Computer simulation of superionic CaF2

Computer simulation of defect motion in model normal and ‘fast ion’ conductors II. SrCl 2

The molecular dynamics method has been applied to the simulation of the fast-ion conductor SrCl 2 . Calculations have been done at increasing temperatures from low temperatures through the normal and high-conductivity regions into the melt. Because of the periodic boundary conditions imposed and the size of the simulation sample (324 ions), the minimum defect concentration of anion Frenkel defects that can exist in the steady state is 1/216 defects per anion lattice site. At low temperatures Frenkel defects introduced into the initial configuration are eliminated, but at higher temperatures Frenkel defects form spontaneously on the anion sublattice. Anions move primarily in <1 0 0> directions by the vacancy mechanism, although more rarely <1 1 0> jumps also occur. At intermediate temperatures interstitials were observed to make non-collinear interstitialcy jumps, but at higher temperatures the motion of interstitials makes little contribution to mass transport. The vacancies appear to be very mobile and interstitials have little or no chance to move before being annihilated by a wandering vacancy. The picture is thus one of a highly dynamic system in which individual defects do not survive for long. However, it must be recognized that the rigid-ion potential used has its limitations and may overemphasize to some extent the vacancy mobility. There is so much motion on the anion sublattice at temperatures close to melting that various indicators such as the anion–anion radial distribution function, the individual ion displacements in a fixed time, and the anion self-correlation function are quite similar below and above the melting temperature. In this sense therefore, the applied description of sublattice melting does not seem to be inappropriate. However, periodic structure is still recognizable on the anion sublattice until it also disappears on the cation sublattice. The small sample molecular dynamics simulation does not predict defect clustering; this is probably because of limitations of the method and so our results are not necessarily in conflict with recent quasi-elastic neutron scattering results.

Computer simulation of defect motion in model normal and ‘fast-ion’ conductors I. NaCl and AgCl

The molecular dynamics simulation technique has been used to study Frenkel defects in crystalline NaCl and AgCl. Partial radial distribution functions, mean square displacements and the velocity autocorrelation function with its Fourier transform have been calculated for cation and anion species. Because the minimum Frenkel defect concentration that can be introduced into the molecular dynamics sample is 1/108, the initial configurations represent a non-equilibrium state and their evolution in time provides an opportunity to study mechanisms of defect migration that are involved in the recombination of Frenkel defects. Direct interstitial jumps, collinear interstitialcy migration, and [110] vacancy jumps have all been observed. The time dependence of the mean square displacements, and particularly the ratio of the square of second moment to the fourth moment of the self-correlation function, are sensitive indicators of the occurrence of defect jumps. Plots of the individual ion displacements in various time windows reveal which ions are in motion, while the projections of the cation and anion sublattices in the simulation cell give a detailed picture of the motion of the defects.

A model for the onset of fast-ion conduction in fluorites

The quasielastic neutron scattering from fast-ion conductors with the fluorite structure has been interpreted alternatively as being due to either clusters of defects or to the motion of defects and their associated strain field. In this paper, we examine the hypothesis that the sharp rise in ionic conductivity that occurs in fluorites near the transition temperature ( T c ) from normal to superionic conductivity, is associated with defect clustering. By using lattice simulation techniques it is shown that a wide variety of clusters are stable in SrCl 2 , CaF 2 , SrF 2 , BaF 2 and β-PbF 2 . A cluster consisting of two vacancies, two relaxed lattice ions and three interstitials (which we shall designate 2| 2| 3) is particularly stable. It is shown that the formation of 2| 2| 3 clusters results in an increase in the concentration of vacancies and that this increase can account quantita­tively for the rise in conductivity near T c . Our concern is thus with the possible connection of the onset of superionic conduction with defect clustering and we do not attempt, in this paper, any interpretation of the quasielastic neutron scattering.