High pressure behavior of complex phosphate K2Ce[PO4]2: Grüneisen parameter and anharmonicity properties

Abstract

Herein we reported structural stability, vibrational and thermal properties of K2Ce[PO4]2, a relatively underexplored complex phosphate of tetravalent Ce4+ from in situ high-pressure Raman spectroscopic investigations up to ~ 28GPa using a diamond anvil cell. The studies identified the soft phonons that lead to a reversible phase transformation above 8GPa, and a phase coexistence of ambient (PI) and high pressure (PII) phases in a wider pressure region 6–11GPa. From a visual representation of the computed eigen vector displacements, the Ag soft mode at 82cm−1 is assigned as a lattice mode of K+ cation. Pressure-induced positional disorder is apparent from the substantial broadening of internal modes and the disappearance of low frequency lattice and external modes in phase PII above 18GPa. Isothermal mode Grüneisen parameters γi of the various phonon modes are calculated and compared for several modes. Using these values, thermal properties such as average Grüneisen parameter, and thermal expansion coefficient are estimated as 0.47, and 2.5 × 10−6K−1, respectively. The specific heat value was estimated from all optical modes obtained from DFT calculations as 314J-mol−1K−1. Our earlier reported temperature dependence of phonon frequencies is used to decouple the “true anharmonic” (explicit contribution at constant volume) and “quasi harmonic” (implicit contribution brought out by volume change) contributions from the total anharmonicity. In addition to the 81cm−1 Ag lattice mode, several other lattice and external modes of PO43- ions are found to be strongly anharmonic.