Electronic structure and structural stability of LaAl2 and LaAl3—A comparative study

The large structural stability regime of LaAl₂ and LaAl₃ as a function of pressure is investigated by the band structure calculations using the FP-LAPW method. An earlier experimental study has revealed that there is no structural phase transition at ∼35 and ∼30 GPa for LaAl₂ and LaAl₃, respectively. Our calculations indicate that in the density of states curve of LaAl₂, the Fermi level (E_F) lies in a slope between bonding maxima and antibonding minima. At high pressures the E_F moves slightly towards the valley, but this shifting does not affect its structural stability. In LaAl₃, the E_F falls in a flat region in the density of states and does not move even up to 33 GPa. The band dispersion curves for both the compounds show movement of bands under the influence of pressure. Some of them cross the Fermi level leading to so called Lifshitz transitions. However, it is seen that these electronic changes do not manifest into any volume anomaly in LaAl₃ under pressure. Our study clearly shows that the density of states behavior for LaAl₂ and LaAl₃ satisfies the Yamashita-Asano criterion for structural stability. The theoretical equations of state, bulk modulus and its pressure derivative values are compared with the experimental values.     

Search for new iron oxypnictide superconductors by using high-pressure synthesis technique

Here we report on the invention of the Co-doped induced superconductivity in PrFe_1−xCo_xAsO (x = 0 and 0.075). Polycrystalline samples were prepared by high-pressure synthesis method. The PrFe_1−xCo_xAsO (x = 0 and 0.075) crystallizes with the ZrCuSiAs type crystal structure (P4/nmm). The PrFeAsO is a parent compound with non-superconducting phase which can be transformed to superconductor by replacement 7.5% of Fe by Co. The magnetic susceptibility measurement shows the onset of superconducting transition temperature at 15 K.     

Numerical analysis on auto-ignition of a high pressure hydrogen jet spouting from a tube

In this study, a direct numerical simulation based on compressible flow dynamics has been applied to the autoignition and extinction of a high-pressure hydrogen jet spouting from a tube. The diameter of the tube is 4.8 mm. The length of the tube is 71 mm. At the inlet, pressure is set at 3.6, 5.3 and 21.1 MPa, and temperature is set at 300 K for all cases. To explore the autoignition of hydrogen jet, two-dimensional axisymmetric Navier–Stokes equations with a detailed chemical kinetics and rigorous transport properties have been employed. The hydrogen jet through the tube is choked. The numerical results show that the high-pressure hydrogen jet produces a semi-spherical shock wave in the ambient air at the early time of jetting. The shock wave heats up the air to a high temperature and causes the autoignition of the hydrogen and air mixture in the tube as well as at the tube exit.     

Transport properties of undoped and Br-doped PbTe sintered at high-temperature and pressure ≥4.0 GPa

The thermoelectric properties of nominally undoped PbTe and Br doped PbTe materials sintered at high-pressure and high-temperature (HPHT) have been studied. All samples show n-type semiconducting behavior with negative thermopower. For undoped PbTe, four different HPHT treatments were performed at pressures between 4.0 and 6.5 GPa. PbTe doped with Br at 0.5, 1.0, 2.0, 3.0×10¹⁹ cm⁻³ was HPHT treated at 4.0 GPa and 1045 °C. As the dopant concentration increases, the absolute thermopower decreases, thermal conductivity increases, and electrical resistivity decreases. At a nominal dopant concentration of 1.0×10¹⁹ cm⁻³, carrier mobility of 1165 cm²/V s and dimensionless thermoelectric figure-of-merit, ZT, of around 0.27 at 300 K were obtained. These results demonstrate that HPHT post-processing is a viable and controllable way of tuning the thermoelectric properties of PbTe-based materials.     

Structural and physical properties of 1:2 B-site-ordered perovskite Ba3CaIr2O9

The high-pressure phase of iridium-based compound Ba₃CaIr₂O₉ was synthesized using high-pressure sintering. Being different from the distorted hexagonal BaTiO₃ structure of the ambient Ba₃CaIr₂O₉, the high-pressure phase crystals into the 1:2 B-site-ordered perovskite structure with the space group P-3m1 (Z=1). Through fitting the X-ray powder diffraction (XRD) data with Rietveld analysis, in which the obtained R_p, R_wp, and R_exp factors are 7.49%, 11.4%, and 4.82%, respectively, the lattice parameters are a=5.8296(1) Å and c=7.1659(2) Å. The atomic coordinates and the main interatomic distances and bond angles were also obtained. The relationship of electrical resistivity versus temperature shows that the high-pressure phase of Ba₃CaIr₂O₉ is a semiconductor in the temperature range of 5-300 K. The measurement of temperature dependence of magnetic susceptibility indicates that it is paramagnetic.     

Pressure-induced coordination changes in LiBO2

The coordination and structure changes in LiBO₂ have been studied at high pressure and temperature up to 5 GPa and 1500 °C using in-situ high-pressure differential thermal analysis, infrared absorption spectra and X-ray diffraction. The layer framework structure of α-LiBO₂ is found to be compressed easily along the direction of c-axis, resulting in the formation of tetra-coordinated BO₄ units. The phase transition boundaries between α- and γ-LiBO₂ as well as between amorphous LiBO₂ hydrate and γ-LiBO₂ have negative pressure–temperature slopes. The conditions for transformation from α- to γ-LiBO₂ are lower than that necessary to transform amorphous LiBO₂ hydrate to γ-LiBO₂. Moreover, the melting curve of LiBO₂ has also been determined and has a positive pressure–temperature slope. Upon quenching from high pressure, LiBO₂ may not contain ^[3]B–O–^[3]B rings but contain more fraction of ^[4]B units with increasing pressure.     

Combinative use of high-pressure, metal-templating and sulfur-nucleophilicity towards dithiacyclophane synthesis and its complex intermediates

Combined use of elevated pressure in the liquid phase (15 kbar), a metal template and the sulfur nucleophilicity of [Pt₂(μ-S)₂(P-P)₂] (P-P = diphosphine or 2 · monophosphine) facilitates the one-pot synthesis of 3,8-dibenzo-1,6-dithiacyclodecane. Under r.t.p., nucleophilic addition of [Pt₂(μ-S)₂(P-P)₂] [P-P = 2 · PPh₃; Ph₂P(CH₂)_nPPh₂, n = 2, 1,2-bis(diphenylphosphino)ethane (dppe), 3, 1,3-bis(diphenylphosphino)propane (dppp)] with α-α′-dichloro-o-xylene would terminate as a dithiolato bridged cation viz. [Pt₂(μ-SCH₂C₆H₄CH₂S)(P-P)₂]²⁺. Under high pressure (15 kbar) at r.t., these stoichiometric reactions progress via a “catalytic-like” pathway to yield 3,8-dibenzo-1,6-dithiacyclodecane (up to 35%), and a series of mechanistically relevant intermediates and byproducts. The dithiolated intermediates [Pt₂(μ-SCH₂C₆H₄CH₂S)(P-P)₂]²⁺ for PPh₃ and dppp have been isolated as complexes and their crystal structure determined. The formation of 3,8-dibenzo-1,6-dithiacyclodecane demonstrates a convenient synthetic strategy over the multi-step synthesis of this macrocyclic dithioether.     

Effects of the vacancy point-defect on the refractive index and equation of state (EOS) of LiF at high pressure: A first principles investigation

By the first-principles method, the refractive-index and density of LiF crystal without and with charged Li or F vacancy were calculated within 102 GPa, whose results were used to explore effects of shock-induced vacancy point-defects on its refractive-index and EOS at high pressure. Our data indicate that the calculated refractive-index of a perfect LiF crystal increases more rapidly with increasing pressure than those determined by shock experiments and above ∼50-60 GPa there is also a similar behavior in density-pressure curve. It is found that Li¹⁺ vacancy-induced decreases in refractive-index and density are supposed to be a possible source causing these differences. Our results support that the vacancy-defect concentrations should increase with increasing shock-pressure.     

Comment on “Impedance spectroscopy study and ground state electronic properties of In(Mg1/2Ti1/2)O3 (Physica B 406 (2011) 1081–1087)” and related works

In this comment, we demonstrated that some reports about synthesis and properties of double In-based perovskites, In(Mg_1/2Ti_1/2)O₃, In(Ni_1/2Zr_1/2)O₃, and In(Co_1/2Ti_1/2)O₃, are incorrect. Mixtures of different known oxides were investigated instead of the claimed new compounds. We also tried to prepare In(Ni_1/2Zr_1/2)O₃, In(Ni_1/2Ti_1/2)O₃, and In(Zn_1/2Mn_1/2)O₃ using a high-pressure method (at 6 GPa and 1580 K) and found that these compounds are not formed. Only in the In₂O₃–NiO–MnO₂ system, a new perovskite phase is formed using the high-pressure preparation method.     

Pressure-dependent dynamical properties of Zn-based II–VI semiconductors

The effect of pressure on optical phonons and polaron properties in ZnS, ZnSe, and ZnTe II–VI compound semiconductors has been investigated. The calculations are performed in the framework of ab initio pseudopotential approach based on the density functional perturbation theory. At zero pressure, a reasonable degree of agreement is generally found between our results and data available in the literature. It is found that when pressure is increased the phonon modes at Г in the Brillouin zone are shifted towards high energies. The pressure dependence of features such as Fröhlich coupling parameter, the Debye temperature of the longitudinal optical phonon frequency and the polaron effective mass showed that the polaron properties are sensitive to the pressure effect.