6533b851fe1ef96bd12aa014

RESEARCH PRODUCT

High-pressure stability and compressibility ofAPO4(A=La, Nd, Eu, Gd, Er, and Y) orthophosphates: An x-ray diffraction study using synchrotron radiation

subject

description

Room-temperature angle-dispersive x-ray diffraction measurements on zircon-type ${\text{YPO}}_{4}$ and ${\text{ErPO}}_{4}$, and monazite-type ${\text{GdPO}}_{4}$, ${\text{EuPO}}_{4}$, ${\text{NdPO}}_{4}$, and ${\text{LaPO}}_{4}$ were performed in a diamond-anvil cell up to 30 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 at 19.7 GPa in ${\text{YPO}}_{4}$ and 17.3 GPa in ${\text{ErPO}}_{4}$. In ${\text{LaPO}}_{4}$ a nonreversible transition is found at 26.1 GPa and a barite-type structure is proposed for the high-pressure phase. For the other three monazites studied, their structures were found to be stable up to 30 GPa. Evidence for additional phase transitions or chemical decomposition of the materials was not 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 the zircon structure. This fact is attributed to the higher packing efficiency of monazite versus zircon. The differential bond compressibility of different polyhedra is also reported and related to the anisotropic compressibility of both structures. Finally, the sequence of structural transitions and compressibilities are discussed in comparison with other orthophosphates.