X-ray diffraction and Raman spectra of merrillite at high pressures

ABSTRACT The compressibility and effect of pressure on the vibrations of merrillite, Ca9NaMg(PO4)7, were studied by using diamond anvil cell at room temperature combined with in-situ synchrotron X-ray diffraction and Raman spectroscopy up to about 18 and 15 GPa, respectively. The pressure-volume data was fitted by a third-order Birch–Murnaghan equation of state to determine the isothermal bulk modulus as K0  = 87.2(32) GPa with pressure derivative K0′ = 3.2(4). If K0′ = 4, the isothermal bulk modulus was obtained as 81.6(10) GPa. The axial compressibility was estimated and an axial elastic anisotropy exists since a-axis is less compressible than the c-axis. The Raman frequencies of all observed modes for merrillite continuously increase with pressure, and the pressure dependences of stretching modes (v 3 and v 1) are larger than those of the bending modes (v 4 and v 2) and external modes. The isothermal mode Grüneisen parameters and intrinsic anharmonicity of merrillite were also calculated.