First-principles calculations of defect-induced lattice relaxation in ionic systems

Local-density-approximation methods are used to investigate defect-induced lattice relaxation in LiCl:Cu⁺ and for the F center in MgO. Calculations are carried out on large finite clusters of atoms using a linear combination of atomic orbitals (LCAO)--type basis set, and defect-related properties are deduced from a comparison of results for related pure and defect clusters. The outer atomic shells of these clusters are allowed to relax, to respond to effects of truncating the clusters at finite sizes. We show that, with this relaxation, the calculated electronic and structural properties of the pure clusters are good approximations to the corresponding bulk properties. Using the finite-cluster procedure, we find essentially no relaxation of the defect near-neighbor ions in LiCl:Cu⁺, but an inward relaxation of 1.3% of the near-neighbor ions around the F center in MgO.