Cao Qi-Long

Phase transition and thermodynamic properties of BiFeO3 from first-principles calculations

The first-principles projector-augmented wave method employing the quasi-harmonic Debye model, is applied to investigate the thermodynamic properties and the phase transition between the trigonal R3c structure and the orthorhombic Pnma structure. It is found that at ambient temperature, the phase transition from the trigonal R3c phase to the orthorhombic Pnma phase is a first-order antiferromagnetic-nonmagnetic and insulator-metal transition, and occurs at 10.56 GPa, which is in good agreement with experimental data. With increasing temperature, the transition pressure decreases almost linearly. Moreover, the thermodynamic properties including Gruneisen parameter, heat capacity, entropy, and the dependences of thermal expansion coefficient on temperature and pressure are also obtained.

Properties of Liquid Nickel along Melting Lines under High Pressure

We report a molecular dynamics study of structural and transport properties of liquid nickel under high pressures. Pressure dependencies of pair distribution function and pair correlation entropy along the melting line indicate that the configuration change along melting lines decreases with increasing pressure. The calculated diffusion coefficients and viscosity by using entropy-scaling laws with modified parameters and ideal parameters are compared with those extracted from mean-square displacement or the Stokes–Einstein relation. The results suggest that the entropy-scaling laws hold well for liquid nickel under high-pressure conditions, and the diffusion coefficients and viscosity increase moderately with pressure along melting lines.

Crystallization of polymer chains induced by graphene: Molecular dynamics study

The present work is devoted to a study of the molecular mechanisms of the crystallization of polymer chains induced by graphene by using molecular dynamics (MD) simulations. From the atomic configuration translation, the number distribution of the atoms, and the order parameter S, the crystallization process can be summarized in two steps, the adsorption and the orientation. By analyzing the diffusion properties of the polymer chains, we find that a graphene substrate has a great adsorption for the polymer molecules and the polymer molecules need more time to adjust their configurations. Therefore, the adsorption step and the orientation step are highly cooperative.

Transport Properties and the Entropy-Scaling Law for Liquid Tantalum and Molybdenum under High Pressure

Molecular dynamics simulations are applied to study the transport properties, including the self-diffusion coefficient and viscosity, of liquid tantalum and molybdenum under high pressure conditions. The temperature dependence of self-diffusion coefficient, viscosity and the pair correlation entropy under high pressure conditions are investigated. Our results show that the Arrhenius law well describes the temperature dependence of self-diffusion coefficients and viscosity under high pressure, and the diffusion activation energy decreases with increasing pressure, while the viscosity activation energy increases with increasing pressure. The temperature dependence of the pair correlation entropy is well described by 1/T scaling. Furthermore, we find that the entropy-scaling laws, proposed by Rosenfeld for self-diffusion coefficients and viscosity in simple liquids under ambient pressure, still hold well for liquid tantalum and molybdenum under high pressure conditions.

Phase transition and thermodynamic properties of ThO2: Quasi-harmonic approximation calculations and anharmonic effects

The thermodynamic properties and the phase transition of ThO2 from the cubic structure to the orthorhombic structure are investigated using the first-principles projector-augmented wave method. The vibrational contribution to Helmholtz free energy is evaluated from the first-principles phonon calculations. The anharmonic contribution to quasi-harmonic free energy is accounted for by using an effective method (2010 Phys. Rev. B 81 172301). The results reveal that at ambient temperature, the phase transition from the cubic phase to the orthorhombic phase occurs at 26.45 GPa, which is consistent with the experimental and theoretical data. With increasing temperature, the transition pressure decreases almost linearly. By comparing the experimental results with the calculation results, it is shown that the thermodynamic properties of ThO2 at high temperature improve substantially after including the anharmonic correction to quasi-harmonic free energy.

Thermodynamics and elastic properties of Ta from first-principles calculations

Within the framework of the quasiharmonic approximation, the thermodynamics and elastic properties of Ta, including phonon density of states (DOS), equation of state, linear thermal expansion coefficient, entropy, enthalpy, heat capacity, elastic constants, bulk modulus, shear modulus, Youngs modulus, microhardness, and sound velocity, are studied using the first-principles projector-augmented wave method. The vibrational contribution to Helmholtz free energy is evaluated from the first-principles phonon DOS and the Debye model. The thermal electronic contribution to Helmholtz free energy is estimated from the integration over the electronic DOS. By comparing the experimental results with the calculation results from the first-principles and the Debye model, it is found that the thermodynamic properties of Ta are depicted well by the first-principles. The elastic properties of Ta from the first-principles are consistent with the available experimental data.

Effect of Mg and Fe Doping on Optical Absorption of LiNbO3 Crystal through First Principles Calculations

Using first principles calculations, we investigate the structural, optical, and electronic properties of LiNbO3 (LN) and M doped LN (M=Mg, Fe). The density of states are calculated to analyze the effect of doping Mg and Fe ions on the absorption spectra and electronic properties of LN. The results show an ultraviolet shift in the optical absorption edge of Mg-doped LN compared with that of intrinsic LN. On the contrary, the absorption edge of Fe-doped LN crystal reveals a red shift. The optical absorption spectra show an improved optical response in the visible range for Mg-doped LN, which significantly differs from that obtained for Fe-doped LN. The electronic excitations from the valence band to the conduction band of LN leads to an improved optical absorption response in the visible region as observed experimentally. The obvious changes of the doped LN crystal are found in some cases, which provide a helpful guide for preparing doped LN crystal.

Diffusion activation energy versus the favourable energy in two-order-parameter model::A molecular dynamics study of liquid Al

In the preent work, we find that both diffusion activation energy Ea(D) and Ea(Sex) increase linearly with pressure and have the same slope (0.022±0.001 eV/GPa) for liquid Al. The temperature and pressure dependence of excess entropy is well fitted by the expression −Sex(T,P)/kB = a(P)+b(P)T+c(P) exp (Ef/kBT), which together with the small ratio of Ef/kBT leads to the relationship of excess entropy to temperature and pressure, i.e. Sex −cEf/T, where c is about 12 and Ef ( = ΔE − PΔV) is the favourable energy. Therefore, there exists a simple relation between Ea(Sex) and Ef i.e. Ea(Sex) cEf.