  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.
