Density-functional theory for the solid alkali cyanides

Abstract

Structural properties of the solid alkali cyanides were studied with a recently reported ab initio theory based on the Gordon–Kim electron gas model. The low temperature crystal structures of LiCN, NaCN, KCN, RbCN, and CsCN were examined. The average error in lattice parameters is 2.6% and in lattice dissociation energy 1.7%. For the NaCN and KCN systems, the energy differences between the ferroelectric and antiferroelectric structures were examined and the antiferroelectric structure was found to be more stable at zero temperature by about 0.5–0.8 kcal/mol. The high pressure phases of KCN were studied and the phase diagram partially mapped out at 0 K. The crystal structure of RbCN at low temperatures was studied, and the orthorhombic Pmmn structure found to be more stable than the monoclinic Aa structure. The heat of formation of RbCN was calculated at 0 K to be about 31.4±7 kcal/mol. The cyanide electron density was found to shift from the C atom towards the N atom in all crystals studied, when the free space CN− ion is incorporated into a crystal.