ELECTRONIC ORIGIN OF NONSTOICHIOMETRY IN EARLY-TRANSITION-METAL CHALCOGENIDES

Using cluster models and extended Huckel calculations on rocksalt supercells, we demonstrate that the transformation of square-planar S atoms to cis-divacant ''butterflies'' drives the ordering of Sc vacancies in to alternating (111) metal planes in Sc1-xS. Charge-iteration calculations show that metal vacancy formation arises from a combination of first-order shifts in the Sc d band with increasing nonstoichiometry and second-order mixing of the Sc 3d and S 3p bands. Such a mechanism explains why it is precisely for the early-transition-metal sulfides that this kind of nonstoichiometry is observed. Our extended Huckel results are corroborated by ideas from the method of moments and pair potential calculations.