HIGH-PRESSURE PHASES OF PBF2 - A JOINT EXPERIMENTAL AND THEORETICAL-STUDY

We report a joint experimental and theoretical study of the dynamical and structural properties of lead fluoride (PbF2). Specifically, we ha ve performed detailed measurements of the room-temperature Raman-activ e phonon modes to 310 kbar. We verify the transition at about 4 kbar f rom the cubic (beta) to the orthorhombic (alpha) phase and the recover y of the alpha phase at ambient conditions. At approximately 147 kbar, we observe a transformation from the alpha phase to a modification th at we label gamma. The data indicate that the gamma phase is structura lly similar to the orthorhombic alpha phase. Moreover, we have carried out first-principles calculations which support the existence of a tr ansition 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, beta and alpha. In the case of the alpha phase, we have made a complete structural determination by calculating the pressure-dependent values of all six internal struc tural parameters as well as the axial ratios, c/a and b/a, for pressur es ranging from zero up to 2 Mbar. Using the calculated alpha-phase eq uation of state, we also extract Gruneisen parameters from the data fo r several optical phonon modes.