The calculation of dipole moments associated with defects in ionic crystals

Abstract

Computer simulation methods have been used to calculate the dipole moments of first- second- and third-neighbour complexes of cation vacancies with the divalent impurity ions Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Cd²⁺ and Pb²⁺ in NaCl, KCl and KBr, and also for Zn²⁺, Mn²⁺, Cd²⁺, Sr²⁺ and Pb²⁺ in AgCl and AgBr. The results show, when the relaxation of the lattice ions is accounted for, that the effective dipole moment is significantly less than the value which would be given by a point-ion calculation for a perfect lattice. The decrease in dipole moment due to lattice relaxation is about 30% for the alkali halides and about 35% for the silver halides. For the nearest-neighbour and next-nearest-neighbour complexes a small decrease in the dipole moment is found as the radius of the impurity ion increases.