High-pressure polymorphism of PbF2 to 75 GPa

Abstract

Lead fluoride, PbF2, was investigated experimentally in the laser-heated diamond anvil cell by x-ray diffraction to pressures of 75 GPa at room temperature and to 64.5 GPa and 2430 K, as well as through first-principles density functional theory calculations up to 70 GPa. During room temperature compression, no discontinuous changes in the x-ray diffraction pattern or volume were observed, but the lattice parameters displayed highly anomalous trends between 10–22 GPa with enhanced compressibility along the a direction and reduced or even negative compressibility along b and c. Theoretical calculations of valence electron densities at 22 GPa showed that α-PbF2 underwent a pressure-induced isosymmetric phase transition to a postcotunnite Co2Si structure and also revealed the detailed atomic rearrangements associated with the development of an extra Pb-F bond in the high-pressure phase. Our x-ray results and theoretical calculations are consistent with an isosymmetric phase transition smoothly occurring over 10–22 GPa rather than abruptly as previously suggested. The characteristic values for the cell constants a/c and (a + c)/b, which are used to distinguish among cotunnite-, Co2Si-, and Ni2In-type phases, require modification based on our results. An equation of state fit yields a bulk modulus, K0, of 72(3) GPa for the cotunnite-type, and an ambient-pressure volume, V0, of 182(2) °A3, andK0 = 81(4) GPa for the Co2Si-type phase when fixing the pressure derivative of the bulk modulus, K0 = 4. Upon heating above 1200 K at pressures at or above 25.9 GPa, PbF2 partially transformed to the hexagonal Ni2In-type phase but wholly or partially reverted back to Co2Si-type phase upon temperature quench. From 43–65 GPa, nearly complete transformation to the Ni2In-type PbF2 was observed at high temperature, but the material partially transformed back to the orthorhombic phase upon temperature quench. Our results show that high-pressure behavior of PbF2 is distinct from that of the alkaline earth fluorides with similar ionic radii. Our results also have relevance to understanding the behavior of lanthanide and actinide dioxides, which have been predicted theoretically to exhibit similar isosymmetric transitions at Mbar pressures.