Intercluster and intracluster interactions in Ge1−xSnxSe2

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Journal of Non-Crystalline Solids 114 (1989) 64-66 North-Holland

INTERCLUSTER AND INTRACLUSTER INTERACTIONS IN Gel.xSnxSe2 Lo E. MCNEIL and M. J. PETERS Dept. of Physics and Astronomy, Univ. of North Carolina, Chapel Hill, NC 27599-3255 USA MSssbauer and Raman spectroscopy and X-ray diffraction have been applied to Ge 1.xsnxse2 crystals and glasses. In the ternary crystals Sn is found to substitute for Ge. Applied pressure causes larger changes in the asymmetric Sn sites in the glass than in the symmetric sites. Ion bombardment of the crystal drives the structure toward to that of the melt-quenched glass. 1. INTRODUCTION In the chalcogenide compound GeSe 2 one can

ternary crystals were found to be virtually identical to that of GeSe 2 save for some addi-

study interactions for a variety of length scales and degrees of order by comparing the glassy and crys'talline forms, as the local bonding is similar in the two 1,2. When some of the Ge in the compound is replaced by Sn, the Sn atoms substitute for Ge atoms 3, preferentially occupying the cluster-edge sites in the glass4. This study uses several techniques to examine two aspects of the ternary compounds: the location of the Sn atoms in crystalline forms of Gel.xSnxSe2; and the interaction of the clusters in the glass.

tional weak lines which could be indexed to SnSe 2. The intensity ratios of the lines indi-

2. CRYSTALS To study the location of Sn atoms in crystalline G e l . x S n x S e 2, crystals with x = 0, 0.2 and 0.3 were grown using the Bridgman method. The resulting boules consisted of grains ranging in dimension from a few mm to 1 cm or more. The grains were easily cleaved parallel to the basal plane and were yellowish in color. X-ray diffraction measurements of powders ground from these crystals were made using the e-2e configuration, and the pattern for GeSe 2 matched the calculated pattern tensity tion of allel to The

well save for some differences in inratios due to the preferred orientabasal planes of the powder grains parthe sample mount. X-ray diffraction patterns of the

0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

cated that the fraction of the total Sn concentration which was present in the form of SnSe 2 was larger in the Geo.TSno.3Se 2 crystal than in the Geo.8Sno.2Se 2, but in neither case was the phase separation complete. This suggests that the solid solubility of Sn in GeSe 2 is somewhat less than 20%. There was no indication of preferential occupation of a subset of the four crystallographically distinct Ge sites by Sn. It appears therefore that Sn atoms primarily substitute randomly on the Ge sites without disturbing the GeSe 2 crystal structure or producing a significant change in the lattice parameters. A room temperature, atmospheric pressure 119Sn MSssbauer spectrum of the Geo.8Sno.2Se 2 crystal shows a single line with isomer shift IS = 1.396 + 0.068 mm/s (relative to Cal19SnO3), characteristic of Sn in a tetrahedral site. In a glass of the same composition IS = 1.471 + 0.032 mm/s. In both cases Sn atoms predominantly occupy substitutional Ge sites. Room temperature Raman spectra of the three crystals are shown in Figure 1 in the region of the Alg tetrahedral breathing mode at 210 cm -1. The addition of Sn induces two

L.E. McNeil, M.J. Peters /Intercluster and intracluster interactions in Get_=Sn=Se2

new Raman lines at 189 and 199cm -1 as indicated in the figure. In both spectra the integrated intensity of each of these lines is approximately 5% of that of the 210 cm -1 line. In neither does the dominant5 185 cm -1 line of SnSe 2 appear, nor is there any significant change in the low energy (< 100 cm -1) layer-layer modes.

3. GLASS--HIGH PRESSURE Hydrostatic high pressure 119Sn M6ssbauer measurements of glassy Geo.5Sno.5Se 2 were made using a diamond anvil cell. The glass is made up of two structural components 2,3, and thus the spectra show two quadrupole-split lines, corresponding to two Sn sites: Sn atoms in ethane-type sites, Ge2(Se1/2)6; and in tetrahedral sites, Ge(Sel/2) 4 within a fragment of the crystalline layer 3.

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FIGURE 1 Raman spectra of Gel.xSnxSe 2 crystals• The appearance of the new modes at energies close to that of the breathing mode of the Ge(Sel/2)4 tetrahedron upon the addition of Sn, together with the X-ray and M6ssbauer evidence, leads tc the conclusion that these modes represent Alg vibrations of Sn substituted for Ge on a lattice site. Calculations 4 indicate that the replacement of Ge by Sn leads to a softening of this mode by approximately 25 cm q, as is observed here. The two Sn-centered modes may represent Sn occupying sites at the center of the corner- and edge-sharing tetrahedra, although the energy separation of the two is larger than for the Ge-centered tetrahedra (5 cm-1).

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FIGURE 2 Pressure dependence of effective thickness (ET, arb. units), quadrupole splitting (QS, mm/s), and isomer shift (IS, mm/s relative to Ca 119Sn 03) in glassy Geo.5Sno.sSe 2 (atm. pressure points taken at 77 K). As shown in Fig. 2, all changes with pressure are much larger in the ethane site. The changes in isomer shift show similar behav-

L.E. McNeil, M.J. Peters/ Intercluster and lntracluster interactions in Ge,_=Sn=Se2

66

ior in the two sites, but these changes are irreversible in the ethane site. The changes in quadrupole splitting differ in sign on the two sites, with the large decrease in the splitting on the ethane site indicating that the charge surrounding the Sn there has become more symmetrical, while the tetrahedral site develops more asymmetry. The effective thickness is proportional to the recoilless fraction, and for the ethane site this increases rapidly with pressure. This suggests that the chains undergo a structural alteration that leads to tighter bonding of the Sn atoms. The two structural components of the glass clearly respond differently to pressures below 36 kbar, with the ethane chains being more sensitive than the layer fragments. At higher pressures the structure is transformed to a more fully three-dimensional form, which is more rigid. This transformation is at least partially irreversible, indicating that new bonds have been formed.

4. ION BOMBARDMENT-CRYSTAL TO GLASS To examine the transition from the crystalline to the glassy state, the crystals were bombarded with 250 keV Ar + to a dose of 1015 ions/cm 2. As can be seen in Fig. 3, the Raman spectrum of the bombarded region of the crystal (ion penetration depth = 170 nm) begins to resemble that of the glass as the structure becomes more disordered, with a peak at 198 cm -1 (the Alg mode of the glass) appearing. 5. CONCLUSIONS These measurements have shown that Sn occupies a tetrahedral layer-like site in both the crystalline and glassy forms. The ethanelike chains which also occur in the glass tend to cross-link under pressure to a more threedimensional structure. ACKNOWLEDGEM ENTS The authors would like to thank Dr. C.S. Smith for invaluable assistance in analyzing the X-ray data, and Dr. K. Whatley for providing low-temperature M6ssbauer data. Support for this work was provided by the U.S. ARO under grant no. DAAL03-86-K-0069, and by the Research Corp. REFERENCES 1. G. Dittmar and H. Sch&fer, Acta Cryst..B 32 (1976) 2726.

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2. J.M. Griffiths, G.P. Espinosa, J.P. Remeika, and J.C. Phillips, Phys. Rev. B25 (1982) 1272. . P. Boolchand, J. Grothaus, W.J. Bresser and P. Suranyi, Phys. Rev. B25 (1982) 2975.

FIGURE 3 Raman spectra of ion-bombarded crystalline GeSe 2 and melt-quenched glass.

. J.M. Mikrut and L.E. McNeil, J. Non-Cryst Sol. 109 (1989) 237. . A.J. Smith, P.E. Meek and W.Y. Liang, J. Phys. C10 (1977) 1321.