High-pressure stability and compressibility of APO4 (A = La, Nd, Eu, Gd, Er, and Y) orthoposphates: A synchrotron powder x-ray diffraction study

TL;DR

This study investigates the high-pressure behavior of APO4 orthoposphates, revealing reversible zircon-to-monazite phase transitions and providing detailed equations of state and compressibility data for different structures.

Contribution

It provides new experimental data on phase transitions and compressibility of APO4 orthoposphates up to 27 GPa, including the discovery of a non-reversible high-pressure phase in LaPO4.

Findings

Reversible zircon to monazite transition near 17-20 GPa.

Non-reversible barite-type phase in LaPO4 around 26 GPa.

Monazite structure is less compressible than zircon.

Abstract

Room temperature angle-dispersive x-ray diffraction measurements on zircon-type YPO4 and ErPO4, and monazite-type GdPO4, EuPO4, NdPO4, and LaPO4 were performed in a diamond-anvil cell up to 27 GPa using neon as pressure-transmitting medium. In the zircon-structured oxides we found evidence of a reversible pressure-induced structural phase transformation from zircon to a monazite-type structure. The onset of the transition is near 17-20 GPa. In LaPO4 a non-reversible transition is found around 26 GPa, being a barite-type structure proposed for the high-pressure phase. In the other three monazites, this structure is found to be stable up to highest pressure reached in the experiments. No additional phase transitions or evidences of chemical decomposition are found in the experiments. The equations of state and axial compressibility for the different phases are also determined. In particular, we found that in a given compound the monazite structure is less compressible than zircon structure, being this fact related to the larger packing efficiency of monazite compared with zircon. The differential bond compressibility of different polyhedra is also reported and related the anisotropic compressibility of both structures. Finally, the sequence of structural transitions and compressibilities are discussed in comparison with other orhtophosphates.