Zhonglong Zhao

Pressure-induced metallization of dense (H2S)2H2 with high-Tc superconductivity

The high pressure structures, metallization, and superconductivity of recently synthesized H2-containing compounds (H2S)2H2 are elucidated by ab initio calculations. The ordered crystal structure with P1 symmetry is determined, supported by the good agreement between theoretical and experimental X-ray diffraction data, equation of states, and Raman spectra. The Cccm structure is favorable with partial hydrogen bond symmetrization above 37 GPa. Upon further compression, H2 molecules disappear and two intriguing metallic structures with R3m and Im-3m symmetries are reconstructive above 111 and 180 GPa, respectively. The predicted metallization pressure is 111 GPa, which is approximately one-third of the currently suggested metallization pressure of bulk molecular hydrogen. Application of the Allen-Dynes-modified McMillan equation for the Im-3m structure yields high Tc values of 191 K to 204 K at 200 GPa, which is among the highest values reported for H2-rich van der Waals compounds and MH3 type hydride thus far.

Nitrogen concentration driving the hardness of rhenium nitrides.

The structures and properties of rhenium nitrides are studied with density function based first principle method. New candidate ground states or high-pressure phases at Re:N ratios of 3:2, 1:3, and 1:4 are identified via a series of evolutionary structure searches. We find that the 3D polyhedral stacking with strong covalent N-N and Re-N bonding could stabilize Re nitrides to form nitrogen rich phases, meanwhile, remarkably improve the mechanical performance than that of sub-nitrides, as Re3N, Re2N, and Re3N2. By evaluating the trends of the crystal configuration, electronic structure, elastic properties, and hardness as a function of the N concentration, we proves that the N content is the key factor affecting the metallicity and hardness of Re nitrides.

Modulated T carbon-like carbon allotropes: an ab initio study

The structural stability, mechanical properties, and dynamical properties of T carbon-like structures were extensively studied by first-principles calculations using density functional theory. A novel modulated T carbon-like carbon allotrope (T-II carbon) is predicted by means of first principles calculations. This structure has 8 atoms in the unit cell, possesses the Pnm space group, and can be derived by stacking up two T carbons together. T-II carbon is a semiconductor with band gap 0.88 eV and has a higher hardness (27 GPa) than that of T carbon (5.6 GPa). The calculations of ideal strength and the electron localization function indicate that T-II carbon has better ability to resist shear strain than T carbon.

Miscibility and ordered structures of MgO-ZnO alloys under high pressure.

The MgxZn1−xO alloy system may provide an optically tunable family of wide band gap materials that can be used in various UV luminescences, absorption, lighting, and display applications. A systematic investigation of the MgO-ZnO system using ab initio evolutionary simulations shows that MgxZn1−xO alloys exist in ordered ground-state structures at pressures above about 6.5 GPa. Detailed enthalpy calculations for the most stable structures allowed us to construct the pressure-composition phase diagram. In the entire composition, no phase transition from wurzite to rock-salt takes place with increasing Mg content. We also found two different slops occur at near x = 0.75 of Eg-x curves for different pressures, and the band gaps of high pressure ground-state MgxZn1−xO alloys at the Mg concentration of x > 0.75 increase more rapidly than x < 0.75.

First-principles study on the structural and electronic properties of metallic HfH2 under pressure

The crystal structures and properties of hafnium hydride under pressure are explored using the first-principles calculations based on density function theory. The material undergoes pressure-induced structural phase transition I4/mmm→Cmma→P21/m at 180 and 250 GPa, respectively, and all of these structures are metallic. The superconducting critical temperature Tc values of I4/mmm, Cmma, and P21/m are 47–193 mK, 5.99–8.16 K and 10.62–12.8 K at 1 atm, 180 and 260 GPa, respectively. Furthermore, the bonding nature of HfH2 is investigated with the help of the electron localization function, the difference charge density and Bader charge analyses, which show that HfH2 is classified as a ionic crystal with the charges transferring from Hf atom to H.

High pressure structures and superconductivity of AlH3(H2) predicted by first principles

Motivated by the potential high-temperature superconductivity in hydrogen-rich materials, the high-pressure structures of AlH3(H2) in the pressure range of 25–300 GPa were extensively explored by using a genetic algorithm. We found an insulating P1 phase, a semiconducting P phase and an intriguing sandwich-like metallic phase with a space group of P21/m-Z (containing Z shape net layers of Al atoms). We found that the H2 molecules in the environment of AlH3 became metallic and showed a molecular semi-molecular phenomenon. The application of the Allen–Dynes to modify the McMillan equation yields remarkably high superconducting temperatures of 132–146 K at 250 GPa, which is among the higher values reported so far for phonon-mediated superconductors. In this paper, we reveal a unique superconducting mechanism, which shows that the direct interactions between H2 and AlH3 at high pressure play a major role in the high superconductivity, while the contribution from the H2 vibration is minor.

Ab initio study of germanium-hydride compounds under high pressure

Motivated by the potential high-temperature superconductivity in hydrogen-rich materials and phase transitions, germanium-hydride compounds under high pressure were studied by a genetic algorithm. Enthalpy calculations suggest that the Ge and H will form Ge3H, Ge2H, GeH3, and GeH4 at about 32, 120, 280, and 280 GPa, respectively. These four germanium-hydride compounds are all stable up to at least 300 GPa. For Ge3H, the most stable structure is P-Ge3H at 32–220 GPa and P63/m-Ge3H at 220–300 GPa. All the germanium-hydride compounds are metallic phases as demonstrated by the band structure and density of states.

A Novel High-Density Phase and Amorphization of Nitrogen-Rich 1H-Tetrazole (CH 2 N 4 ) under High Pressure

The high-pressure behaviors of nitrogen-rich 1H-tetrazole (CH2N4) have been investigated by in situ synchrotron X-ray diffraction (XRD) and Raman scattering up to 75 GPa. A first crystalline-to-crystalline phase transition is observed and identified above ~3 GPa with a large volume collapse (∼18% at 4.4 GPa) from phase I to phase II. The new phase II forms a dimer-like structure, belonging to P1 space group. Then, a crystalline-to-amorphous phase transition takes place over a large pressure range of 13.8 to 50 GPa, which is accompanied by an interphase region approaching paracrystalline state. When decompression from 75 GPa to ambient conditions, the final product keeps an irreversible amorphous state. Our ultraviolet (UV) absorption spectrum suggests the final product exhibits an increase in molecular conjugation.

Ideal stoichiometric technetium nitrides under pressure: A first-principles study

Technetium nitrides with various ideal stoichiometries have been investigated with the first-principle method at the pressures between 0–60 GPa. It have been found that there could be many stable technetium nitrides including Tc3N, Tc2N, TcN, Tc2N3, TcN2, TcN3, and TcN4, among which Tc3N and Tc2N subnitrides are synthesizable at zero pressure and could be applied to nuclear waste management, such as separate radioactive 99Tc from nuclear fuel cycle. Moreover, N-rich TcN3 and TcN4 exhibit remarkable bulk properties and can be potential ultrastiff and hard materials.

High-pressure phase transition of MH3 (M: Er, Ho)

Motivated by the potential high temperature superconductivity in hydrogen-rich materials, high-pressure structures of ErH3 and HoH3 were studied by using genetic algorithm method. Our calculations indicate that both ErH3 and HoH3 transform from P-3c1 structure to a monoclinic C2/m structure at about 15 GPa, and then transforms into a cubic Fm-3m structure at about 40 GPa. ErH3 and HoH3 adopt the same P6₃/mmc structure with space group P6₃/mmc at above about 220 and 196 GPa, respectively. For ErH3, the P6₃/mmc phase is stable up to at least 300 GPa, while for HoH3, a phase transformation P6₃/mmc → Cmcm occurs at about 216 GPa, and the Cmcm phase is stable up to at least 300 GPa. The P-3c1 ErH3 and HoH3 are calculated to demonstrate non-metallic character, and the other phases are all metallic phases.