Dynamical Simulations of Superionic Conductors

Abstract

Superionic conductors are ionic solids in which one ion species is disordered andhighly mobile. Molecular dynamics (MD) simulation gives a powerful way of studying models of superionic conductors. We explain the MD technique and describe how it is applied to ionic systems. We than show how simulation of fluorite superionic conductors has successfully reproduced a range of experimental measurements on quantities including the diffusion coefficient, the spatial distribution of disordered ions, the van Hove self-function and the dynamical structure factors. The simulations have been used to provide a unified interpretation of the observations. This shows that superionic conduction in fluorites occurs by the motion of vacancy and interstitial defects, both of which jump between regular lattice sites, the coherent quasielastic peak recently observed by neutron scattering is shown to arise from the motion of these defects. Ionic Solids at High Temperatures Downloaded from www.worldscientific.com by NANYANG TECHNOLOGICAL UNIVERSITY on 09/30/15. For personal use only. 170 IntroductionAll crystalline materials show some degree of disorder at high temperatures. This disorder is of two kinds: firstly, there are vibrational displacements of the ions from their regular sites, and secondly, there are point defects. The presence of point defects -vacancies and interstitials -is responsible for the diffusion of ions that is observed in all ionic crystals. Vibrational motions are usually treated in the harmonic approximation, which assumes that the displacements are small. As one goes to higher temperatures, this approximation becomes less and less valid and anharmonic effects become important: the lattice parameter depends on temperature, the constant-pressure and constant-volume specific heats differ, and the vibrational modes become damped. A more sophisticated theoretical description is then called for -an example is the self-consistent phonon approximation described by Ball elsewhere in this book. The point-defect disorder is also easiest to discuss when the temperature is not too high. One can then use energy-minimization techniques to determine the energies of formation and migration of the defects, and harmonic theory to calculate the associated entropies. These highly successful methods are discussed in the article by Harding. Again, at very high temperatures one encounters difficulties, especially if the defect concentration becomes high and the defects begin to interact with each other.

Shell-model molecular dynamics simulation of superionic conduction in CaF2

“…Our results confirm the conclusions of previous work (e.g. Gillan 1989) that the anion distribution function is strongly concentrated on the regular anion sites all the way up to the melting point, in spite of the very high mobility of the anions at this temperature (see below). However, the distribution spreads out into a wide region 1028 of the unit cell, reflecting the fact that the mobile ions take a range of paths as they jump between the regular sites.…”

“…The ions carry full ionic charges, and the short-range interaction between ionic shells is modelled by a Born-Mayer-Huggins potential (equation ( 2)). The rigid-ion model of CaF, with which we shall make comparisons is that developed by Dixon and Gillan (1980), and used in simulations by Gillan (1986Gillan ( , 1989, by Brass (1989), by Pontikis (1989, 1991) and by Lindan and Gillan (1991). We have made harmonic calculations to study how well the rigid-ion and shell-model potentials reproduce experimental results for the phonon dispersion relations (Elcombe and Pryor 1970).…”

Disordering phenomena in superionic conductors

“…It can be quickly appreciated that MD simulation is a very useful method for investigating superionic conductors (see, for example, [43]). The atom positions may be used to extract structural parameters (average positions and instantaneous correlations) and atom trajectories may be traced to investigate diffusion pathways and mechanisms.…”