COMPRESSION OF 7 VACANCY-ORDERED PHASES OF SCXS TO 50 GPA

The ScxS system. (0.8<x less than or equal to 1.0) has a lattice based on the cubic rocksalt structure with vacancies only in the metal subl attice. Our compression experiments reveal that the zero-pressure isot hermal bulk modulus increases with increasing vacancy content from abo ut 100 Cpa to about 150 GPa. This is due to enhanced Sc 3d-S 2p orbita l interaction and resulting stronger Sc-S bonds in the samples with hi gh vacancy content. The fourth-order Birch-Murnaghan equations of stat e obtained for each composition converge above 40 GPa, and samples wit h a larger Sc deficiency decompressed from SQ GPa to below 20 GPa show a slight increase in volume. We propose the formation of S-2(-2) dumb bell pairs (similar to those in transition-metal disulfides) from the dimerization of S-2 ions above 20 GPa in the compositions with more va cancies as an explanation of these observations.