Fast-ion conduction in fluoriteCaF2studied by equilibrium and nonequilibrium molecular dynamics

We present the results of a direct determination of the superionic conductivity of ${\mathrm{CaF}}_2$, by using equilibrium and nonequilibrium molecular dynamics at constant temperature. The conductivity values obtained by both techniques are comparable to and in good agreement with the experimental data. Our results suggest that the failure of similar previous attempts should be attributed to an inefficient sampling of the phase space in computing the expectation value of the conductivity. Moreover, the nonequilibrium calculations show that the response of the studied system is linear over a surprisingly wide interval of external forces up to a value ${10}^7$ eV/cm. By computing independently the anionic conductivity and diffusivity, we found a Haven ratio H≊0.35, a value consistent with previous work, which established that superionic diffusion and conductivity in fluorites result from numerous correlated hopping processes involving many anions.