Xiaojing Sha

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.

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.

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.

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.

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.

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.

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.

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.