Collective dynamics in super-ionic CaF2. II. Defect interpretation

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Abstract
For pt.I see ibid., vol.19, p.3391 (1986). The author continues a study of the quasi-elastic peak observed by coherent neutron scattering in super-ionic CaF2, and investigates here the connection between this peak and the motion of point defects. The work is based on molecular dynamics simulations which the author found in a previous paper to agree well with experiment. The point defects in the simulated system are examined using a technique due to Dixon and Gillan (1978), which the author has extended so that fluctuations in their density can be monitored for different wave-vectors. It is argued that the motion of the defects will govern the low-frequency fluctuations of the ion densities, and the author shows that the correctness of this idea can be explicitly tested by an examination of the cross-correlations between defect and ion densities. Numerical calculations demonstrate that the quasi-elastic peak does arise entirely from the motion of the defects. The collective fluctuations of the ions are influenced in an important way by the lattice distortion surrounding the defects, and the strong wave-vector dependence of the quasi-elastic intensity arises directly from this distortion.