POLARIZABILITY OF THE OXIDE ION IN CRYSTALLINE BEO

The polarisability of an oxide ion in the BeO lattice is obtained from all-electron coupled Hatree-Fock calculations on an embedded ion cluster. The ion is nearly isotropic, with calculated mean polarisability alphaBAR = 8.5 au and a small positive anisotropy. The result agrees to within 3% with the value of 8.77 au derived from an extended Lorentz-Lorenz treatment of the refractive index of solid BeO, thus supporting an ionic picture of the electronic structure of this material. The unstability of the Gladstone-Dale equation for determination of ionic and molecular polarisability is discussed.