The γ-tricalcium phosphate phase (γ-TCP), γ-Ca3(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 γ-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 γ-TCP phase a density of ρ0 = 3.461(1) g/cm3, an isothermal bulk modulus of KT = 100.2(13) GPa, and first pressure derivative of KT = 5.48(16). When KT is fixed at 4, the derived KT is 113.1(12) GPa.

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