Direct Computation of the Superionic Conductivity in CaF ₂ by Constant-Temperature Equilibrium and Nonequilibrium Molecular Dynamics

We present the results of the first direct calculation of the superionic conductivity of CaF 2 , using constant temperature equilibrium and nonequilibrium molecular dynamics in the linear response regime based on a rigid-ion potential. We found σ = (5.1 ± 0.2) mho/cm at T = 1700 K ≈ T melting , in good agreement with available experimental data and that the response of the system remains linear under external forces up to F = 10 7 eV/cm. The Haven's ratio, resulting from independent computations of conductivity and diffusivity at this temperature, H = 0.34 ± 0.05, suggests that the anionic sublattice is still crystalline and that mass transport and ionic conduction are not due to a simple vacancy or interstitialcy mechanism for which higher H values are expected.