Diffusion of F− ions in CaF2

Abstract

It is shown that in a recently reported model of the dynamics of CaF2, the decay with time of the Fourier transform, 〈exp(ik⋅u(t)) 〉, of u(t), the F− displacement, for various wave vectors k can only arise from diffusive motion occurring between the simple cubic anion sites of the fluorite structure. Most of the jumps (∼80%) occur between near neighbor sites (i.e., they are in the 100 direction). The motion of F− ions can be understood in terms of a spontaneous creation of anti-Frenkel pairs which diffuse and eventually get annihilated. At the temperature of this calculation (1590 K) the average number of such pairs at any instant is found to be 7.9 in an assembly of 216 anions. A detailed breakdown of the nature of various types of atomic jumps arising out of the dynamics of these pairs is also presented.