Electronic charge distribution and the degree of ionicity in crystalline NaF, NaCl, SrF2, and SrCl2as found by the local-orbitals Hartree-Fock method

Abstract

We present the local orbitals for NaF, Sr${\mathrm{F}}_{2}$, and Sr${\mathrm{Cl}}_{2}$. We use these results, along with previously reported local orbitals for NaCl, to calculate the electron density. We find that NaF has 65% more electron density at the density minimum located along the nearest-neighbor axis than NaCl. In addition, Sr${\mathrm{F}}_{2}$ has 79% more density at the minimum than Sr${\mathrm{Cl}}_{2}$. The latter result is in agreement with recently reported experimental evidence indicating more dynamical charge overlap in Sr${\mathrm{F}}_{2}$ than Sr${\mathrm{Cl}}_{2}$. We find the NaCl electron density agrees extremely well with experimental results obtained from x-ray scattering measurements. The implications of the above result are discussed in terms of relative ionicity. Dividing the charge density into ions along the surfaces of density minima (i.e., along the ionic radii) yields a net charge per ion which is inconsistent with the actual Coulomb potential in the solids. The reasons for this are discussed.