Binhua Chu

Mechanical and metallic properties of tantalum nitrides from first-principles calculations

The phase stability, mechanical properties and metallic properties of tantalum nitrides are extensively studied by means of first principles calculations. The relationship between nitrogen concentration and physical properties of tantalum nitrides has been systematically investigated. With the nitrogen concentration increasing, it is found that the feature of covalent bonding enhances and the directionality of the covalent bonding and hardness of tantalum nitrides reduce. While these make the ductility of tantalum nitrides improve with the nitrogen concentration increasing. The intensity of metallic properties of tantalum nitrides can be effectively adjusted by controlling the nitrogen concentration and pressure. When the tantalum: nitrogen ratio reaches Ta:N = 1:3, remarkable nitrogen–nitrogen bonds are found in TaN3. The hardness of TaN3 abnormally increases with reference to that of the preceding composition Ta3N5-II. The potential synthesis routes of tantalum nitrides are suggested.

Cubic C96: a novel carbon allotrope with a porous nanocube network

A novel cubic porous carbon allotrope C96 carbon with intriguing physical properties was predicted. It has 96 atoms in the conventional cell, possessing a Pmm space group. The basic building block of C96 carbon is a planar six-membered carbon ring. The structural stability, mechanical properties, and dynamical properties of C96 carbon were extensively studied. It is a semiconductor (1.85 eV) with a lower density (2.7 g cm−3) and a larger bulk modulus (279 GPa) and is stable under ambient conditions. The hardness of C96 carbon (25 GPa) is larger than that of T carbon (5.6 GPa). Due to the structural porous feature and lower density, C96 carbon can also be expected to be a good hydrogen storage material.

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.

Ab initio structure determination of n-diamond

A systematic computational study on the crystal structure of n-diamond has been performed using first-principle methods. A novel carbon allotrope with hexagonal symmetry R32 space group has been predicted. We name it as HR-carbon. HR-carbon composed of lonsdaleite layers and unique C3 isosceles triangle rings, is stable over graphite phase above 14.2 GPa. The simulated x-ray diffraction pattern, Raman, and energy-loss near-edge spectrum can match the experimental results very well, indicating that HR-carbon is a likely candidate structure for n-diamond. HR-carbon has an incompressible atomic arrangement because of unique C3 isosceles triangle rings. The hardness and bulk modulus of HR-carbon are calculated to be 80 GPa and 427 GPa, respectively, which are comparable to those of diamond. C3 isosceles triangle rings are very important for the stability and hardness of HR-carbon.

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.

Structural, mechanical, and electronic properties of Rh2B and RhB2: first-principles calculations.

The crystal structures of Rh2B and RhB2 at ambient pressure were explored by using the evolutionary methodology. A monoclinic P21/m structure of Rh2B was predicted and donated as Rh2B-I, which is energetically much superior to the previously experimentally proposed Pnma structure. At the pressure of about 39 GPa, the P21/m phase of Rh2B transforms to the C2/m phases. For RhB2, a new monoclinic P21/m phase was predicted, named as RhB2-II, it has the same structure type with Rh2B. Rh2B-I and RhB2-II are both mechanically and dynamically stable. They are potential low compressible materials. The analysis of electronic density of states and chemical bonding indicates that the formation of strong and directional covalent B-B and Rh-B bonds in these compounds contribute greatly to their stabilities and high incompressibility.

Ab initio investigation of CaO-ZnO alloys under high pressure

CaxZn1–xO alloys are potential candidates to achieve wide band-gap, which might significantly promote the band gap engineering and heterojunction design. We performed a crystal structure search for CaO-ZnO system under pressure, using an ab initio evolutionary algorithm implemented in the USPEX code. Four stable ordered CaxZn1–xO structures are found in the pressure range of 8.7–60 GPa. We further constructed the pressure vs. composition phase diagram of CaO-ZnO alloys based on the detailed enthalpy calculations. With the increase in Ca concentration, the CaO-ZnO alloy first undergoes a hexagonal to monoclinic transition, and then transforms back to a hexagonal phase. At Above 9 GPa, there is no cubic structure in the alloys, in contrast to the insostructural components (B1-B1). The band gap of the CaxZn1–xO alloy shows an almost linear increase as a function of the Ca concentration. We also investigated the variation regularity of the band gap under pressure.

The crystal structure of IrB2: a first-principle calculation

First-principle calculations were performed to investigate the structural, elastic, and electronic properties of iridium diboride (IrB2). It was demonstrated that the new phase of IrB2 belongs to the monoclinic C2/m space group, and we have named it m-IrB2. Its structure is energetically much superior to the recently proposed Pmmn-type IrB2. Further calculations of phonon and elastic constants confirm that m-IrB2 is dynamically and mechanically stable. The calculated high shear modulus reveals that it is a potentially a material of low compressibility. An analysis of the density of its states and chemical bonding show that the strongly directional covalent B–B and B–Ir bonds in m-IrB2 make a considerable contribution to its stability.

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.

Ab initio study on the stability of N-doped ZnO under high pressure

We perform first-principles density functional theory calculations to examine the stability of nitrogen-doped wurtzite ZnO under pressure. Our calculations indicate that both the stability of the nitrogen-doped ZnO and the defect concentration increase with pressure. As the pressure increases from 0 to 9 GPa, the density of states at the Fermi level decreases, and the states have a tendency to move to lower energy levels. Electron-localization function and Bader charge analysis have been used to understand the pressure effect on the defect. Under the basic growth conditions (using e-N2 for nitrogen atoms), the calculated formation enthalpies decrease with pressure, which suggests a rise in the defect concentration. Applying pressure has great impact on the nitrogen-doped defects, and can be used as an efficient approach to form p-type ZnO.