Cui-E Hu

Dynamical stability of Mo under high pressure and high temperature

Considering the phonon-phonon interactions, we obtain the high temperature phonons of Mo under high pressure. The dynamically stable regions of bcc and fcc Mo in the phase diagram are predicted. By comparing the anharmonic free energy, we determine the bcc-fcc boundary. The bcc Mo is the stable phase up to 700 GPa. Around 210 GPa, there is no bcc-fcc phase transition, which is different with the results from quasiharmonic approximation.

High Pressure Structural Instability and Thermal Properties of Rutile TiO2 from First-principles

We report a first-principles calculation to investigate the structural instability of rutile TiO2. The high pressure structural parameters are well reproduced. The calculated phonon dispersion curves agree with experiments at zero pressure. Under compression, we capture a large softening around Г point, which indicates the structural instability. From the high pressure elastic constants, we find that the rutile TiO2 is unstable when the applied pressure is larger than 17.7 GPa. Within the quasi-harmonic approximation, the thermal equation of state, thermal expansion coefficient, bulk modulus, and entropy are well reproduced. The thermal properties confirm the available experimental data and are extended to a wider pressure and temperature range.

Magnetism and Sound Velocities of Iron Carbide (Fe3C) under Pressure

The elastic property and sound velocity of Fe3C under high pressure are investigated by using the spin-polarized generalized gradient approximation within density-functional theory. It is found that the magnetic phase transition from the ground ferromagnetic (FM) state to the nonmagnetic (NM) state occurs at ∼73 GPa. Based on the predicted Hugoniot of Fe3C, we calculate the sound velocities of FM-Fe3C and NM-Fe3C from elastic constants. Compared with pure iron, NM-Fe3C provides a better match of compressional and shear sound velocities with the seismic data of the inner core, supporting carbon as one of the light elements in the inner core.

First-principles investigations on elastic, thermodynamic and lattice thermal conductivity of topological insulator LaAs

Abstract Topological insulators are always a hot topic owing to their various peculiar physical effects, which are useful in spintronics and quantum information processing. Herein, we systematically investigate the elastic, thermodynamic and lattice thermal conductivity of a new typical topological insulator LaAs by combining the first-principles approach and an iterative solution of the Boltzmann transport equation. The obtained elastic constants and other lattice structural parameters of LaAs are well consistent with the experimental and other theoretical results. For the first time, the lattice thermal conductivity (5.46 W/(m∙K)) and mean free path (14.4 nm) of LaAs are obtained，which manifests that the LaAs is more likely to be a desirable thermoelectric material. It is noted that the obtained mode-averaged Grüneisen parameters by different ab initio simulation packages are very similar, suggesting that our results are rather responsible. From the phonon scattering rates of LaAs, we speculate that the reduction of acoustic-optical gap and the larger phonon scattering may jointly result in reduction of thermal conductivity for LaAs. Meanwhile, the temperature dependence curves of the lattice thermal conductivity, heat capacity and phonon mean free path are also presented. We expect our work can provide more information for further experimental studies.

First-Principles Investigations on Structural, Elastic, Dynamical, and Thermal Properties of Earth-Abundant Nitride Semiconductor CaZn2N2 under Pressure

Abstract We presented a detailed first-principal calculation to study the structural, elastic, dynamical, and thermal properties of a new synthetic ternary zinc nitride semiconductors CaZn2N2 using the generalised gradient approximation (GGA) method. The obtained lattice parameters of CaZn2N2 at 0 K and 0 GPa are in good agreement with the experimental data and other theoretical findings. The pressure dependences of the elastic constants Cij together with other derived mechanical properties of CaZn2N2 compound have also been systematically investigated. The results reveal that CaZn2N2 is mechanically stable up to 20 GPa. The calculated the phonon curves and phonon density of states under different pressures indicate that the CaZn2N2 compound maintains its dynamical stability up to 20 GPa. An analysis in terms of the irreducible representations of group theory obtained the optical vibration modes of this system, and we obtained the frequencies of the optical vibrational modes at Г points together with the atoms that contributed to these vibrations of CaZn2N2. Meanwhile, the pressure dependencies of the frequencies Raman-active and IR-active modes at 0–20 GPa have been studied. The quasi-harmonic approximation (QHA) was applied to calculate the thermal properties of CaZn2N2 as functions of pressures and temperatures such as the heat capacity, thermal expansions, the entropy, and Grüneisen parameter γ.

Elastic, electronic and thermal properties of topological insulator SmB6: first-principles calculations

Abstract The pressure dependence of the structural, elastic, electronic and thermal properties of Kondo insulator SmB6 have been systematically studied by density functional theory combined with the quasi-harmonic Debye model. The calculated structure at zero pressure is in good agreement with the available experimental results at low temperature. The obtained elastic constants, bulk modulus and shear modulus indicate that SmB6 is mechanically stable and behaves in a brittle manner under the applied pressure 0–20 GPa, consistent with available experimental data. In addition, the elastic-relevant properties, Young’s modulus and the Poisson ratio manifest that increasing pressure results in an enhancement in the stiffness of the compound. It is found that unlike temperature, pressure has little effect on the heat capacity of SmB6. What more important is that we observed an insulator to metal phase transition at about 5.5 GPa through the disappearance of the band gap, well consistent with the experimental data. This transition has little effect on the physical properties of SmB6.