Crystal structure of hydrous wadsleyite with 2.8% H 2 O and compressibility to 60 GPa

ABSTRACT Hydrous wadsleyite (β-Mg 2 SiO 4 ) with 2.8 wt% water content has been synthesized at 15 GPa and 1250 °C in a multi-anvil press. The unit-cell parameters are: a = 5.6686(8), b = 11.569(1), c = 8.2449(9) Å, β = 90.14(1)°, and V = 540.7(1) Å 3 , and the space group is I2/m. The structure was refined in space groups Imma and I2/m. The room-pressure structure differs from that of anhydrous wadsleyite principally in the increased cation distances around O1, the non-silicate oxygen. The compression of a single crystal of this wadsleyite was measured up to 61.3(7) GPa at room temperature in a diamond anvil cell with neon as pressure medium by X-ray diffraction at Sector 13 at the Advanced Photon Source, Argonne National Laboratory. The experimental pressure range was far beyond the wadsleyite-ringwoodite phase-transition pressure at 525 km depth (17

Cubic to hexagonal structural transformation in Gd2O3 at high pressure

High-pressure X-ray diffraction studies on gadolinium sesquioxide (Gd2O3) have been carried out up to a pressure of 25 GPa in a diamond-anvil cell at room temperature. Gadolinium oxide, which has a cubic or bixbyite structure under ambient conditions, undergoes an irreversible structural phase at around 12 GPa. The high-pressure phase has been identified as a hexagonal La2O3-type structure. The bulk modulus and its pressure derivative of this phase have been calculated

Hydrogen bond symmetrization and equation of state of phase D

[1] We have synthesized phase D at 24 GPa and at temperatures of 1250-1100°C in a multianvil press under conditions of high silica activity. The compressibility of this high-silica-activity phase D (Mg 1.0 Si 1.7 H 3.0 O 6 ) has been measured up to 55.8 GPa at ambient temperature by powder X-ray diffraction. The volume (V) decreases smoothly with increasing pressure up to 40 GPa, consistent with the results reported in earlier studies. However, a kink is observed in the trend of V versus pressure above ∼40 GPa, reflecting a change in the compression behavior. The data to 30 GPa fit well to a third-order Birch-Murnaghan equation of state (EoS), yielding V o = 85.1 ± 0.2 Å 3 ; K o = 167.9 ± 8.6 GPa; and K′ o = 4.3 ± 0.5, similar to results for Fe-Al-free phase D reported b