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 {~}0.9 GPa and temperatures up 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 {β} phase to the cotunnite structured {α} phase occurs at p=0.61(5)GPa with an 8.5(1)% decrease in volume. On heating, the irreversible →α{β} transition at ambient pressure occurs at a temperature of 601(4) K, prior to the superionic →β{{{β}}}*transition at ~710 K. Increasing pressure has no measurable effect on the concentration of the Frenkel defects which characterize the intrinsic disorder within the superionic{{{β}}}*$ phase. In contrast to previous studies, we find no evidence for significant lattice disorder within the denser {α} phase at elevated temperatures and, on increasing temperature at pressures in excess of {~}0.5 GPa, the superionic transition occurs abruptly at the →α{{{β}}}*$ transition.