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

摘要

Room temperature angle-dispersive x-ray diffraction measurements onzircon-type YPO4 and ErPO4, and monazite-type GdPO4, EuPO4, NdPO4, and LaPO4were performed in a diamond-anvil cell up to 27 GPa using neon aspressure-transmitting medium. In the zircon-structured oxides we found evidenceof a reversible pressure-induced structural phase transformation from zircon toa monazite-type structure. The onset of the transition is near 17-20 GPa. InLaPO4 a non-reversible transition is found around 26 GPa, being a barite-typestructure proposed for the high-pressure phase. In the other three monazites,this structure is found to be stable up to highest pressure reached in theexperiments. No additional phase transitions or evidences of chemicaldecomposition are found in the experiments. The equations of state and axialcompressibility for the different phases are also determined. In particular, wefound that in a given compound the monazite structure is less compressible thanzircon structure, being this fact related to the larger packing efficiency ofmonazite compared with zircon. The differential bond compressibility ofdifferent polyhedra is also reported and related the anisotropiccompressibility of both structures. Finally, the sequence of structuraltransitions and compressibilities are discussed in comparison with otherorhtophosphates.