7 - Defects, Surfaces and Interfaces

Abstract

This chapter discusses the methods of simulating defects and interfaces using classical models. The chapter focuses on static and quasi-harmonic calculations. Most calculations of the internal energy of a defect process use classical potential models. Two kinds of approaches are used to calculate defect processes: first, that based on the Mott–Littleton approximation and second, the supercell method. Calculations of the entropy of formation of defects are also performed. The calculation of defect entropies at constant volume is essentially a calculation of the effect of the defect on the lattice phonon spectrum. Again, this problem may be approached either by a supercell method or by a two-region approach. All these methods assume that a static lattice calculation has already been performed and a set of ion positions in the relaxed configuration is available. The calculation of interfaces uses the two-region strategy. Planes close to the interface are explicitly relaxed. Planes further away are held rigid, but the stack is allowed to move as a block; this permits surface dilatation. The two-dimensional Coulomb sums are then performed.