  We report here the combined results of angle-dispersive x-ray diffraction
experiments performed on ThGeO4 up to 40 GPa and total-energy
density-functional theory calculations. Zircon-type ThGeO4 is found to undergo
a pressure-driven phase transition at 11 GPa to the tetragonal scheelite
structure. A second phase transition to a monoclinic M-fergusonite type is
found beyond 26 GPa. The same transition has been observed in samples that
crystallize in the scheelite phase at ambient pressure. No additional phase
transition or evidence of decomposition of ThGeO4 has been detected up to 40
GPa. The unit-cell parameters of the monoclinic high-pressure phase are a =
4.98(2) A, b = 11.08(4) A, c = 4.87(2) A, and beta = 90.1(1), Z = 4 at 28.8
GPa. The scheelite-fergusonite transition is reversible and the
zircon-scheelite transition non-reversible. From the experiments and the
calculations, the room temperature equation of state for the different phases
is also obtained. The anisotropic compressibility of the studied crystal is
discussed in terms of the differential compressibility of the Th-O and Ge-O
bonds.
