Theoretical Study of the High-Pressure Isosymmetric Phase Transition in Lead Fluoride, PbF2

Under compression many AX2 compounds including the alkaline earth fluorides follow a canonical transition sequence from the fluorite-type (Coordination Number, CN = 8) to the cotunnite-type (CN = 9) to the Ni2In-type phase (CN = 11). Lead fluoride is instead reported to undergo an unusual isosymmetric phase transition from the cotunnite phase to a "Co2Si-like" phase (9-fold coordination) at 10 GPa1. Previous theoretical calculations2 have reported that the Ni2In-type structure should be stable, but this phase has not been observed experimentally. In our previous experimental study, we examined the high-pressure behavior of PbF2 using in-situ synchrotron x-ray diffraction to 70 GPa. Here we use theoretical density functional theory calculations to better understand the high-pressure behavior of this material. The calculated lattice parameters display highly anomalous trends from 9-21 GPa, consistent with experiments. Experimental evidence for a transition to a Co2Si-like phase was based solely on lattice parameter systematics. Here we use electron density calculations, to directly show that the high-pressure phase is Co2Si-type. Our calculations also reveal the detailed atomic rearrangements associated with the development of an extra Pb-F bond in the high-pressure phase. At P > 9 GPa, both the lead and fluorine ions begin to reorient themselves as an additional fluorine atom, initially outside the 9-fold coordination polyhedron of cotunnite, approaches it. The transition regime from 9 to 21 GPa is marked by continuous atomic movements leading to anomalous behavior of the lattice parameters and compressibilities. At P 21 GPa, the transformation to the Co2Si-type structure is completed with an increase in the coordination number from 9 to 10. References1Haines, J. M. Léger, and O. Schulte, Phys. Rev. B 57, 7551 (1998). 2A. Costales, M. A. Blanco, R. Pandey, and J. M. Recio, Phys. Rev. B 61, 11359 (2000). 3W. Jeitschko, Acta Crystallogr. Sect. B 24, 930 (1968).