Ionic conductivity and activation volumes in the lead halides Pb${\mathrm{Cl}}_2$, Pb${\mathrm{Br}}_2$, and Pb${\mathrm{I}}_2$

The effect of hydrostatic pressure on the ionic conductivities of Pb${\mathrm{Cl}}_2$, Pb${\mathrm{Br}}_2$, and Pb${\mathrm{I}}_2$ has been investigated. Activation volumes for vacancy motion in Pb${\mathrm{Cl}}_2$ and Pb${\mathrm{Br}}_2$ are found to be 4.7 and 4.4 ${\mathrm{cm}}^3$/mole, respectively. Defect-formation volumes are found to be large in Pb${\mathrm{Cl}}_2$ and Pb${\mathrm{Br}}_2$, and consistent with the dominance of Schottky defects in those materials. The activation volumes for Pb${\mathrm{I}}_2$ are all found to be small and consistent with the dominance of Frenkel defects in both the low-pressure layered hexagonal phase and in the high-pressure phase. Transition temperatures and pressures are given for the high-pressure phase transition in Pb${\mathrm{I}}_2$ and are found to agree with values reported elsewhere.