HR: 1340h
AN: MR43C-1522    [Abstracts]
TI: Phase transitions and equations of state of alkaline earth fluorides CaF₂ and SrF₂ to 95 GPa
AU: * Dorfman, S M
EM: dorfman@princeton.edu
AF: Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08540,
AU: Jiang, F
EM: fumingj@princeton.edu
AF: Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08540,
AU: Mao, Z
EM: zhumao@princeton.edu
AF: Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08540,
AU: Kubo, A
EM: akubo@cars.uchicago.edu
AF: CARS, University of Chicago, 9700 S Cass Ave., Bldg. 434E, Argonne, IL 60439,
AU: Prakapenka, V
EM: prakapenka@cars.uchicago.edu
AF: CARS, University of Chicago, 9700 S Cass Ave., Bldg. 434E, Argonne, IL 60439,
AU: Duffy, T S
EM: duffy@princeton.edu
AF: Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08540,
AB: AX₂ compounds include a wide range of oxides and salts of broad interest in geoscience, materials science and chemistry, such as SiO₂ and CaF₂, and have in common a sequence of phase transitions dependent on ionic size ratio and electronic properties (Leger and Haines, 1997). Shock compression studies have shown that CaF₂ transforms to a highly incompressible phase above 1 Mbar along the Hugoniot (Nellis, 2007). In this study we examine phase transitions and equations of state of the alkaline earth fluorides CaF₂ and SrF₂ to 95 GPa. Angle-dispersive x-ray diffraction experiments were performed on CaF₂ and SrF₂ samples in laser-heated diamond anvil cells at beamlines X17B3 of the National Synchrotron Light Source and 13-ID-D of the GSECARS sector at the Advanced Photon Source. We confirmed that both materials undergo a phase transition from the cubic fluorite structure to the orthorhombic cotunnite-type structure at pressures less than 10 GPa. Both materials further transform to a hexagonal Ni₂In-type structure at 84 and 36 GPa, respectively, following laser heating. This finding is consistent with theoretical calculations and the behavior of the analog compound BaF₂ (Leger et al., 1995). For SrF₂, the Ni₂In-type phase was confirmed by Rietveld refinement. On decompression with heating, we found that Ni₂In-type SrF₂ passes through an intermediate orthorhombic phase at 28 GPa before returning to cotunnite structure at 22 GPa. This transition appears analogous to the isosymmetric phase transition to the Co₂Si-type structure reported in PbF₂ by Haines et al. (1998). Unit cell parameters and volumes were determined as a function of pressure for the new phases. We also constrained the equation of state of the cotunnite phase of CaF₂ to 82 GPa. Fitting the data to a Birch-Murnaghan equation of state yields a zero-pressure bulk modulus of 97.9 GPa with a pressure derivative of 5.6 for cotunnite-type CaF₂. This work represents the first synthesis and characterization of the Ni₂In-type phase for these compositions and the first report of Co₂Si structure in an alkaline earth fluoride.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3900 MINERAL PHYSICS
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting