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ArXiv (2021, In Press)
Disordered eight-coordinated Mg2GeO4
In this work, we have studied the high-pressure behavior of Mg2GeO4, an excellent analogue for planetary silicates to ultrahigh pressures of 275 GPa using the laser-heated diamond anvil cell technique coupled with synchrotron X-ray diffraction. Our work shows that post-perovskite MgGeO3+ MgO recombines into a novel Th3P4-type phase at ~160 GPa and remains stable up to the peak pressure. The intrinsically disordered nature of the phase not only opens a new realm of crystal chemistry for high-pressure oxides, but has geophysical implications for exoplanet interiors if it is stable in the silicates e.g. reduced thermal conductivity. The phase is also the first report of an eight-fold coordinated phase in any silicate or its analogues.
Post-cotunnite phases in SnO2, PbO2 and HfO2
DFT calculations predict a nine-coordinated cotunnite-type structure to be stable in SiO2 at ~750 GPa. We studied the pressure-induced phase transitions in three binary oxides SnO2, PbO2and HfO2 using LHDAC experiments and theoretical calucaltions to 200 and 600 GPa respectively. The cotunnite-type phase was found to be stable up to the maximum pressure in SnO2 and PbO2, while the Fe2P-type was observed in HfO2.The transition sequences can be explained by the energetic competition of stationary electronic flat bands and a pressure-induced shift of electronic states to lower energies.
Post-post-perovskite phases in NaMgF3
Neighborite, NaMgF3 crystallizes in the orthorhombic perovskite structure (same as bridgmanite, MgSiO3) under ambient conditions. Our work elucidates for the first time the complex sequence of high-pressure phases beyond post-perovskite in the ABX3 system. We show the presence of a post-post-perovskite phase, followed by a partial and then complete dissociation into binary fluorides. This work is the first experimental report of a complete sequence of post-post-perovskite phases in any known ABX3 system.
High-pressure behavior of FeGeO3 to 120 GPa
Germanates serve as excellent analogs for silicates of the deep Earth. Laser-heating experiments were conducted at three pressures (33, 54 and 123 GPa) chosen to cover the stability regions of different GeO2 polymorphs. In all cases, we observe that FeGeO3 dissociates into GeO2+ FeO at high-pressure and temperature conditions.Neither the perovskite nor the post-perovskite phase was observed up to 127 GPa at ambient or high temperatures.
Equation of State of GeO2 to 120 GPa
The high-pressure behavior of crystalline and amorphous germania has been of wide interest as an analog for silica and for understanding the structural response of AX2 compounds generally. We studied the equation of state of the four different phases of germania up to ~120 GPa both experimentally and theoretically. We also calculated the theoretical Hugoniot and compared our results to the densities of germania and silica glass.
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