Coulomb Energy in Pseudobinary Alloys

A tight-binding theory is derived for Coulomb electron-electron interactions in tetrahedrally coordinated pseudobinary alloys of the form A0.5B0.5C Coulomb contributions to the mixing enthalpy are driven by the bond polarity mismatch, as distinct from the bond length mismatch customarily considered. The Coulomb energy shows a strong dependence on the alloy configuration, which arises from the configuration dependence of charge fluctuations. The mixing enthalpy is of order 0.10 eV, commensurate with that of the strain contribution customarily considered.