ROLE OF INDUCED QUADRUPOLES IN THE SIMULATION OF INTRINSIC POINT-DEFECTS IN AGCL AND NACL

The paper presents a detailed formulation of the energetics of point defects in ionic crystals that takes into account the quadrupolar deformation of the ions surrounding the defect within a polarizable-point-ion picture. The dipolar polarizabilities are modeled to represent correctly the static dielectric response of the crystal. Consideration of the axial symmetry of the electric-field-gradient tensor, the residual site symmetries in the defect crystal, and the symmetry of the quadrupolar-polarizability tensor leads to a cylindrical symmetry for the induced quadrupoles for both the vacancy and interstitial type of defects. These quadrupoles give rise to an additional polarization contribution to the defect formation besides affecting the dipolar energy. Computer codes have been developed based on this model and results are presented for vacancies and interstitials in AgCl and NaCl and discussed in detail. The calculations use a two-body central-force potential with well-represented van der Waals interactions. It is found that there is a substantial contribution from the quadrupoles to the defect enthalpy. The investigations reveal the comparative importance of accurate modeling of the polarization features vis a vis subtle refinements in the interionic potential.