Qi-Jun Liu

Structural, elastic, electronic and optical properties of various mineral phases of TiO2 from first-principles calculations

Titanium dioxide is well known as a semiconductor material, which attracts a great deal of attention for promising applications in many fields due to its outstanding physical and chemical properties. To investigate the structural, elastic, mechanical, electronic and optical properties of various TiO2 phases systematically, we present the ultrasoft pseudopotential planewave method within local density approximation and generalized gradient approximation, as well as the norm-conserving pseudopotential within hybrid functional B3LYP by first-principles calculations on fluorite, pyrite, rutile, anatase, hollandite, brookite, columbite, cotunnite, bronze and baddeleyite TiO2 phases. The structural parameters of ten phases are calculated, which are shown to be consistent with previous theoretical and experimental data. We obtain the elastic constants of ten phases and then estimate the bulk, shear and Youngs moduli, Poissons coefficient and Lames constants using the Voigt–Reuss–Hill approximation. The energy band structures, density of states and charge populations of ten phases were obtained and indicated there is covalency in TiO2. Moreover, the complex dielectric function, refractive index and extinction coefficient of the ten phases were calculated; this data can aid future experimental research.

A Density Functional Theory Study of Codoping Characteristics of Sulfur with Alkaline Earth in Delafossite CuAlO2

The structural, electronic properties and formation energies of sulfur and alkaline earth codoped delafossite CuAlO2 have been investigated using the first-principles density functional theory calculations. Our results reveal that the volume of codoping systems increases with the increasing atomic radius of metal atoms. The formation energies under different growth conditions have been calculated, showing that the codoping systems are formed easily under O-rich growth conditions. Electronic band structures and density of states have been obtained. The decreased bandgaps, enhanced covalence and appearance of electron acceptors after codoping are all good for p-type conductivity.

Mechanical properties, Born effective charge tensors and high frequency dielectric constants of the eight phases of BaTiO 3

We have preformed the first-principles calculations for the mechanical properties, Born effective charge tensors and high frequency dielectric constants of the eight phases of BaTiO3. The independent elastic constants, bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio were obtained, which were consistent with the available theoretical and experimental values. The mechanical stability and brittle/ductile behaviors of the eight phases of BaTiO3 have been discussed. The calculated results indicated that the eight phases were all mechanically stable and behaved in a brittle manner. The calculated Born effective charge tensors shown the covalent Ti–O bond and ionic Ba–O bond. Moreover, the high frequency dielectric constants have been given.

First-principles study of the structural, elastic, mechanical, electronic, and optical properties of cubic Mg2TiO4

We have performed ab-initio total energy calculations using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT) to study structural, elastic, mechanical, electronic, and optical properties of cubic Mg2TiO4. The calculated lattice parameter a is in good agreement with the experimental values. The independent elastic constants are calculated. The mechanical properties including bulk, shear and Young’s modulus, Poisson’s coefficient, compressibility and Lamé’s constants are obtained using the Voigt-Reuss-Hill method. Debye temperature is estimated using the Debye-Grüneisen model. Band structure, density of states and charge densities are shown and analyzed. In order to clarify the mechanism of optical transitions of cubic Mg2TiO4, the complex dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, loss function and complex conductivity function are calculated.

Vibrational spectrum of condensed H2O in hydrogen-bonding environment: an ab initio simulation study

Local hydrogen-bonding environments have important influences on the intra-molecular O–H stretchings of H2O molecules. The relationship between the contributions of intra-molecular O–H stretching and the local hydrogen-bonding environments is investigated using ab initio simulation for a condensed H2O system at 300 K, 1.7 g/cm3, and calculated/estimated pressure of approximately 9 GPa. The calculation results demonstrate that the local hydrogen-bonding environments around the two intra-molecular hydrogen atoms are not always similar. The existence of asymmetric local hydrogen environments will result in decoupling of the intra-molecular O–H stretchings in the molecule; thus, the broad O–H stretching band may be decomposed into a sum that includes isolated intra-molecular O–H stretchings but not symmetric stretching and asymmetric stretching. This research serves as a reminder to pay attention to the influence of an asymmetric local hydrogen-bonding environment on the vibrational details of the H2O molecular system and will facilitate the interpretation of measurements of the infrared and Raman spectra of the condensed H2O systems.

Structural, Electronic, and Optical Properties of Cubic Y 2 O 3 : First-Principles Calculations

Structural, electronic, and optical properties of cubic Y2O3 were studied using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The ground-state properties were calculated and these results were in good agreement with the previous work. Furthermore, in order to understand the optical properties of cubic Y2O3, the complex dielectric function, refractive index, extinction coefficient, optical reflectivity, absorption coefficient, energy-loss function, and complex conductivity function were calculated, which were in favorable agreement with the theoretical and experimental values. We explained the origin of the absorption peaks using the theories of crystal-field and molecular-orbital bonding and investigated the relation between electronic structure and optical properties.

Electrical resistivity of fluid methane multiply shock compressed to 147 GPa

ABSTRACT Shock wave experiments were carried out to measure the electrical resistivity of fluid methane. The pressure range of 89–147 GPa and the temperature range from 1800 to 2600 K were achieved with a two-stage light-gas gun. We obtained a minimum electrical resistivity value of 4.5 × 10−2 Ω cm at pressure and temperature of 147 GPa and 2600 K, which is two orders of magnitude higher than that of hydrogen under similar conditions. The data are interpreted in terms of a continuous transition from insulator to semiconductor state. One possibility reason is chemical decomposition of methane in the shock compression process. Along density and temperature increase with Hugoniot pressure, dissociation of fluid methane increases continuously to form a H2-rich fluid.

Calculations of Structural, Electronic, Chemical Bonding, and Optical Properties of Orthorhombic CsAlTiO4

Structural parameters, electronic, chemical bonding and optical properties of orthorhombic CsAlTiO4 are studied using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The equilibrium lattice constants, bulk modulus and electronic structure are obtained. To our knowledge, no data are available in literature of orthorhombic CsAlTiO4 with Pnma space group for comparison. Electronic and chemical bonding properties have been studied from the calculations of band structure, density of states and charge densities. The complex dielectric functions are calculated and we have explained the origins of spectral peaks.

Vacancy defects in delafossite СuАlO2: First-principles calculations

Electronic properties and formation energies of vacancy defects in delafossite CuAlO2 have been investigated by using the first-principles density functional theory. The band structures and density of states of various vacancy defects have been obtained and analyzed. The results show that the VCu systems with different charge states influence the type of conductivity. The introduced vacancy defects enhance the hybridization between O-2p and Cu-3d states, which is good for p-type conductivity. The calculated formation energies indicate that the Cu vacancy is relatively easy to form and it trends to have positive charge.

Theoretics-directed effect of copper or aluminum content on the ductility characteristics of Al-based (Al3Ti, AlTi, AlCu, AlTiCu2) intermetallic compounds

First-principle simulations have been applied to investigate the effect of copper (Cu) or aluminum (Al) content on the ductility of Al3Ti, AlTi, AlCu, and AlTiCu2 alloys. The mechanical stable and elastic properties of Al-based intermetallic compounds are researched by density functional theory with the generalized gradient approximation (DFT-GGA). The calculated lattice constants are in conformity with the previous experimental and theoretical data. The deduced elastic constants show that the investigated Al3Ti, AlTi, AlCu, and AlTiCu2 structures are mechanically stable. Shear modulus, Youngs modulus, Poissons ratio, and the ratio B/G have also been figured out by using reckoned elastic constants. A further analysis of Youngs modulus and Poissons ratio reveals that the third added element copper content has significant effects on the Al–Ti-based ICs ductile character.