Phase transitions and equations of state of alkaline earth fluorides CaF[subscript 2], SrF[subscript 2], and BaF[subscript 2] to Mbar pressures

Journal Article · Mon Jun 14 00:00:00 EDT 2010 · Phys. Rev. B

DOI:https://doi.org/10.1103/PhysRevB.81.174121· OSTI ID:1002454

Dorfman, Susannah M; Jiang, Fuming; Mao, Zhu; Kubo, Atsushi; Meng, Yue; Prakapenka, Vitali B; Duffy, Thomas S ^[1]

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1. Princeton

Phase transitions and equations of state of the alkaline earth fluorides CaF{sub 2}, SrF{sub 2}, and BaF{sub 2} 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{bar 3} m) fluorite structure to the orthorhombic (Pnam) cotunnite-type structure. This work has characterized an additional phase transition in CaF{sub 2} and SrF{sub 2}: these materials were observed to transform to a hexagonal (P6{sub 3}/mmc) Ni{sub 2}In-type structure between 63-79 GPa and 28-29 GPa, respectively, upon laser heating. For SrF{sub 2}, the Ni{sub 2}In-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 CaF{sub 2} and SrF{sub 2}, 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 Ni{sub 2}In-type phase is {approx}5%. The percentage increase in the bulk modulus ({Delta}K) across the transition is greater when the cation is smaller. While for BaF{sub 2}, {Delta}K is 10-30%, {Delta}K values for SrF{sub 2} and CaF{sub 2} are 45-65% and 20-40%. Although shock data for CaF{sub 2} 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.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
• Sponsoring Organization: USDOE
• OSTI ID: 1002454
• Journal Information: Phys. Rev. B, Vol. 81, Issue (17) ; 2010; ISSN 1098-0121
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

36 MATERIALS SCIENCE
EQUATIONS OF STATE
CRYSTAL-PHASE TRANSFORMATIONS
CALCIUM FLUORIDES
STRONTIUM FLUORIDES
BARIUM FLUORIDES
PRESSURE RANGE GIGA PA
X-RAY DIFFRACTION