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Using our newly developed particle swarm optimization algorithm on crystal structural prediction, we characterized the pressure-induced structural transition sequence of gallane (GaH 3 ). As has been observed in alane (AlH 3 ), enthalpy calculations reveal that the Pm3n structure of GaH 3 becomes stable above 160 GPa, below which it is unstable with respect to elemental decomposition. Interestingly, the Pm3n structure is metallic, and the application of the Allen-Dynes modified McMillan equation reveals a high superconducting transition temperature (T c ), which reaches 86 K at 160 GPa and increases with decreasing pressure (T c = 102 K at 120 GPa). Our band structure calculations demonstrate that GaH 3 within the Pm3n structure is a highly ionic solid, where the ionicity of H atoms plays an important role in the predicted high temperature superconductivity.

Being a giant bulk Rashba semiconductor, the ambient-pressure phase of BiTeI was predicted to transform into a topological insulator under pressure at 1.7−4.1 GPa [Nat. Commun. 2012, 3, 679]. Because the structure governs the new quantum state of matter, it is essential to establish the high-pressure phase transitions and structures of BiTeI for better understanding its topological nature. Here, we report a joint theoretical and experimental study up to 30 GPa to uncover two orthorhombic high-pressure phases of Pnma and P4/nmm structures named phases II and III, respectively. Phases II (stable at 8.8−18.9 GPa) and III (stable at >18.9 GPa) were first predicted by our first-principles structure prediction calculations based on the calypso method and subsequently confirmed by our high-pressure powder X-ray diffraction experiment. Phase II can be regarded as a partially ionic structure, consisting of positively charged (BiTe) + ladders and negatively charged I − ions. Phase III is a typical ionic structure characterized by interconnected cubic building blocks of Te−Bi−I stacking. Application of pressures up to 30 GPa tuned effectively the electronic properties of BiTeI from a topological insulator to a normal semiconductor and eventually a metal having a potential of superconductivity.

Using the angle-dispersive synchrotron x-ray powder diffraction technique in a diamond anvil cell, the high-pressure behaviors of antimony telluride (Sb(2)Te(3)) are explored up to 52.7 GPa at room temperature. Three high-pressure phases have been observed, at about 8.0 GPa, 13.2 GPa and above 21.6 GPa, respectively. Furthermore, the crystalline structures of these high-pressure phases are determined as monoclinic sevenfold C2/m phase, eightfold C2/c phase and disordered body-centered cubic structure (space group Im - 3m) respectively. The phase-transition sequences and pressures observed are well explained by first-principles calculations. The pressure dependence of the volume of all high-pressure phases is described by a third-order Birch-Murnaghan equation of state. All the high-pressure phases are metallic and the metallic character for β-, γ- and δ-Sb(2)Te(3) increases in turn based on the results of the electronic density of states calculated for each high-pressure phase.

We measure the electrical resistivity of hcp iron up to ∼170 GPa and ∼3000 K using a four-probe van der Pauw method coupled with homogeneous flattop laser heating in a DAC, and compute its electrical and thermal conductivity by first-principles molecular dynamics including electron-phonon and electronelectron scattering. We find that the measured resistivity of hcp iron increases almost linearly with temperature, and is consistent with our computations. The results constrain the resistivity and thermal conductivity of hcp iron to ∼80 AE 5 μΩ cm and ∼100 AE 10 Wm −1 K −1 , respectively, at conditions near the core-mantle boundary. Our results indicate an adiabatic heat flow of ∼10 AE 1 TW out of the core, supporting a present-day geodynamo driven by thermal and compositional convection.

We report joint theoretical and experimental research on the high-pressure structures of bismuth selenide (Bi2Se3) up to 50 GPa. Our first-principles structure prediction via calypso methodology meets our high-pressure X-ray diffraction experiments performed in diamond anvil cell. We established that the ambient-pressure rhombohedral phase transforms to a monoclinic C2/m structure at 9.8 GPa, and then to a monoclinic C2/c structure at 12.4 GPa. Above 22.1 GPa, we were able to identify that Bi2Se3 develops into a novel 9/10-fold structure, which was not taken by its other family members Bi2Te3 and Sb2Te3. The large differences in atomic core and electronegativity of Bi and Se are suggested to be the physical origin of the stabilization of this 9/10-fold structure. Our research work allows us to reveal a rich chemistry of Bi in the formation of 6, 7, 8, and 9/10-fold covalent bond with Se at elevated pressures.

We studied the Nb-H system over extended pressure and temperature ranges to establish the highest level of hydrogen abundance we could achieve from the resulting alloy. We probed the Nb-H system with laser heating and x-ray diffraction complemented by numerical density functional theory-based simulations. New quenched double hexagonal close-packed (hcp) NbH2.5 appears under 46 GPa, and above 56 GPa cubic NbH3 is formed as theoretically predicted. Nb atoms are arranged in close-packed lattices which are martensitically transformed in the sequence: face-centered cubic (fcc) → hcp → double hcp (dhcp) → distorted body-centered cubic (bcc) as pressure increases. The appearance of fcc NbH2.5−3 and dhcp NbH2.5 cannot be understood in terms of enthalpic stability, but can be rationalized when finite temperatures are taken into account. The structural and compressional behavior of NbHx>2 is similar to that of NbH. Nevertheless, a direct H-H interaction emerges with hydrogen concentration increases, which manifests itself via a reduction in the lattice expansion induced by hydrogen dissolution

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