Abstract

We report here the combined results of angle-dispersive x-ray diffractionexperiments performed on ThGeO4 up to 40 GPa and total-energydensity-functional theory calculations. Zircon-type ThGeO4 is found to undergoa pressure-driven phase transition at 11 GPa to the tetragonal scheelitestructure. A second phase transition to a monoclinic M-fergusonite type isfound beyond 26 GPa. The same transition has been observed in samples thatcrystallize in the scheelite phase at ambient pressure. No additional phasetransition or evidence of decomposition of ThGeO4 has been detected up to 40GPa. 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 28.8GPa. The scheelite-fergusonite transition is reversible and thezircon-scheelite transition non-reversible. From the experiments and thecalculations, the room temperature equation of state for the different phasesis also obtained. The anisotropic compressibility of the studied crystal isdiscussed in terms of the differential compressibility of the Th-O and Ge-Obonds.