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Superionic conductors are characterized by the fact that one type of ion, out of the two or more ionic species which the material contains, is mobile while the system as a whole behaves as a solid. CaF2 is one such material. The talk will be concerned with a computer simulation of CaF2 to study the nature of certain structural and dynamical correlations in that material. Using well known methods for treating ionic systems [1], the calculation was made for a temperature of 1 590 K and density of 2.525 g/cc, using the potential functions given by Kim and Gordon [2]. The calculation was initiated with the 108 Ca++'s and the 216 F-'s occupying the sites of the fluorite structure. The results show [3] quite unambiguously that under the above conditions the Ca++ ions perform thermal motions around sites of an f.c.c. lattice but the F- ions have no permanent mean position and have a liquid like constant of self-diffusion, namely, 2.6 x 10-5 cm2. s-1. From the pair correlation between the octahedral sites of the Ca++ f.c.c. lattice and the F-'s, it was inferred that the F-'s make considerable use of the octahedral sites during their diffusive motion. In later work [4], it has been shown that the Chudley-Elliott jump diffusion model [5], suitably modified, is able to account for the long time behaviour of the function < exp(ik.u(t)) >, u(t) is the displacement of an F- ion. Probabilities of jumps in the 100 and 110 directions are 0.80 and 0.20, respectively, and the residence time 6 x 10-l2 s. It is possible, by direct measurement, to determine the flight time as well ; this work is in progress. There are indications that the F- diffusion is very sensitively dependent on the density of the system. Also, the substitution of a Mg++ in place of one of the 108 Ca++ makes a large increase in the constant of self-diffusion. These studies are in progress

DIFFUSIVE MOTION OF IONS IN SUPERIONIC CONDUCTORS : A COMPUTER SIMULATION STUDY OF CaF2

DIFFUSIVE MOTION OF IONS IN SUPERIONIC CONDUCTORS : A COMPUTER SIMULATION STUDY OF CaF₂