Similarity of optical properties of hydrides and semiconductors for antireflection coatings

The electronic structure and optical properties of the complex hydrides Ca₂FeH₆, Ca₂RuH₆, Mg₂FeH₆, Mg₂RuH₆, Sr₂FeH₆, Sr₂RuH₆ and Sr₂OsH₆ were studied using first-principles calculations. Optical spectra of the hydrides were compared with those of SiN _x , In₂O₃ and ZnO determined from theoretical calculations and measured experimentally. Based on an analysis of band structure, it is found that the electrical conductivity of the hydrides is expected to be poor. However, optical properties of the hydrides in the energy range 0–3 eV are found to be almost the same as those of SiN _x , In₂O₃, ZnO and TiO₂. Hydrides are suggested to be used as antireflection layers and for passivation of surface and bulk defects. This finding could be useful for electronic device technology including solar cells.     

Electronic structure and transport of Ca3Co2O6 and Ca3CoNiO6

We have calculated the band structure of Ca₃Co₂O₆ and Ca₃CoNiO₆ by using the self-consistent full-potential linearized augmented plane-wave method within density function theory and the generalized gradient approximation for the exchange and correlation potential. The spin-orbit interaction is incorporated in the calculations using a second variational procedure. The relation of these band structure calculations to thermoelectric transport is discussed. The results illustrate that transport is highly anisotropic with much larger mobility in the a-b plane than out of the a-b plane, and the introduction of Ni in Ca₃Co₂O₆ alters its electronic structure and its thermoelectric transport properties.     

Thermal diffusivity of double perovskite Sr2MMoO6 (M=Fe, Mn and Co) and La doping effects studied by mirage effect

The thermal diffusivity has been investigated in double perovskite Sr₂MMoO₆ (M=Fe, Mn and Co) by means of the mirage effect. We have found that the thermal diffusivity of metallic Sr₂FeMoO₆ is 0.39 cm²/s, which is larger than that (0.33 cm²/s) of insulating Sr₂MnMoO₆ and Sr₂CoMoO₆. We further investigate the substitution effects of the La³⁺ ions for the Sr²⁺ ions in Sr₂FeMoO₆ and Sr₂MnMoO₆, and have found that the thermal diffusivities of both samples significantly increase with the La concentration. Such an enhancement of the thermal diffusivities has been ascribed to occupation of the extra itinerant electrons on the conduction Mo4d band.     

Evidence for the Griffiths phase in pure and Y-, Ca- and Cr-doped LaSr2Mn2O7 manganites

The paramagnetic susceptibility measured on La_1−xY_xSr_2−yCa_y Mn₂O₇ (x, y or z=0, 0.03 and 0.1) manganites does not obey the Curie-Weiss law. The Curie constant exceeds the expected values by more than order of magnitude. Based on strong disorder we have compared the experimental data with predictions based on Griffith's singularity model. We find that this model allows for consistent interpretation of the data in contrary to a model assumes that magnetic polarons make contribution akin to cluster to susceptibility. In particular the values of Curie constant agree excellently with those theoretically expected, whereas the magnetic polarons model is less satisfactory.     

Influence of preparation method on SrMoO4 impurity content and magnetotransport properties of double perovskite Sr2FeMoO6 polycrystals

The magnetic and electric properties of the Sr₂FeMoO₆ compound produced under different preparation conditions were studied. Depending on the preparation condition, a strong variation in the nonmagnetic SrMoO₄ impurity content was found, which in turn determined the metallic or semiconducting behavior of the resistivity of the Sr₂FeMoO₆ compound. There was also evidence that SrMoO₄ played a crucial role in modifying the low magnetic field intergrain tunneling magnetoresistance in Sr₂FeMoO₆. In addition, we have established a simple method to prepare the single phase Sr₂FeMoO₆ polycrystals.     

Structural studies of rhodium doped Sr2RuO4

The influence of Rh doping on the structure of Sr₂RuO₄ has been investigated using neutron powder diffraction methods. The metallic Ru rich compounds adopt a regular K₂NiF₄-type structure, space group I4/mmm, with Ru-O-Ru bond angles of 180°. The structures of the nonmetallic Rh rich compounds crystallise in space group I4/acd and are characterised by tilting of the MO₆ octahedra reducing the Ru-O-Ru angle to about 160°. Irrespective of Rh content the MO₆ polyhedra are not regular octahedra but are elongated along the c direction. The temperature dependence of the structure of Sr₂Ru_0.9Rh_0.1O₄ was investigated and revealed this elongation to be weakly temperature dependent.     

Structure and magnetic properties of perovskite Sr2CuNbO6−δ

Single phase perovskite Sr₂CuNbO_6−δ with a high proportion of Cu¹⁺ ions and oxygen vacancies was synthesized by solid-state reaction. The structure was determined by Rietveld method with space group Pm3m. Isotropic g value was evaluated from electron spin resonance (ESR) measurements. The ESR result is consistent with that of magnetic susceptibility.     

Infrared absorption and reflectivity of double perovskite Sr2FeWO6

In this work we present temperature dependent infrared reflectivity and absorption of Sr₂FeWO₆ between 700 and 17 K measured from 40 to 10000 cm⁻¹. The reflectivity spectra show well defined phonon bands peaking at 143, 227, 377 and 625 cm⁻¹ assigned to overlapping vibrational modes split from those active in cubic perovskite. We have also verified that this compound is structurally stable in the whole temperature range and that its optical gap at ∼750 cm⁻¹ (95 meV) undergoes only a minor high temperature decrease ascribed to new thermally accessible levels.     

Band structure and photocatalytic properties of perovskite-type compound Ca2NiWO6 for water splitting

An oxide semiconductor Ca₂NiWO₆, with double-perovskite crystal structure, was synthesized by solid-state reaction method. The compound Ca₂NiWO₆ was characterized by X-ray diffraction, UV-visible diffuse reflectance, and photoluminescence. The photocatalytic properties of the compound for water splitting were investigated under UV and visible light irradiation. The results showed H₂ evolution was not observed over the compound under visible light irradiation (λ>420 nm) with a 300 W xenon arc lamp when using methanol (CH₃OH) as electron donor, although the compound was responsive to visible light region. Based on the experimental results, a possible band structure was proposed through theoretical calculation of the electronic structure by using the full potential-linearized augmented plane wave (F-LAPW). The band structure and photocatalytic properties were attributed to the special crystal and electronic structures. Due to the oxygen vacancies in the compound, which worked as electron-hole recombination centers, the photocatalytic activity of the compound was low.     

Density functional theory study on two-peak emission of Eu activators in Sr2SiO4

The supercells of pure and Eu-doped Sr₂SiO₄ were theoretically analyzed by density functional theory (DFT) calculations to investigate the typical two-peak emission of Sr₂SiO₄:Eu²⁺, which originates from two different Sr²⁺ (or Eu²⁺) sites in the Sr₂SiO₄ host structure. The Perdew-Wang generalized-gradient approximation (GGA) functional and the double numerical plus d-functions (DND) basis set with effective core potentials (ECP) were employed in the calculations of electronic properties. The electron transfer between Eu²⁺ ions placed at two different crystallographic Sr²⁺ sites was understood based on the accurate assignment of deconvoluted peaks of the two-peak emission to their corresponding crystallographic sites. This study ought to be instructive as a basic guideline to improve the color chromaticity of Sr₂SiO₄:Eu²⁺ for use in white light emitting diodes (WLEDs).     

Electronic and magnetic structure studies of double perovskite Sr2CrReO6 by first-principles calculations

First-principles calculations have been performed to study the electronic band structure and ferromagnetic properties of the double perovskite Sr₂CrReO₆. The density of states (DOS), the total energy, and the spin magnetic moment were calculated. The calculations reveal that the Sr₂CrReO₆ has a stable ferromagnetic ground state and the spin magnetic moment per molecule is 1.0 μ_B, in good agreement with the experimental value. By analysis of the band structure, we propose that the ordered double perovskite Sr₂CrReO₆ is a strong candidate for half-metallic ferromagnet.     

Analysis of low temperature specific heat in Nd0.5Sr0.5MnO3 and R0.5Ca0.5MnO3 (R=Nd,Sm, Dy and Ho) compounds

We report on the specific heat C(T) of doped manganites Nd_0.5Sr_0.5MnO₃, Nd_0.5Ca_0.5MnO₃, Sm_0.5Ca_0.5MnO₃, Dy_0.5Ca_0.5MnO₃ and Ho_0.5Ca_0.5MnO₃ in the temperature range 2?T?300 K using modified rigid ion model (MRIM). The present specific heat results are in general satisfactory agreement with experimental data except at very low temperatures (i.e. T?12 K). Also a sharp peak observed in the experimental results for these compounds around 5 K could not be revealed by our computed results as they arise due to Schottky-like anomaly. Besides, we have reported the cohesive and the thermal properties of these compounds. The results obtained by us are discussed in detail.     

Electronic structure and chemical bonding of phosphorus-contained sulfides InPS4, TI3PS4, and Sn2P2S6

The electronic structure of phosphorus-contained sulfides InPS₄, Tl₃PS₄, and Sn₂P₂S₆ was investigated experimentally with X-ray spectroscopy and theoretically by quantum mechanical calculations. The partial densities of electron states calculated with the ab initio multiple scattering FEFF8 code correspond well to their experimental analogues—the X-ray K- and L_2,3-spectra of sulfur and phosphorus. The good agreement between theory and experiment was also achieved for K-absorption spectra of S and P in the investigated sulfides. In spite of the difference in the crystallographic structure of InPS₄, TI₃PS₄, and Sn₂P₂S₆ that influence the form of K-absorption spectra, the electronic structure of their valence bands are rather similar. This is due to the strong interaction of the P and S atoms, which are the nearest neighbors in the compounds studied. The electron densities of p- and s-states of phosphorus are shifted by about 3 eV to lower energies in comparison to the analogous electron states of sulfur. This is connected with the greater electro-negativity of sulfur, and is confirmed by the calculated electron charge transfer from P to S.     

Calculations of the vibrational frequency and isotopic shift of UF6 and U2F6

Molecular structure, vibrational frequency and infrared intensity of UF6 are investigated by using the revised Perdew Burke-Enzerhof function with the triple-zeta polarized basis set. The calculation results are in good agreement with the experimental values and indicate the existence of a stable U2F6 molecule with a multiple bonded U2 unit. The calculation results also predict that the D3d symmetry of U2F6 is more stable than D3h. The optimized geometries, vibrational frequencies, and infrared intensities are also reported for U2F6 molecules in D3d symmetry. In addition, the isotopic shift of vibrational frequencies of the two molecules under isotopic substitution of uranium atom are also investigated with the same method. The U2F6 molecule is predicted to be better than UF6 for laser uranic isotope separation.     

The anharmonic force fields of PH3, PHF2, PF3, PH5, and H3PO

The cubic and quartic force fields of the title compounds are determined from ab initio SCF calculations using 6-31G^** and TZP/TZ2P basis sets. The computed geometries, vibration-rotation interaction constants, l-doubling constants, anharmonicity constants, and vibrational wavenumbers are compared with the available experimental data, especially for PH₃ and PF₃. Many experimentally unknown spectroscopic constants are predicted. A scaling procedure based on calculated harmonic and anharmonic force fields is proposed for predicting the vibrational wavenumbers of unknown molecules such as PH₅.     

Dielectric properties for the ordered-perovskite cuprate Sr2Cu(Re0.69Ca0.31)O6

Dielectric properties are reported on polycrystalline cubic ordered-perovskite cuprate Sr₂Cu(Re_0.69Ca_0.31)O₆ in the frequency range 10 Hz-100 kHz at temperature from 300 to 500 K. Both the dielectric permittivity and dielectric loss factor are found to be frequency and temperature dependent. The enhanced value of the low frequency dielectric permittivity is associated to ionic polarization and interfacial phenomena. The material is found to possess significantly high dielectric permittivity. The calculated ac conductivity suggests semiconducting behaviour for the Sr₂Cu(Re_0.69Ca_0.31)O₆.     

Atomistic computer simulation studies of Sr2RuO4 and Ca2RuO4

Atomistic static computer simulation techniques have been applied to investigate the energetics of defects and dopants in Sr₂RuO₄ (SRO) and Ca₂RuO₄ (CRO). Interatomic potentials have been derived which reproduced the crystal structures of these systems. Solution energies are calculated for different dopant ions to ascertain the site occupied by the dopant ion in the host lattice. Monovalent and divalent ions are predicted to substitute preferentially at the alkaline-earth site in both the systems. Trivalent cations of smaller ionic radii substitute at the Ru sites while those having larger ionic radii prefer to substitute at the Sr or Ca sites in SRO or CRO systems, respectively. In addition, there is a possibility of self-compensation, where a trivalent cation can substitute at both Sr(Ca) and Ru sites. Tetravalent dopants are found to substitute at the ruthenium sites in both systems.     

X-ray study of phase transitions in the system of solid solutions K2Pb4Nb10O30-Na2Pb4Nb10O30-K6W4Nb6O30

Boundaries of morphotropic phase transitions region in the system of solid solutions K₂Pb₄Nb₁₀O₃₀-Na₂Pb₄Nb₁₀O₃₀-K₆W₄Nb₆O₃₀ with the structure of the tetragonal tungsten bronze have been specified. Presence of the second morphotropic phase transition, perpendicular to the first one has been revealed. The temperature dependences of the structural parameters of some compounds have been investigated. The compounds with high values of Curie temperatures and working temperatures have been obtained.     

Influence of V substitution for Fe on the transport and magnetic properties of Sr2FeMoO6

We have investigated the crystal structure, magnetization and magnetoresistance of the double perovskite compounds Sr₂(Fe_1−xV_x)MoO₆ (0≤x≤0.1). The lattice constants and the cation ordering decrease monotonously with the V content. The Curie temperature, saturation magnetization and low field magnetoresistance of the compounds decrease with increasing x due to the reduced degree of ordering. The resistivity of Sr₂FeMoO₆ and lightly doped samples shows semiconductive behavior, while the samples with higher doping levels exhibit a semiconductor-metal transition around 80 K.