THE STRUCTURE OF AMORPHOUS AS-X-I(X-S,SE)

The short-range structure of amorphous As(40-x)X(60)I(x) (X: S, Se) wi th x = 0-25 has been investigated by Raman scattering and X-ray diffra ction. The Raman spectra for both systems are separated into three com ponents, comprising AsX(3/2) connecting units, discrete AsI3 units and ring- or chain-clusters of chalcogens. The A(1) mode energy for these units depends little on the I concentration, suggesting that mixed-an ion units, which have been observed in amorphous Ge-X-I systems, are n ot formed in the present As chalcohalide systems. Values for the struc tural parameters of these units, determined from a least-square analys is of the X-ray intensity function, have indicated that all of the I a toms added contribute to the formation of AsI3 units and that the exce ss X atoms participate in the formation of AsX(3/2) connecting units, particularly at higher x, i.e., the breakdown of AsX(3/2) units bridge d by corner-shared X atoms is promoted with increasing I content. This is supported by the fact that the first sharp diffraction peak in the structure factor of amorphous As(40)X(60) disappears in the I rich re gion.