Lattice dynamic stability and electronic structures of ternary hydrides La1-x Y x H3 via first-principles cluster expansion.

Lanthanum hydride compounds LaH3 become stabilized by yttrium substitution under the influence of moderate pressure. Novel materials with a wide range of changes in the structural properties as a function of hydrogen are investigated by means of the first-principles cluster expansion technique. Herein, the new compounds La1-x Y x H3, where 0 ≤ x ≤ 1, are determined to adopt tetragonal structures under high-pressure with the compositions La0.8Y0.2H3, La0.75Y0.25H3, and La0.5Y0.5H3. The corresponding thermodynamic and dynamical stabilities of the predicted phases are confirmed by a series of calculations including, for example, phonon dispersion, electronic band structure, and other electronic characteristics. According to the band characteristics, all hydrides except that of I41/amd symmetry are semiconductors. The tetragonal La0.5Y0.5H3 phase is found to become semi-metallic, as confirmed by adopting the modified Becke-Johnson exchange potential. The physical origins of the semiconductor properties in these stable hydrides are discussed in detail. Our findings provide a deeper insight into this class of rare-earth ternary hydrides.

Effect of substitution on the superconducting phase of transition metal dichalcogenide Nb(Se[Formula: see text]S[Formula: see text])[Formula: see text] van der Waals layered structure.

By means of first-principles cluster expansion, anisotropic superconductivity in the transition metal dichalcogenide Nb(Se[Formula: see text]S[Formula: see text])[Formula: see text] forming a van der Waals (vdW) layered structure is observed theoretically. We show that the Nb(Se[Formula: see text]S[Formula: see text])[Formula: see text] vdW-layered structure exhibits minimum ground-state energy. The Pnnm structure is more thermodynamically stable when compared to the 2H-NbSe[Formula: see text] and 2H-NbS[Formula: see text] structures. The characteristics of its phonon dispersions confirm its dynamical stability. According to electronic properties, i.e., electronic band structure, density of states, and Fermi surface indicate metallicity of Nb(Se[Formula: see text]S[Formula: see text])[Formula: see text]. The corresponding superconductivity is then investigated through the Eliashberg spectral function, which gives rise to a superconducting transition temperature of 14.5 K. This proposes a remarkable improvement of superconductivity in this transition metal dichalcogenide.

Route to high-[Formula: see text] superconductivity of [Formula: see text] via strong bonding of boron-carbon compound at high pressure.

We have analyzed the compositions of boron-carbon system, in which the [Formula: see text] compound is identified as structural stability at high pressure. The first-principles calculation is used to identify the phase diagram, electronic structure, and superconductivity of [Formula: see text]. Our results have demonstrated that the [Formula: see text] is thermodynamically stable in the diamond-like [Formula: see text] structure at a pressure above 244 GPa, and under temperature also. Feature of chemical bonds between B and C atoms is presented using the electron localization function. The strong chemical bonds in diamond-like [Formula: see text] structure are covalent bonds, and it exhibits the s-p hybridization under the pressure compression. The Fermi surface shape displays the large sheet, indicating that the diamond-like [Formula: see text] phase can achieve a high superconducting transition temperature ([Formula: see text]). The outstanding property of [Formula: see text] at 250 GPa has manifested very high-[Formula: see text] of superconductivity as 164 K, indicating that the carbon-rich system can induce the high-[Formula: see text] value as well.

Pressure-Induced Formation of Quaternary Compound and In-N Distribution in InGaAsN Zincblende from Ab Initio Calculation.

We present the effects of In-N distribution and high pressure on the zincblende phase (0-5 GPa) of In x Ga1-x As0.963N0.037 (x=0.074, 0.111 and 0.148). Structural, electronic, and optical properties are analyzed, and it is found that non-isotropic distribution of In-N (type C) possesses the minimum free energy for the InGaAsN conventional cell system. An increasing indium content reduces the formation enthalpy of InGaAsN. The formation enthalpy, conduction band minimum, strength of covalent bonds, and electron density differences in free space of InGaAsN are decreased under high-pressure conditions. The dielectric performance and static permittivity of InGaAsN are lower than that of GaAs, for which the dielectric performance transforms to conductor performance at high frequency. The optimum photoabsorption coefficient is found at the composition of In0.111Ga0.889As0.963N0.037 (3In-N), which very well relates to the literature.

Low temperature preparation of oxygen-deficient tin dioxide nanocrystals and a role of oxygen vacancy in photocatalytic activity improvement.

The introduction of oxygen vacancies (Vos) into tin dioxide crystal structure has been found as an effective method to improve its photocatalytic performance. Herein, oxygen-deficient tin dioxide (SnO2-x) nanocrystals were successfully prepared via a facile, one-step hydrothermal method at the temperature lower than those reported previously. The effect of hydrothermal temperature on phase composition and Vos content was also firstly investigated. Due to its high oxygen vacancy concentration, the SnO2-x prepared at 80 °C provides the best photocatalytic degradation of methyl orange under UV-visible light. Scavenger trapping and nitroblue tetrazolium experiments also show that the Vos act as electron trapped sites and molecular oxygen adsorption sites, therefore increasing the production of active O2- radical which is the main species governing the photocatalytic activity of SnO2-x nanocrystals. Raman spectroscopy, X-ray photoelectron spectroscopy, photoluminescence measurement and electron spin resonance investigation clearly indicate that increasing the hydrothermal temperature results in the coexistence of SnO2-x and Sn3O4 phases and the reduction of Vos concentration which are detrimental to the photocatalytic performance. Density functional theory calculations also reveal that the presence of Vos is responsible for the upshift of valence band maximum and an extended conduction band minimum, hence a valence band width broadening and band gap narrowing which consequently enhance the photocatalytic performance of the oxygen-deficient SnO2-x.