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) with x = 0-25 has been investigated by Raman scattering and X-ray diffraction. The Raman spectra for both systems are separated into three components, 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-anion units, which have been observed in amorphous Ge-X-I systems, are not formed in the present As chalcohalide systems. Values for the structural parameters of these units, determined from a least-square analysis of the X-ray intensity function, have indicated that all of the I atoms added contribute to the formation of AsI3 units and that the excess 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 bridged 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 region.