  When monoclinic monazite-type LaVO4 (space group P21/n) is squeezed up to 12
GPa at room temperature, a phase transition to another monoclinic phase has
been found. The structure of the high-pressure phase of LaVO4 is indexed with
the same space group (P21/n), but with a larger unit-cell in which the number
of atoms is doubled. The transition leads to an 8% increase in the density of
LaVO4. The occurrence of such a transition has been determined by x-ray
diffraction, Raman spectroscopy, and ab initio calculations. The combination of
the three techniques allows us to also characterize accurately the pressure
evolution of unit-cell parameters and the Raman (and IR)-active phonons of the
low- and high-pressure phase. In particular, room-temperature equations of
state have been determined. The changes driven by pressure in the crystal
structure induce sharp modifications in the color of LaVO4 crystals, suggesting
that behind the monoclinic-to-monoclinic transition there are important changes
of the electronic properties of LaVO4.
