High-pressure phases of ${\mathrm{PbF}}_2:$ A joint experimental and theoretical study

We report a joint experimental and theoretical study of the dynamical and structural properties of lead fluoride $\left({\mathrm{PbF}}_2\right).\mathrm{}$ Specifically, we have performed detailed measurements of the room-temperature Raman-active phonon modes to 310 kbar. We verify the transition at about 4 kbar from the cubic (β) to the orthorhombic (α) phase and the recovery of the α phase at ambient conditions. At approximately 147 kbar, we observe a transformation from the α phase to a modification that we label γ. The data indicate that the γ phase is structurally similar to the orthorhombic α phase. Moreover, we have carried out first-principles calculations which support the existence of a transition in this pressure range, although the crystal structure of the new phase remains undetermined. We have also calculated the equations of state for the two low-pressure phases, β and α. In the case of the α phase, we have made a complete structural determination by calculating the pressure-dependent values of all six internal structural parameters as well as the axial ratios, $c/a$ and $b/a$, for pressures ranging from zero up to 2 Mbar. Using the calculated α-phase equation of state, we also extract Grüneisen parameters from the data for several optical phonon modes.