Experimental and theoretical investigations on ThGeO4 at high pressure

TL;DR

This study combines experimental x-ray diffraction and density-functional theory calculations to investigate high-pressure phase transitions in ThGeO4, revealing two pressure-induced structural changes up to 40 GPa.

Contribution

It provides the first combined experimental and theoretical analysis of ThGeO4's high-pressure behavior, identifying phase transitions and their reversibility.

Findings

ThGeO4 transitions from zircon to scheelite at 11 GPa

Further transition to monoclinic M-fergusonite at 26 GPa

No decomposition observed up to 40 GPa

Abstract

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.