Electron density in NaF and KCl crystals in the self-consistent local-density-functional approximation (LDA)

Abstract

The electron density ρ(r) in NaF and KCl crystals - composed of 'isoelectronic' alkali and halogen ions - has been computed self-consistently by the method of augmented spherical waves in the local-density- functional approximation (LDA). Calculations were also carded out of the heat of formation of the salts from the alkali metal and the halogen molecule from the pertinent total energies as well as the diamagnetic susceptibility of the solid. The structure factors were then calculated for the low-order 'difference' and 'sum' reflections, the former of which are particularly sensitive to ionic deformations. The computed structure factors are compared with the structure factors obtained from experiment. The latter are also compared with the structure factors calculated from the Hartree-Fock free-ion atomic factors. The physical conclusion of the paper is that in the NaF and KC1 crystals the anion 'contracts' and the cation 'expands' in passing from the free-ion state to the state of ion in crystal. Independent evidence of this type of effect is provided by the Tessman, Kahn & Shockley [Phys. Rev. (1953), 92, 890-895] 'ionic polarizabilities' and by the Fumi & Tosi [J. Phys. Chem. Solids (1964), 25, 31-43; Tosi & Fumi (1964). J. Phys. Chem. Solids, 25, 45-52] 'crystal ionic radii'.