Relaxation and Activation Energies for an Interstitial Neutral Defect in an Alkali Halide Lattice. II

The distortion around and the relaxation energy for a Cl⁰ interstitial atom in a NaCl lattice has been obtained for various positions of the Cl⁰ by a method involving the exact calculation of electrostatic, polarization, dipole-dipole, and repulsive energies. The relaxation of from 14 to 23 neighboring ions has been taken into account, using up to nine independent parameters, in minimizing the energy relative to the perfect lattice. The following positions of the Cl⁰ were investigated: the cube center, the face center, and various positions along the cube and face diagonals. The cube-center and cube-diagonal positions were always found to be stable relative to the face-center and face-diagonal positions. There are two energy minima of almost the same value along the body diagonal, one in the center of the cube and one in which the Cl⁰ and Cl^- are located on either side of a normal Cl^- lattice position. These two minima are separated by a potential barrier of about 0.2 eV. The energy differences for the various configurations along the face diagonal are small with the face center the position of lowest energy. The face-center position is metastable with respect to the body-center position by about 0.03 eV. The energy difference between the body-center and face-center positions is 0.44 eV, giving an activation energy along this line of motion of 0.47 eV.