Electronic Structure and Thermodynamics of Scandium Monosulfide

摘要

The self-consistent, nonrelativistic band structure calculations, employing the KKR Green's function method, were performed for stoichiometric Sc sub 1 sub 00 S and a hypothetical ordered defect compound of composition Sc sub 3 S sub 4 . When metal atom vacancies are introduced, (a) the wavefunctions associated with the sulfur atom near a vacancy are redistributed, inducing nonbonding p-states and (b) the covalency of the metal-nonmetal bonding interaction is enhanced, resulting in an unchanged scandium valency. Analysis of the charge density suggests that the primary metal-metal bonding interaction is directed through the octahedral faces of the sulfur polyhedron, while a secondary interaction is directed through the edges. Vacancy formation and stabilization are thought to be energetically driven but the exact nature has not been determined. The high temperature vaporization of a scandium-rich monosulfide was investigated by the mass loss Knudsen effusion method. The homogeneity range was found to extend into the metal-rich region, as far as Sc sub 1 sub 14 S, as determined by the rapid decrease in the Sc partial pressure at the start of a run. The activity of ScS in Sc sub 0 sub 8065 S(s) was determined to be 0.48 +- 0.05 and is constant at 1950 to 2100 exp 0 K. The energy required to create approximately 20% scandium vacancies in ScS is 25.0 +- 1.8 kcal mol exp -1 at 298 exp 0 K. A third-law temperature independent enthalpy of atomization at 298 exp 0 K of ScS is 255.7 +- 2.6 kcal mol exp -1 and the enthalpy of formation at 298 exp 0 K of ScS is -98.7 +- 2.8 kcal mol exp -1 . (ERA citation 09:044037)