Neutron powder diffusion study of the fast-ion transition and specific heat anomaly in beta -lead fluoride

A powder sample of beta -PbF 2 has been studied by neutron diffraction using the high resolution powder diffractometer at the Rutherford-Appleton Laboratory. Rietveld profile refinements have been performed on data collected at roughly 100 temperatures from ambient to well above the transition to the fast-ion phase, which occurs at T c approximately 710 K. The lattice parameter, a 0 (T), the concentration of Frenkel defects, and thermal parameters for both the fluorine and the lead ions have been determined as a function of temperature. Precise values of a 0 (T) enable the linear expansivity to be calculated, for which there is a pronounced peak centred on T c . It is possible to calculate contributions to the specific heats, C p , from harmonic and anharmonic lattice vibrations, Frenkel disorder an lattice dilation; a broad peak is obtained centred on T c , which is in good agreement with the directly determined experimental data.