Phase transitions and equations of state of alkaline earth fluorides CaF2, SrF2, and BaF2 to Mbar pressures

Phase transitions and equations of state of the alkaline earth fluorides CaF2, SrF2, and BaF2 were examined by static compression to pressures as high as 146 GPa. Angle-dispersive x-ray diffraction experiments were performed on polycrystalline samples in the laser-heated diamond-anvil cell. We confirmed that at pressures less than 10 GPa all three materials undergo a phase transition from the cubic (Fm (3) over barm) fluorite structure to the orthorhombic (Pnam) cotunnite-type structure. This work has characterized an additional phase transition in CaF2 and SrF2: these materials were observed to transform to a hexagonal (P6(3)/mmc) Ni2In-type structure between 63-79 GPa and 28-29 GPa, respectively, upon laser heating. For SrF2, the Ni2In-type phase was confirmed by Rietveld refinement. Volumes were determined as a function of pressure for all high-pressure phases and fit to the third-order Birch-Murnaghan equation of state. For CaF2 and SrF2, the fluorite-cotunnite transition results in a volume decrease of 8-10 %, while the bulk modulus of the cotunnite-type phase is the same or less than that of the fluorite phase within uncertainty. For all three fluorides, the volume reduction associated with the further transition to the Ni2In-type phase is similar to 5%. The percentage increase in the bulk modulus (Delta K) across the transition is greater when the cation is smaller. While for BaF2, Delta K is 10-30 %, Delta K values for SrF2 and CaF2 are 45-65 % and 20-40 %. Although shock data for CaF2 have been interpreted to show a transition to a highly incompressible phase above 100 GPa, this is not consistent with our static equation of state data.