Experimental and theoretical investigation ofThGeO4at high pressure

Abstract

We report here the combined results of angle-dispersive x-ray diffraction experiments performed on ThGeO 4 up to 40 GPa and total-energy density-functional theory calculations. Zircon-type ThGeO 4 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 ThGeO 4 has been detected up to 40 GPa. The unit-cell parameters of the monoclinic high-pressure phase are a = 4.98͑2͒ Å, b = 11.08͑4͒ Å, c = 4.87͑2͒ Å, and ␤ = 90.1͑1͒, Z = 4 at 28.8 GPa. The scheelite-fergusonite transition is reversible and the zircon-scheelite transition nonreversible. 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.

Experimental and Theoretical Investigations on Structural and Vibrational Properties of Melilite-Type Sr2ZnGe2O7 at High Pressure and Delineation of a High-Pressure Monoclinic Phase

“…Similar features have been observed in pressure-induced scheelite to fergusonite and also in monazite to post-monazite structural transitions due to the close similarity between the original and transformed structures. [39][40][41] Thus, the observed changes in the XRD patterns at around 12.8 GPa can be attributed to a possible structural change, which will be latter commented and confirmed by our HP-RS measurements.…”

High-pressure stability and compressibility ofAPO4(A=La, Nd, Eu, Gd, Er, and Y) orthophosphates: An x-ray diffraction study using synchrotron r

“…Apparently, upon compression they behave more similar to orthovanadates [23], orthosilicates [33,34], and orthogermanates [35] than to the rest of the compounds of their own family. The distinctive behavior of LuPO 4 , ScPO 4 , and YbPO 4 can be related with that fact that in these compounds the ionic radius of the A cations is small relative to that of the PO 4 tetrahedra.…”

Compression of scheelite-type SrMoO4 under quasi-hydrostatic conditions: Redefining the high-pressure structural sequence

“…The volume difference is typical of density-functional theory (DFT) calculations at the B3LYP level. [43][44][45] Calculations also provide a good description of the pressure dependence of structural parameters. In particular, by fitting the energy vs. volume curve to a second-order Birch-Murnaghan EOS 39 a bulk modulus (B 0 ) of 74.7 GPa is obtained.…”

“…19 These differences between calculations and experiments are typical of DFT calculations. 22,[40][41][42][43][44][45][46] However, they usually describe well the pressure dependence of E g .…”