Band structure of ZrB2, VB2, NbB2, and TaB2 hexagonal diborides: Comparison with superconducting MgB2

The band structure and Fermi surface parameters of ZrB₂, VB₂, NbB₂, and TaB₂ hexagonal diborides are considered in the framework of the self-consistent full-potential LMTO method in comparison with the relevant parameters of a MgB₂ isostructural superconductor. The factors responsible for the superconducting properties of AlB₂-like diborides are analyzed, and the results obtained are compared with previous calculations and available experimental data.     

The band structures of superconducting MgB2 and the isostructural compounds CaGa2, AgB2, AuB2, ZrBe2, and HfBe2

This paper reports on a self-consistent, full-potential LMTO calculation of the band structure of the medium-T _c superconductor MgB₂ and of the isostructural hexagonal phases of CaGa₂, ZrBe₂, HfBe₂, AgB₂, and AuB₂. The factors responsible for the superconducting properties of magnesium diboride are considered. The results obtained are compared with previous calculations and available experimental data.     

Inelastic neutron scattering and lattice dynamics of ZrO2, Y2O3 and ThSiO4

Zirconia (ZrO₂), yttria (Y₂O₃) and thorite (ThSiO₄) are ceramic materials used for a wide range of industrial applications. The dynamical properties of these materials are of interest as they exhibit numerous interesting phase transitions at high temperature and pressure. Using a combination of inelastic neutron scattering and theoretical lattice dynamics we have studied the phonon spectra and thermodynamic properties of these compounds. The experimental data validate the theoretical model, while the model enables microscopic interpretations of the observed data. The calculated thermodynamic properties are in good agreement with the experimental data.      

Ground state and lattice dynamical study of ionic conductors CaF2, SrF2 and BaF2 using density functional theory

The present paper reports a comprehensive and complementary study on structural, electronic and phonon properties of face centered cubic fluorites, namely CaF₂, BaF₂ and SrF₂, using first principles density functional calculations within the generalized gradient approximation. The calculated lattice constants and bulk modulus are in good agreement with available experimental data. The analysis of band structure and density of states confirms the ionic character for all the three fluorides. The phonon dispersion curves and corresponding phonon density of states obtained in the present work are consistent with the available experimental and other theoretical data. The LO-TO splitting is maximum for CaF₂, which confirms that the ionicity is maximum in the case of CaF₂. The phonon properties for SrF₂ have been calculated for the first time.     

Structural and vibrational properties of Ca2FeH6 and Sr2RuH6

The structural and vibrational properties of the isostructural compounds Ca₂FeH₆ and Sr₂RuH₆ are determined by periodic DFT calculations and compared with their previously published experimental crystal structures as well as new experimental vibrational data. The analysis of the vibrational data is extended to the whole series of alkaline-earth iron and ruthenium hydrides A₂TH₆ (A=Mg, Ca, Sr; T=Fe, Ru) in order to identify correlations between selected frequencies and the T-H bond length. The bulk moduli of Ca₂FeH₆ and Sr₂RuH₆ have also been determined within DFT. Their calculated values prove to compare well with the experimental values reported for Mg₂FeH₆ and several other compounds of this structure.     

Optical properties of 1T and 2H phase of TaS2 and TaSe2

We have calculated the anisotropic frequency dependent dielectric function for the 1T and 2H phases of TaS₂ and TaSe₂ using the linear muffin tin orbital method within the atomic sphere approximation. We find significant anisotropy in the frequency dependent dielectric function for the 1T and 2H phases at low energies (less than 4 eV). Unfortunately there are no experimental data to compare with. The averaged dielectric function agrees with the available experimental data except that the calculated peak heights are underestimated and shifted to higher energies by 1–2eV.     

Electronic and optical properties of ceramic Sc2O3 and Y2O3: Compton spectroscopy and first principles calculations

We present the first-ever experimental Compton profiles (CPs) of Sc₂O₃ and Y₂O₃ using 740 GBq ¹³⁷Cs Compton spectrometer. The experimental momentum densities have been compared with the theoretical CPs computed using linear combination of atomic orbitals (LCAO) within density functional theory (DFT). Further, the energy bands, density of states (DOS) and Mulliken's population (MP) data have been calculated using LCAO method with different exchange and correlation approximations. In addition, the energy bands, DOS, valence charge density (VCD), dielectric function, absorption coefficient and refractive index have also been computed using full potential linearized augmented plane wave (FP-LAPW) method with revised functional of Perdew–Becke–Ernzerhof for solids (PBEsol) and modified Becke Johnson (mBJ) approximations. Both the ab-initio calculations predict wide band gaps in Sc₂O₃ and Y₂O₃. The band gaps deduced from FP-LAPW (with mBJ) are found to be close to available experimental data. The VCD and MP data show more ionic character of Sc₂O₃ than Y₂O₃. The ceramic properties of both the sesquioxides are explained in terms of their electronic and optical properties.     

67Cu(67Zn) Mössbauer emission spectroscopy of Tl2Ba2CuO6 and Tl2Ba2CaCu2O8

The parameters of the crystal electric-field gradient at the copper sites of Tl₂Ba₂CuO₆ and Tl₂Ba₂CaCu₂O₈ have been determined by Mössbauer emission spectroscopy using the ⁶⁷Cu(⁶⁷Zn) isotope, and calculated in the point-charge approximation. The results obtained are analyzed using the available ⁶³Cu nuclear quadrupole resonance data. The experimental and calculated data can be brought in agreement if one assumes that the holes appearing after part of the thallium atoms lower their valency become localized primarily at the oxygen sites lying in the plane of the copper ions.     

Far infrared absorption of N2-H2 gaseous mixtures

The rototranslational absorption spectrum of N₂-H₂ gaseous mixtures has been measured at five different temperatures from 298 to 90 K in the frequency range 80–850 cm^-1. The absorption spectra due to N₂-H₂ interactions have been analyzed considering only the quadrupolar induction mechanism. With this assumption, the experimental data are fairly well accounted for, thereby providing a procedure for predicting the N₂-H₂ spectrum over a wide temperature range.     

Lattice dynamical and thermodynamical properties of ReB2, RuB2, and OsB2 compounds in the ReB2 structure

Structural and lattice dynamical properties of ReB2,RuB2,and OsB2 in the ReB2 structure are studied in the framework of density functional theory within the generalized gradient approximation.The present results show that these compounds are dynamically stable for the considered structure.The temperature-dependent behaviors of thermodynamical properties such as internal energy,free energy,entropy,and heat capacity are also presented.The obtained results are in good agreement with the available experimental and theoretical data.     

High-temperature heat capacity of Sm2CuO4 and Ho2Cu2O5

The heat capacities of Sm₂CuO₄ and Ho₂Cu₂O₅ have been measured in the temperature ranges 329–839 and 359–751 K, respectively. The experimental data have been used to determine the thermodynamic properties of the oxide compounds.     

Analysis of the paramagnetic susceptibility of Fe(HCOO)2·2H2O

The paramagnetic susceptibility of powdered Fe(HCOO)₂·2H₂O is analysed by the combined method of high temperature expansion and molecular field approximation. The experimental data are well explained by the consistent model obtained in the previous specific heat analysis.     

Crystal field interactions in DyFe2Si2 and DyFe2Ge2

Two sets of crystal field (CF) parameters have been proposed for DyFe₂Si₂, none of which could provide a simultaneous explanation of the available experimental data, particularly at low temperatures (below 100 K). The set derived from magnetic studies could not even explain the thermal variation of the magnetic specific heat reported in the same work. Although the set of CF parameters, obtained from a fit to the Mossbauer spectra, could provide a fairly good explanation of the thermal variation of the magnetic susceptibilities along the c-axis, it could not explain the observed thermal variation of other reported experimental findings. In the present work, an appraisal of the CF parameters proposed earlier has been done and a set of CF parameters has been derived, which provide a simultaneous explanation of all the available experimental data. The effect of substitution of Ge for Si on the magnetic properties and the magnetic specific heat of DyFe₂Si₂ has been studied in the framework of one electron crystal field model. The inelastic neutron scattering studies and EPR measurements are required to check the predicted Stark energies and the paramagnetic resonance g-values.     

Melting temperature of H<Subscript>2</Subscript>, D<Subscript>2</Subscript>, N<Subscript>2</Subscript> and CH<Subscript>4</Subscript> under high pressure

The melting temperatures of H₂, D₂; N₂ and CH₄ are analysed. The computed results are in very good agreement with the experimental data in each solid. Further, the analysis indicates the presence of the melting maximum in these solids.     

Energy band structure of the compounds AgGaS2, AgGaSe2, and AgGaTe2

Using the pseudopotential method the energy-band structure of the compounds AgGaS₂, AgGaSe₂, AgGaTe₂ is calculated. Calculations are performed with and without consideration of the displacement of anion atoms not changing the crystalline-lattice symmetry. It is established that anion displacement affects the conduction band only slightly and has a more significant effect on the valence band. Available experimental data are evaluated on the basis of the calculations performed.     

Phonon dispersion and thermodynamical properties in ZrB2, NbB2, and MoB2

The structural and lattice dynamical calculations are performed on ZrB₂, NbB₂, and MoB₂ compounds using the first-principles of total energy calculations. Generalized gradient approximations (GGA) are used to model exchange-correlation effects. Structural results of these calculations are consistent with past experimental data and other theoretical findings. Our lattice dynamical results regarding phonon dispersion curves and temperature-dependent behavior of thermodynamical properties (entropy, heat capacity, internal energy, and free energy) contribute to the existing literature on these compounds.     

Spin glass state in crystals of barium ferrigermanate Ba2Fe2GeO7

The temperature dependence of the magnetization and elastic neutron scattering spectra of Ba₂Fe₂GeO₇ barium ferrigermanate polycrystals are studied. The magnetization is found to depend on the magnetic prehistory of a Ba₂Fe₂GeO₇ sample below T = 8 K. Analysis of the neutron scattering spectra does not reveal long-range magnetic order down to 2 K. Our experimental data indicate the existence of a spin glass state in Ba₂Fe₂GeO₇ polycrystals.     

Calculation of thermophysical properties for CO2 gas using an ab initio potential model

The density, isochoric heat capacity, shear viscosity and thermal conductivity of CO₂ gas in the pressure range of 1–50 atm and 300 K are calculated based on a five-centre potential model obtained from ab initio calculations of the intermolecular potential of a CO₂ dimer. The quantum effects of the intramolecular motion are included in a model by the Monte Carlo (MC) Method. Without using any experimental data, the present model achieves excellent agreements between the calculated thermophysical properties and experimental data for all simulated CO₂ densities except the highest one at 135 kg/m³ (3 mol/L). The contributions of potential to the thermophysical properties of the moderate dense CO₂ gas and their dependence on density are investigated in detail.     

On the role of trajectory modelling in the C2H2 infrared line-broadening computation

Infrared C₂H₂–C₂H₂ and C₂H₂–N₂ line-broadening coefficients are computed by means of a novel trajectory model based on the exact solutions of the classical equations of motion. The traditional (parabolic and straight-line) trajectory models are readily obtained as limiting cases of this new approach, allowing a meaningful comparison of all three models on the basis of a common interaction potential. The latter is taken as a sum composed of long-range quadrupole–quadrupole interactions and short-range atom–atom interactions. The line-broadening coefficients are reported for the R branch at room (296K) and low (173.4K) temperatures, and are found to compare favourably with most experimental data.     

Pseudopotential band structure of solid solutions SnSxSe2−x

A very satisfying agreement with the experimental optical data is obtained from a priori pseudopotential calculations for SnS_xSe_2?x solid solutions. The band structure of SnSe₂ is also computed and previous results for SnS₂ are improved.