CRYSTAL-GROWTH AND ATOMIC-STRUCTURE OF AS2(SE,S)3 COMPOUNDS

The experimental conditions requirod for the growth of crystals of arsenic chalcogenides have been investigated, as a preliminary to the preparation of crystals of the mixed compounds As2(Se, S)3. Crystals of these compounds As2(Se1−xSx)3 have been prepared for 0 < x < 0·5, the size of the crystals produced decreasing rapidly with sulphur content. The crystal structure and site occupation of these mixed compounds have been examined by X-ray and neutron diffraction methods. The mixed compounds have the same monoclinic structure as the parent materials, but with neither a random solid solution nor an As2Se2S superlattice. Instead it appears that the S-As-S (or Se-As-Se) chain structure is important in both aspects of our study. In the crystal growth this structure provides a growth spiral which favours long needle-shaped crystals. On the other hand the geometrical similarity between this chain and the cradle-shaped molecule of AsS also appears to influence the site occupation in the mixed compounds, in a way that is consistent with the tendency towards segregation of AsS. © 1979 Taylor & Francis Ltd.