NEUTRON POWDER DIFFRACTION STUDY OF THE FAST-ION TRANSITION AND SPECIFIC-HEAT ANOMALY IN BETA-LEAD FLUORIDE

A powder sample of beta-PbF2 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, a0(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 a0(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 heat, C(p), from harmonic and anharmonic lattice vibrations, Frenkel disorder and lattice dilation; a broad peak is obtained centred on T(c), which is in good agreement with the directly determined experimental data.