Polyanionic and octet phases in the K-Sb system. I. Crystalline intermetallic compounds

Abstract

The crystal structure, chemical bonding, and electronic properties of intermetallic compounds in the K-Sb system have been investigated using first-principle--local-density-functional calculations including generalized gradient corrections. It is shown that the chemical bonding obeys a generalized Zintl principle, i.e., a formally complete electron transfer from K to Sb. The stable crystal structure is determined for the stoichiometric octet compound ${\mathrm{K}}_{3}$Sb by the formation of an ionic lattice and at the equiatomic composition by the formation of covalently bonded ${\mathrm{Sb}}_{\ensuremath{\infty}}^{\ensuremath{-}}$ helices in close analogy to the isoelectronic chalcogen elements. At intermediate compositions the Sb atoms cluster together to form chainlike polyanion radicals, the electrons provided by the excess alkali metal serving to partially saturate the dangling bonds at the chain ends. It is demonstrated that density-functional theory describes the crystal structure of all compounds with high accuracy. The overbinding characteristic of the local-density approximation is most pronounced in the alkali-rich limit, but merely causes a scaling of all interatomic distances without distorting the structure. Gradient corrections substantially improve the prediction at large K content, but tend to overshoot in the Sb-rich range.