Structural and electronic properties of clathrate-like hydride: MH6 and MH9 (M = Sc, Y, La).

CONTEXT: The addition of central metal atoms to hydrogen clathrate structures is thought to provide a certain amount of "internal chemical pressure" to offset some of the external physical pressure required for compound stability. The size and valence of the central atoms significantly affect the minimum pressure required for the stabilization of hydrogen-rich compounds and their superconducting transition temperature. In recent years, many studies have calculated the minimum stable pressure and superconducting transition temperature of compounds with H24, H29, and H32 hydrogen clathrates, with centrally occupied metal atoms. In order to investigate the stability and physical properties of compounds with H cages in which the central atoms change in the same third group B, herein, based on first-principles calculations, we systematically investigated the lattice parameters, crystal volume, band structures, density of states, Mulliken analysis, charge density, charge density difference, and electronic localization function in I m 3 ¯ m -MH6 and P63/mmc-MH9 systems with different centered rare earth atoms M (M = Sc, Y, La) under a series of pressures. We find that for MH9, the pressure mainly changes the crystal lattice parameters along the c-axis, and the contributions of the different H atoms in MH9 to the Fermi level are H3 > H1 > H2. The density of states at the Fermi level of MH6 is mainly provided by H 1 s. Moreover, the size of the central atom M is particularly important for the stability of the crystal. By observing a series of properties of the structures with H24 and H29 cages wrapping the same family of central atoms under a series of pressures, our theoretical study is helpful for further understanding the formation mechanism of high-temperature superconductors and provides a reference for future research and design of high-temperature superconductors. METHODS: The first principles based on the density functional theory and density functional perturbation theory were employed to execute all calculations by using the CASTEP code in this work.

Structural, elastic, mechanical, electronic, and optical properties of cubic K2Pb2O3 from first-principle study.

CONTEXT: Based on first principles, the structure, elasticity, mechanics, electronics, and optical properties of cubic K2Pb2O3 were studied. The structural parameters calculated by this method are close to the previous theoretical results. The elastic constant, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and mechanical stability are studied, and it is shown that cubic K2Pb2O3 is mechanically stable, isotropic, and brittleness. The electrical conductivity and chemical bonding of cubic K2Pb2O3 were analyzed based on the calculated band structure, density of states (DOS), and bond populations. The dispersion of optical functions, including the dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, and loss function, is displayed and analyzed. METHODS: All computations have been carried out based on density functional theory (DFT) as implemented in the CASTEP code. The norm conservation pseudopotential method is used to exchange correlation functionals within the generalized gradient approximation (GGA).

The structural, mechanical and electronic properties of Ti-Al-based compounds by first-principles calculations.

The first-principles calculations with density functional theory were performed to investigate the effects of transition metal elements (Mo, Cu, Fe, Ni and Nb) on the physical properties of the Ti-Al-based compounds. Our optimized crystal parameters are in good agreement with the previous theoretical and experimental values. The mechanical stability is verified by the independent elastic constants. The B/G and Poisson's ratio ν both show that Al6TiMo is brittle, while other compounds exhibit ductility. The values of compression anisotropy of the compounds are small, but the shear anisotropy of AlCu2Ti and AlNi2Ti is much more intense than that of other compounds. The anisotropy in elastic properties of AlFe2Ti and AlNbTi2 is smaller than that of the others. It can be seen that the capacity to compress along c-axis is smaller than that along a-axis and b-axis for AlNbTi2. For AlNbTi2, the anisotropy of the bulk modulus along a-axis relative to b-axis is more insignificant than that along c-axis relative to b-axis. The hardness and Debye temperature verify that AlFe2Ti has the greatest resistance to the plastic deformation and more intense inter-atomic bonding force, respectively. Band structures and DOS are used to investigate the electronic properties. The band structures without band gaps show that these ternary Ti-Al-based compounds are conductors. DOS shows the interactions between elements and gives the bond properties. Density of states and charge density both show the strong covalent properties of AlFe2Ti by the hybridization between Fe-3d and Ti-3d states.

The study of spectroscopy and vibrational assignments of high nitrogen material 1,1'-azobis-1,2,3-triazole.

Benefiting from the new strategy of oxidative azo coupling of the N-NH2 moiety, a series of energetic nitrogen-rich molecules with long catenated nitrogen chains have been successfully synthesized. As one of them, the synthesized 1,1'-azobis-1,2,3-triazole shows excellent thermal stability, great explosive performance, and special photochromic properties, which has caused widespread concern. To further characterize its performance, the structural, electronic, vibrational, mechanical, and thermodynamic properties of 1,1'-azobis-1,2,3-triazole were investigated based on the first-principles density functional theory calculations. The obtained structural parameters are consistent with previous results. We used the band structure, density of states, Mulliken charges, bond populations, and electron density to analyze the electronic properties and chemical bonding. The vibrational frequency regions (396.51-3210.12 cm-1) were assigned to the corresponding vibrational modes. Furthermore, mechanical properties of 1,1'-azobis-1,2,3-triazole are also calculated. Finally, the thermodynamic properties of 1,1'-azobis-1,2,3-triazole were calculated, including the specific heat at constant volume Cv, temperature*entropy TS, enthalpy H, Gibbs free energy G, and Debye temperature ΘD.

Effects of molecular vacancy and ethylenediamine on structural and electronic properties of CH3NO2 surfaces.

The structural and electronic properties of (100) surface for nitromethane (NM) are studied using density functional theory (DFT) with the generalized gradient approximation and Perdew-Burke-Ernzerhof functional (GGA-PBE). Molecular vacancy and ethylenediamine (C2H8N2) substitution are considered in this work. We find that ethylenediamine substitution significantly decreases the band gap, while molecular vacancy increases the band gap slightly. It indicates that ethylenediamine substitution has a positive effect on the impact sensitivity of NM. Also, the formation energies are calculated and the reasons for the decrease of band gap for ethylenediamine substitution and the increase of band gap for CH3NO2 vacancy are explained.

Judging the phase transition pressure of the unknown parent phase if the resulting state is known.

In high-pressure phase transition experiments, the crystal structure of the intermediate phase in some phase transitions is difficult to successfully measure due to the limitations of the experimental conditions. The absence of crystal structure data for the intermediate phase also makes it difficult to calculate the pressure point from the intermediate phase to the new phase by the traditional thermodynamic criterion in theoretical simulations. The Conch Criterion is employed by us to successfully verify the phase transition points by observing the reverse shifts of the DOS (electron density of states) curves for the new phase of Cu2S, PbS, PbSe and PbTe, which breaks through the constraints of the traditional criterion and realizes tracing the source of the phase transition in theoretical calculations.

A new criterion for the prediction of solid-state phase transition in TMDs.

The classical thermodynamic criterion for phase transition predicts whether the phase transition will occur according to whether the nth derivative of the state parameter is discontinuous, and the continuity verification of multi-order derivatives increases the difficulty and complexity of judgment for phase transition to a certain extent. Based on the reverse shifts of the DOS curves near the Fermi level, we propose a new criterion for solid-state phase transition named Conch Criterion, which has been verified in the TMD system. The new criterion can observe the occurrence of phase transition from another perspective besides the thermodynamic properties while mutually confirming the thermodynamic criterion.

Transfer of intensity quantum correlation with twin beams.

We demonstrate experimentally a protocol of transferring nonclassical quantum properties using two pairs of quantum-correlated twin beams in the continuous variable regime. The intensity quantum correlation from one twin beam is transferred to two initially independent idler beams with the help of a displacement transformation. It makes two originally independent beams exhibit an intensity quantum correlation of 0.8 dB below shot-noise level.