Equation of state of γ-tricalcium phosphate, γ-Ca3(PO4)2, to lower mantle pressures

The gamma-tricalcium phosphate phase (gamma-TCP), gamma-Ca-3(PO4)(2), is a high-pressure polymorph of tricalcium phosphate with a potential important implication as the reservoir of rare-earth elements and very large lithophile elements in the deep mantle. In situ synchrotron X-ray diffraction measurements of the gamma-TCP phase have been carried out using a diamond-anvil cell to 40.29 GPa at room temperature, with a methanol-ethanol mixture as the pressure medium. The pressures in the measurements have been determined by using gold metal as the internal pressure calibrant. The third-order Birch-Murnaghan equation of state fitted to the experimentally defined unit-cell parameters suggests for the gamma-TCP phase a density of rho(0) = 3.46 1 (1) g/cm(3), an isothermal bulk modulus of K-T = 100.2(13) GPa, and first pressure derivative of K-T' = 5.48(16). When K-T' is fixed at 4, the derived K-T is 113.1(12) GPa.