Earth's core-mantle boundary shaped by the crystallization of a hydrous terrestrial magma ocean.

Enigmatic anomalous structures of Earth's lowermost mantle may have been incubated at the hydrous magma ocean of the Hadean eon.

Mid-mantle water transportation implied by the electrical and seismic properties of ε-FeOOH.

Water in the mantle transition zone and the core-mantle boundary plays a key role in Earth's stratification, volatile cycling, and core formation. If water transportation is actively running between the aforementioned layers, the lower mantle should contain water channels with distinctive seismic and/or electromagnetic signatures. Here, we investigated the electrical conductivity and sound velocity of ε-FeOOH up to 71 GPa and 1800 K and compared them with global tomography data. An abrupt three-order jump of electrical conductivity was observed above 50 GPa, reaching 1.24(12) × 103 S/m at 61 GPa. Meanwhile, the longitudinal sound velocity dropped by 16.8% in response to the high-to-low spin transition of Fe3+. The high-conductivity and low-sound velocity of ε-FeOOH match the features of heterogenous scatterers in the mid-lower mantle. Such unique properties of hydrous ε-FeOOH, or possibly other Fe-enriched phases can be detected as evidence of active water transportation in the mid-lower mantle.

Evolution of Structural and Electronic Properties of TiSe2 under High Pressure.

A pressure-induced structural phase transition and its intimate link with the superconducting transition was studied for the first time in TiSe2 up to 40 GPa at room temperature using X-ray diffraction, transport measurement, and first-principles calculations. We demonstrate the occurrence of a first-order structural phase transition at 4 GPa from the standard trigonal structure (S.G.P3̅m1) to another trigonal structure (S-G-P3̅c1). Additionally, at 16 GPa, the P3̅c1 phase spontaneously transforms into a monoclinic C2/m phase, and above 24 GPa, the C2/m phase returns to the initial P3̅m1 phase. Electrical transport results show that metallization occurs above 6 GPa. The charge density wave observed at ambient pressure is suppressed upon compression up to 2 GPa with the emergence of superconductivity at 2.5 GPa, with a critical temperature (Tc) of 2 K. A structural transition accompanies the emergence of superconductivity that persists up to 4 GPa. The results demonstrate that the pressure-induced phase transitions explored by the experiments along with the theoretical predictions may open the door to a new path for searching and controlling the phase diagrams of transition metal dichalcogenides.

Phase transition mechanism and bandgap engineering of Sb2S3 at gigapascal pressures.

Earth-abundant antimony trisulfide (Sb2S3), or simply antimonite, is a promising material for capturing natural energies like solar power and heat flux. The layered structure, held up by weak van-der Waals forces, induces anisotropic behaviors in carrier transportation and thermal expansion. Here, we used stress as mechanical stimuli to destabilize the layered structure and observed the structural phase transition to a three-dimensional (3D) structure. We combined in situ x-ray diffraction (XRD), Raman spectroscopy, ultraviolet-visible spectroscopy, and first-principles calculations to study the evolution of structure and bandgap width up to 20.1 GPa. The optical band gap energy of Sb2S3 followed a two-step hierarchical sequence at approximately 4 and 11 GPa. We also revealed that the first step of change is mainly caused by the redistribution of band states near the conduction band maximum. The second transition is controlled by an isostructural phase transition, with collapsed layers and the formation of a higher coordinated bulky structure. The band gap reduced from 1.73 eV at ambient to 0.68 eV at 15 GPa, making it a promising thermoelectric material under high pressure.

Evidence of the pressure-induced conductivity switching of yttrium-doped SrTiO3.

The electrical transport properties of undoped and yttrium-doped strontium titanate (Sr(Ti1 - x Y x )O3 - δ , x = 0, 0.02) under high pressure were investigated with in situ impedance spectroscopy measurements. A pressure-induced conductivity switching for undoped and 2 mole% Y-doped strontium titanate is observed at around ~10.0 and 7.0 GPa respectively, which are caused by a cubic to tetragonal I4/mcm phase transition. The decrease of the phase transition point of 2 mole% Y-doped strontium titanate can be attributed to larger Y(3+) atoms occupying the B-site and the creation of more oxygen vacancies, which lead to octahedra tilting and symmetry breaking. The results of the voltage-bias dependence of grain-boundary impedance of undoped and 2 mole% Y-doped strontium titanate at different pressures revealed that Schottky-type potential barriers formed at grain boundaries are the key factor for the accumulation of oxygen vacancy at the interface under pressure.