An Extended Interionic Potential for Alkali Halides

A ten‐parameter interionic potential is developed for the alkali halide crystals of rocksalt structure within the framework of the shell model for ionic crystals. This potential is an extension of the traditional Born‐Meyer form and takes into account the additional coupling between the repulsive overlap force and dispersion force. The analytical form of the proposed short‐range interaction potential is suggested by the coupled Drude oscillator model: Φii(r) = Bii exp (−r/σii) + Aii[(1/σii)2 − 2/σiir] Bii exp (−r/σii) − Cii/r6 for like ion pairs and Φij(r) = Bij exp (−r/σij) for unlike ion pairs. The four like ion parameters (Bii, σii) are determined by recourse to the Thomas‐Fermi electron gas model. The unlike ion repulsion parameters (Bij, σij) are fixed by the requirement of consistency with the equilibrium lattice constant as well as with the dielectric data as in the shell model. The van der Waals coefficients Cii and the coupling parameters Aii are treated to be crystal‐independent. A common set of these parameters is deduced by fitting to the elastic constants over the family of sixteen crystals. The resulting van der Waals coefficients are found to be generally large for both, cations and anions. The effect of input data on the constructed potentials is studied. Copyright © 1981 WILEY‐VCH Verlag GmbH & Co. KGaA