A Born Model Calculation of the Energies of Schottky Pair Formation and Cation Migration in Potassium Azide

Calculations are presented for the energy of Schottky pair formation and for the activation energy of cation vacancy migration in potassium azide (KN3). Parameters for the repulsive overlap energy of the azide molecule ion have been derived from a Born model treatment of the lattice interactions with the cohesive energy and its first and second derivatives used in the fitting procedure. Three different representations of the repulsive potential are considered. Point defect calculations are performed first in a rigid lattice approximation and then with the effects of polarization and relaxation included. The polarization energy is calculated by a generalized Mott and Littleton method, which accounts for the anisotropy of the lattice, in both zeroth and first order approximations. Relaxations include displacements and rotations of nearest neighbors of the defect configurations. The sensitivity of the results t o the choice of a repulsive potential are discussed. L'energie de formation d'une paire de Schottky et l'energie d'activation pour la migration d'une lacune de cation ont ete calculees pour le nitrure de potassium (KN3). Des parametres pour l'energie de repulsion venant du chevauchement de l'ion-molecule nitrure ont At obtenus en traitant les interactions du reseau a l'aide d'un modiele de Born, et l'energie de cohesion ainsi que ses derivees premiere et seconde ont ete utilisees pour obtener la meilleure correlation. Trois differentes representations du potentiel de repulsion sont considerees. Les calculs sur les defauts ponctuels sont faits d'abord dans I'approximation du reseau rigide, ensuite en tenant compte des effects de polarisation et de relaxation. L'energie de polarisation est calculee par une methode de Mott et Littleton generalisee, qui tient compte de I'anisotropie du reseau dans I'approximation d'ordre zero et dans celle du premier ordre. Les phenomienes de relaxation comprennent les deplacements et les rotations des plus proches voisins du complexe du defaut. La depenidance des resultats au choix des potentiels de repulsion est discutee.

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