Prec. Indian Acad. Sci. (Chem. Sei.), Vol. 90, 1Numlrer 2, April 1981, pp. 83-88. ~) Printed in India.

Infrared absorption spectra of As-Se glasses

R MOHAN and K J RA0* Solid State and Structural Chemistry Unit, Irtdiart Institute of Scierte~, Bangalore 560 012, India

MS received 14 October 1980

Abstract. IR absorption spectra of As-Se glasses have boon studied over a wide range of compositions. Various two-phonort, multiphorton (c,3mbination tortes) and impurity absorptions have been identified. Compositional variation of relative band irttensities has boort exptairted in terms of the chemically ordered n~work model.

Keywords. Irffrared spectra; As-$e glasses.

1. Introduction

Infrared spectroscopy of chalcogenide glasses has attracted wide attention because of their special IR transmitting properties. In particular, As~Se3 glass has been studied in detail by many workers (Edmond and Redfeara !963; Hilton et al 1966; Taylor e t a l 1970; Lucovsky 1972; Moynihan e t a l 1975). ,Recently Moynihan et al (1975) have reported the IR spectra of glassy As~Sea doped with As203 in the region 250 to 4000 cm -1. Spectra of amorphous As (Lucovsky and Knight 1974; Greaves et al 1979), Se (I-iilton et al 1966; Savage and Nielsen 1964; Lueovsky et al 1967; Siemsen and Riccius 1969) and As-Se (Hilton e t a t 1966; Moyrtihan e ta[ 1975) have also been reported in the literature and absorption frequencies of amorphous As, Se, As-Se and As2Sez below 500 cm -1 are summarised in table 1. Molecular vibration calculations have been performed by Lucovsky (1972) and Lucovsky and Martin (1972) who find that the spectrum of As~Se a conform; to that of vibrating AsSea units. In the 200 to 1200 cm -1 region As2S% exhibits several peaks due to multiphonon absorption (Moynihan et al 1975). Some of the rather intense absorption bands appear to be due to the presence of As203 and SeOz impurities. A%Oa having a very high extinction coefficient (Hilton e ta l 1966; Moynihart et al 1975; Jerger and Sherwood 1964) gives rise to bands even at low concentrations of ~ 100 ppm (below the saturation solubility of As2Oz in A%Se3).

Various properties of binary As-Se glasses have been investigated in this labo- ratory (Mohan et al 1980; Rao and Mohan 1980) and the properties support a

* To whom all correspondence should be made.

83

84 R Mohan and K J Rao

Table 1. Literature data on absorption fr~luencie~ below 500 era -1 for amorphous As, So, As-Se and As2Se>

Absorption System frequencies Designation Reference

GITr I v i

As 130 vl 230 v2

Se 256 487

Va

V4

Lucovsky and Knight 1974; Greaves et al 1979

Hilton et al 1966; Savage and Nielsen 1964; Lucovsky et al 1967; Siemson and R.iecius 1969

196 v 5 217 v o

As-So 227 v 7 Hilton et al 1966 250 v s 270 v 0

AsaSe3 100 vx0 230 vlt 480 vl~

Hilton et al 1966; Moynihan et al 1975

chemically-ordered network model (Botts et al 1970 ; Lucovsky et al 1974, 1977). We therefore considered it interesting to examine the IR spectra of these glasses. In this paper we report IR absorption spectra of As-Se glasses in the region 250-1100 cm -x and discuss the spectra in the light of the chemically ordered net- work model.

2. Experimental

As-So glasses were prepared as described earlier (Mohan et a11980) from the elements (As and So) of 5N plrity (Koch-Light Company, U.K.). Thin sections of these glasses were prepared by grinding and polishing (using rouge powder) to less than 1 m:rt thickness (most of them were 0.5 to 0.8 mm thick). They were repeatedly washed with distilled water and dried. Spectra of these glasses, which were prepared under almost identical conditions, were obtained from a Perkin Elmer Model 580 IR sp~trometer. No special efforts were made to prevent oxygen contamination at the preparative stage except evacuation of the quartz tube previously flushed with nitrogen or argon, before seMing.

The spectra of six different As-So compositions are given together in figure 1. Since the thicknesses of the samples were not identical the peak absorbanees cannot be compared. Intensities of the shoulders in the spectra were determined approximately by decomposing the actual absorpt.;on peaks into required number of Gaussian peaks adopting a simple trial and error technique (but not by computer fitting). The actual values of intensities reported may therefore have large uncertainties but would suffice to examine trends in the related series of glasses. In order to check how the intensities vary with composition, the relative variations

Infrared absorption spectra of As-Se glasses 85

,(

1

2 . . . .

,/

I t 700 iO0 1100 1000 900 800 Wavenumber {cm -1)

�9 I I ~ " 500 400 300

Figure 1. IR absorption spectra of As-Se glasses. The numbnrs correspond to the following compositions : (1) As~oSeeo, (2) As~oSeso, (3) AssoS~To, (4) As4oScm, (5) AssoSe:o, and (6) A%oSe4o.

of intensities of various absorption peaks have been determined with respect to the fundamental absorption peak at 365 cm-L We have assumed that the 365cm -~ band is due to a fundamental although doubts have been expressed in the literature (Moynihan etal 1975) with regard to this assignment.

3. Results and discussion

Infrared bands of As-Se glasses are listed in table 2. The relative intensities of band~ with respect to 365 cm -1 band are plotted against the composition in figure 2. The 475, 485 and 495 cm -1 bands which show continuous variation of intensity with the p~rcentage As arise from a two-phonon absorption processes as suggested by the e~rlier workers (Moynihan et al 1975). Since As and Se have almost the sam~ misses, it should be expected that two close lying absorption frequencies dae to As-As and Se-Se vibrations are present in the spectra. In Se-rich glasses the absorption band in this region may be due to the contributions cf 487 cm -1 absorption of Se (Hilton etal 1966; Savage and Nielsen 1964; Lucovsky etal 1967; Siemten and Riccius 1969). In the As-rich compositions it is possible that the intensity of the absorption is due to a two-phonon process involving the 230 cm -x absorption band as seen in amorphous As (Lucovsky and Knight 1974; Greaves et al 1979). It may be noted that IR absorption of pure As and Se are of dynamical origin, since the static lattices do not permit any dipole moment changes (Greaves etal 1979).

The 635, 790 and 1050era -x absorption bands have been attributed by the previous workars (Moynihan etal 1975) to the presence of two different oxide

86 R Mohan and K J Rao

r

g ro

o o o §

q- :#

r

r

11

z II

II

Infrared absorption spectra of As-Se glasses 87

o 1.7 ~L

.I0

o t , 5

u

~ 1,3

c 0,9 0

o 0.5

0.3

-~) cm -1

�9 380 v 475

�9 485

o 495 ,d

/

(

(

(

)

0.1

I I I , I t 1 10 20 30 z,0 50 60

% A s

Figure 2. Composition variation of relative absorbancos (relative to 365em -t absorption l~ak).

(impurity) structures. The 635 and 790cm -1 absorption peaks increase in intensity with increase in As content which is quite reasonable. But the steep decrease in intensity of oxide I~ structure below 30~o of As is interesting. It is either due to the pronounced decrease in the solubility of oxide II in Se-rich glasses or due to a consequence of the greater solubility of oxide II structure in As-rich glasses. The 1050cm -1 absorption is relatively weak and it has been assigned to the oxide I form of A%O3 impurity (Moynihatx etal 1975). Its intensity does not seem to vary sufficiently with composition. The origin of these oxides in A~-$e glasses is not clear; one possibility is a reaction involving silica reaction tubes (Moynihan et al 1975).

The band at 820 cm -1 also shows an increase in intensity with As concentration. This band could arise from multiphonon absorption such as (h + 3v.,). The intensity of the 380 cm -1 band seems to increase with Se concentration in these glasses ; its origin is not clear. One possible assignment of this would be (v3 + vt,) since such combinations are likely to occur in a chemically ordered network. The 555 cm -1 band seems to increase continuously with As content which could be due to (2v 1 + vg).

88 R Mokan and K J Rao

The infrared region investigated by us covers only regions of mul t iphonon absorptions in As-Se glasses. The mnlt iphonon absorption reported here can be roughly assigned employing literature data f rom table 1. The relative intensities of som~ of the absorpt ion bands such as the ones a t 815 era -1, 555 cm -1 and the 380 era -1 vary gradually with the concentration of As or Se. We feel that the lil~ely assignments of tlle3e bands are the combinat ions indicated in table 2. The chemically ordered network possibly permits such combinat ion tones on a network of atoms in whiclx there are quasi-independent vibrating units such as As-As and As-Se in a manner similar to that in glassy As-See (Lucovsky 1972).

Aeknowledgemczts

The authors are very thankful to Professor C N R Rao of Indian Inst i tute of Science for various suggestions and kind encouragement. One of tile authors (P,M) thanks the Depar tment of Science and Technology, Government of India, for financial support .

Rafereaeas

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