High-pressure synchrotron x-ray diffraction and Raman spectroscopic study of plumbogummite

PbAl₃(PO₄)₂(OH,H₂O)₆, an important environmental mineral, is in-situ studied by synchrotron x-ray diffraction (XRD) and Raman scattering combined with diamond anvil cells (DACs) at pressures up to~11.0 GPa and room temperature. The XRD results indicate that plumbogummite does not undergo a phase transition between 0 GPa and 10.9 GPa. Moreover, the c axis is more compressible than the a axis, revealing its anisotropic behavior. The pressure-volume data are fitted to the third-order Birch-Murnaghan equation of state to yield the plumbogummite bulk modulus K₀ of 68(1) GPa and K'₀ of 6.1. ThePO₄^3- andHPO₄^2- Raman vibrational modes exhibit scale nearly linearly as a function of pressure. ThePO₄^3- stretching modes are generally more sensitive to pressure than the bending modes. The Grüneisen parameters range from -0.07 to 1.19, with an arithmetic mean of approximately 0.39.