studles 6 in a-GeSe 2

i n d i c a t e the presence of a peak at k - I A-I which is also present In the a-Aschalcogenides, a-As and a-P and has been assoclated with i n t e r l a y e r Diffraction

measurements 6 in l i q u i d

IRO. 6-8

(1-) GeSe2 also i n d i c a t e a low angle peak

suggesting t h a t IRO order may extend i n t o the l i q u i d phase. studies have addressed the questlon of IRO in l i q u i d s .

Limited Raman

Raman s c a t t e r i n g mea-

surements in l - S e , e . g . , have suggested t h a t a nonrandom dihedral angle distribution

may occur, r e s u l t i n g in vestiges of IRO in the l i q u i d . 9

In

c o n t r a s t , the Raman spectra IO of l i q u i d and amorphous As2S3 do not e x h i b i t sharp s t r u c t u r e which would be i n d i c a t i v e of order beyond short range.

The

present high temperature study in l-GeSe 2 suggests by the presence of the companion l i n e , as well as other features s i m i l a r to those of a-GeSe2, t h a t IRO is present in the l i q u i d phase. *Supported by NSF Grant DMR 8109033

0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan

J.R. Maga~a, J.S. Lannin / Raman scattering and structural order

1056

2. EXPERIMENT First order polarized (HH) and depolarized (VH) Raman spectra were obtained on 1-GeSe2 encapsulated in a heavy-walled, fused s i l i c a tube. contained in a high temperature furnace

The sample was

modified for pseudo backscattering

Raman measurements. A Spex third monochromator system with microcomputer control was employed along with 5145 A Ar laser excitation.

The backscattering

measurement required subtraction of the VH and HH high temperature Raman spectra of the fused s i l i c a background. Crystalline GeSe2 of high stoichiometry was employed along with a small cell dead space to maintain the liquid compositlon. 3. RESULTSAND DISCUSSION The VH and HH Raman spectra of l-GeSe2 obtained at T = 770 C are shown in Fig. I.

Although the main spectral features above 150 cm-1 are clearly observ-

ed, the SiO2 background contributions are relatively large, exceeding the '

I

I

I

I

I

I

I

I

I

I

I

VH

HH

~8

8

o~

F-

~E

6

~4

4

~2

2

]

I

I00

200

0

300

I

I

I00

I

I

200

I

300

~(cm-l)

~(cm-I) FIGURE l VH and HH Raman spectra of liquid GeSe2

liquid maximum intensity by factors of -2.

Subtraction of the cell background,

which is structureless and r e l a t i v e l y monotonic, yields the spectra shown in Fig. 2.

Both the VH and HH spectra indicate considerable detail in the

liquid scattering response that is q u a l i t a t i v e l y similar to that of a-GeSe2.2 The main high frequency polarized band between 150-234 cm-1 exhibits peaks at 193 cm-l and 205 cm- l for the Al and weaker companion modes, respectively.

In

addition to thermal broadening, that may be associated with structural disorder and anharmonic effects in l-GeSe2, these peaks are shifted to lower frequencies by ~3% and 5%, respectively.

The observation of a d i s t i n c t , though

weakened companion line is somewhat surprising given i t s sensitivity to changes in alloy composition2 and external pressure. 11

The VH and HH Raman

spectra also exhibit weak features at ~170 cm-l similar to that observed in

J.R. Maga~a, J.S. Lannin / Rarnan scattering and strtwtural order

I

I

I

I

I

I

I

I

I

1057

I

I

I

HH

VH

8

~6

Z

:~6 cD oc:

~4

~4

H

~2 I

0

I

I

i(3o

I

l

2oo

I

I

3(3o

I I00

I

FIGURE 2 VH and HH Raman spectra of l i q u i d GeSe2 a f t e r

state.

is,

I 300

background s u b t r a c t l o n

a-GeSe 2 as well as a very weak HH peak at ~247 cm- I . peak at 300 cm-I

I 200

~(cm -i}

(cm -I )

however, reduced in i n t e n s i t y

Below the apparent pseudogap at 150 cm- l ,

low frequency phonon e x c i t a t i o n s

The h i g h e s t frequency relative

the l l q u i d

t o the amorphous spectra i n d i c a t e

s i m i l a r to t h a t in the amorphous s t a t e .

A major r e s u l t o f the Raman measurements Is the o b s e r v a t i o n of a companion peak in the l i q u i d

state.

This suggests the IRO i s t o some e x t e n t preserved

in the l i q u i d .

In general IRO may be a s s o c l a t e d w i t h i n t r a l a y e r

unit structural

correlations,

the degree or type o f a s s o c i a t i o n of t e t r a h e d r a . diffraction

The o b s e r v a t i o n of a low angle

peak in neutron s c a t t e r i n g measurements at k - 1 A- l

of i n t e r u n i t

structural

or i n t r a -

where the u n i t or c l u s t e r i s a f u n c t i o n here of

correlatlons.

IS i n d i c a t i v e

The increased width of t h i s

peak in

l-GeSe 2 as well as the observed broadening of the companion l i n e are c o n s l s t e n t w i t h increased s t r u c t u r a l

d i s o r d e r in the l i q u i d .

quency o f the A 1 and companion modes ms p r i m a r i l y s h o r t range c o v a l e n t i n t e r a c t i o n s in f l r s t

and is in accord w i t h a small ]ncrease

neighbor d i s t a n c e in the l i q u i d . 6

Concurrent w i t h reduced covalency

i s an i n c r e a s e in the number of second nelghbors. different

shifts

The r e d u c t i o n o f the f r e a consequence of reduced

This may account f o r the

o f the A1 and companion modes.

The o b s e r v a t i o n of a companion mode i m p l i e s in s e v e r a l models t h a t the l i q uid c o n t a i n s s t r u c t u r a l

units of considerable extent.

Recent Mossbauer measure-

ments 12 appear t o i n d i c a t e the presence of Se-Se bonds, whose presence i s pred i c t e d in the r a f t model. 3 fective

crystalline

This model, which may be viewed as a fragmented, de-

system, i m p l i e s extended s t r u c t u r a l

correlations.

the 2 - r i n g model a l s o would imply such l a r g e c r y s t a l l i n e - l i k e the l i q u i d . 5

Similarly

c o n f i g u r a t i o n s in

Somewhat s m a l l e r , though nonetheless extended s t r u c t u r a l

correla-

t i o n s are a l s o i m p l i e d in the c h e m i c a l l y ordered CRN model of edge s h a r i n g

1058

,LR. Maga~a, J.S. Lannin / Raman scattering and structural order

tetrahedra. 4 Recent cluster calculation results, 5 as well as the presence of Se-Se bonds seem, however, to imply at present that this model is incomplete. The VH Raman spectra of liquid and amorphous GeSe2 both Indicate a f a i r l y strong companion peak or subband. This suggests a similar peak in the phonon density of states, although structural correlations might result in an enhancement of the VH intensity r e l a t i v e to the phonon spectrum. 8 A significant contribution to the integrated density of states suggests that the companion subband is not to be associated with highly specialized modes of low density, such as ring vibrations.

The width of the VH and HH bands and the absence of gaps in

the Raman spectra, as well as the loss of the t h i r d c-GeSe2 rdf peak in a-GeSe2, suggest the presence of bond and dihedral angle disorder.

The Raman spectra and

d i f f r a c t i o n studies, which parallel bulk a-P results, 8 thus suggest an anisot r o p i c , noncrystalline layer or ribbon model for the amorphous and liquid phases. ACKNOWLEDGEMENT We wish to thank J. Griffiths for kindly supplying the GeSe2 material.

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