Collective dynamics in superionic CaF2. I. Simulation compared with neutron-scattering experiment

Recent experiments on superionic CaF 2 and other fluorites using coherent inelastic neutron scattering have revealed a strong quasi-elastic peak in the cross section. This is associated with collective fluctuations which decay on a time scale of order 1 ps. The author describes here an extensive series of molecular dynamics simulations on CaF 2 over a wide range of temperatures spanning the transition to the superionic state. The simulations have been used to calculate the dynamical structure factors, which are directly related to the coherent inelastic cross section. He shows the existence of a quasi-elastic peak in the structure factors and demonstrates that the dependence on temperature and wave-vector of the intensity and frequency width of this peak are in reasonable agreement with the experimental results. He concludes that the collective motions responsible for the peak are essentially the same in the simulated system and in the real material.