Effect of hydrostatic pressure on the crystal structure and superionic behavior of lead (II) fluoride

The structural behavior of lead (II) fluoride has been investigated at pressures up to similar to 0.9 Cpa and temperatures up to similar to 1000 K using the powder neutron diffraction technique. The observed phase transitions between fluorite and cotunnite structured polymorphs are consistent with the published p-T phase diagram for PbF2. An irreversible pressure induced transition from the fluorite structured beta phase to the cotunnite structured alpha phase occurs at p = 0.61(5) GPa with an 8.5(1)% decrease in volume. On heating, the irreversible alpha-->beta transition at ambient pressure occurs at a temperature of 601(4) K, prior to the superionic beta-->beta* transition at similar to 710 K. Increasing pressure has no measurable effect on the concentration of the Frenkel defects which characterize the intrinsic disorder within the superionic beta* phase. In contrast to previous studies, we find no evidence for significant lattice disorder within the denser a phase at elevated temperatures and, on increasing temperature at pressures in excess of similar to 0.5 Cpa, the superionic transition occurs abruptly at the alpha-->beta* transition. [S0163-1829(98)06745-9].