High-pressure structural and vibrational properties of monazite-type BiPO₄, LaPO₄, CePO₄, and PrPO₄

Abstract

Monazite-type BiPO4, LaPO4, CePO4, and PrPO4 have been studied under high pressure by ab initio simulations and Raman spectroscopy measurements in the pressure range of stability of the monazite structure. A good agreement between experimental and theoretical Raman-active mode frequencies and pressure coefficients has been found which has allowed us to discuss the nature of the Raman-active modes. Besides, calculations have provided us information on how the crystal structure is modified by pressure. This information has allowed us to determine the equation of state and the isothermal compressibility tensor of the four studied compounds. In addition, the information obtained on the polyhedral compressibility has been used to explain the anisotropic axial compressibility and the bulk compressibility of monazite phosphates. Finally, we have carried out a systematic discussion on the high-pressure behavior of the four studied phosphates in comparison to results of previous studies.

High-Pressure Elastic, Vibrational and Structural Study of Monazite-Type GdPO4 from Ab Initio Simulations

“…No evidence of soft phonons was found at the Γ point in this pressure range. These results are consistent with the experimental and theoretical reports that confirm that there is no phase transition induced by pressure in the monazite structure up to the highest pressure reached in those studies (30 GPa) [14,17]. At zero pressure, a crystal is mechanically stable when the Born stability criteria are fulfilled [53].…”

“…The calculation of the phonon dispersion curves along the main directions within the BZ was carried out with a supercell of size 2 × 2 × 2. Following the conclusions of our previous studies and those of Blanca-Romero et al [45], in all the calculations, we neglect the spin-orbit interaction, because the structural properties are barely affected [15,17,26,29,30,37].…”

“…Actually, in the monazite-type GdPO 4 , the volume change of the GO 9 polyhedra seems to be responsible for most of the volume reduction induced by pressure within the structure. In fact, the phosphate tetrahedra can be treated as a rigid unit in the monazite-type phosphate [17,18,48]. The P-O distances do not vary greatly within the tetrahedra.…”

“…Therefore, the vibrational spectra of the GdPO4 monazite could be interpreted in term of the modes of the PbO4 tetrahedra, which can be considered as independent units in the monazite structure, and GdO9 polyhedra. Indeed, the vibrational spectrum of monazites ABO4 has been interpreted in terms of internal modes (bending and stretching modes) associated with the tetrahedron BO4 and external modes (translational and rotational) which involves movement of both A and BO4 ions [17,63]. Through group theory analysis, it can be established that the monazite structure has 72 vibrational modes at the zone center: 36 optical Raman-active modes (18Ag + 18Bg), 33 optical IRactive modes (17Au + 16Bu), and 3 acoustic modes (1Au + 2Bu).…”

“…Therefore, the study of these orthophosphates under high pressure is very promising for future practical applications. Although there are several studies focused on pressure-induced transition and on the structural and vibrational properties of monazite phosphates under pressure [7,[14][15][16][17][18], these studies were carried out only up to 30 GPa. A systematic understanding of their properties has not yet been achieved; the study of their elastic and mechanical properties is scarce [5,19,20].…”