X-ray and Mössbauer study of structural changes in K3Na(FeO4)2

Mixed potassium–sodium ferrate(VI), K₃Na(FeO₄)₂, has been synthesized by precipitation from alkaline solution. At room temperature it decomposes spontaneously giving Fe(III) compounds and ferrate(VI) with a structure similar to that of K₂FeO₄, which is confirmed by X-ray diffraction and Mössbauer spectroscopy.     

Study of the electronic structure and spectral features of TiS2,ZrS2 and HfS2 by the Xα discrete variational method

In order to study the electronic structure of TiS₂, ZrS₂ and HfS₂, selfconsistent charge X_α discrete variational calculations of clusters MeS₆^8- (Me = Ti, Zr, Hf) have been performed. The results obtained are consistent with band structure calculations. The covalency of the chemical bonding is shown to increase in the series TiS₂-ZrS₂-HfS₂, which corresponds to the atomization energy in the series. The results are applied in the interpretation of X-ray emission, photoelectron and optical spectra.     

EPR study of charge compensation of chromium centers in the strontium titanate crystal

Chromium centers and their charge compensation in two single strontium titanate crystals, i.e., SrTiO₃: Cr (0.05 at %) and Sr_0.9995TiO₃: Cr_0.0005 grown with strontium deficiency, have been studied by the electron paramagnetic resonance method. The crystals have been investigated both immediately after growth and after oxidation and reduction procedures. Oxidation and reduction are performed by crystal annealing in a corresponding gas atmosphere at high temperature. Chromium centers associated with oxygen vacancy (Cr³⁺-V _O) are detected in the reduced crystals. It is shown that strontium vacancies are formed in the crystal grown with strontium deficiency, which leads to a lowering of the tetragonal symmetry of (Cr³⁺-V _O) and Cr⁵⁺ centers to the orthorhombic symmetry. Possible compensation mechanisms for charges of various chromium centers are considered.     

Absorption spectroscopic study of the conduction band of titanium dichalcogenides

The L _{2, 3} spectra of titanium in the layered compound TiSe₂ and intercalated compounds Fe_1/2TiSe₂, Cr_1/3TiSe₂, and Fe_1/4TiTe₂ are studied. Theoretical calculations of the electronic structure of these compounds are performed. The experimental data and calculations suggest that the intercalation of the Cr and Fe atoms into the TiSe₂ matrix brings about a partial filling of the Ti 3d states and the spin polarization of the Cr 3d and Fe 3d states. Chemical bonds are formed through the hybridization of the d orbitals of intercalated atoms with the Ti 3d-Se 4p states of the matrix.     

EHCO studies on Cis polyacetylene

To date, band structure calculations on cis polyacetylene have been used on its translational symmetry, i.e. C₄H₄ is the unit cell. In a band structure calculation based on helical symmetry, the mer C₂H₂ becomes the unit cell. The merits of band structure calculations based on helical symmetry are discussed, and it is demonstrated that calculations on cis polyacetylene in either mode give comparable results.     

Mechanism of thermal decomposition of K<Subscript>2</Subscript>FeO<Subscript>4</Subscript> and BaFeO<Subscript>4</Subscript>: A review

This paper presents thermal decomposition of potassium ferrate(VI) (K₂FeO₄) and barium ferrate(VI) (BaFeO₄) in air and nitrogen atmosphere. Mössbauer spectroscopy and nuclear forward scattering (NFS) synchrotron radiation approaches are reviewed to advance understanding of electron-transfer processes involved in reduction of ferrate(VI) to Fe(III) phases. Direct evidences of Fe ^V and Fe ^IV as intermediate iron species using the applied techniques are given. Thermal decomposition of K₂FeO₄ involved Fe ^V, Fe ^IV, and K₃FeO₃ as intermediate species while BaFeO₃ (i.e. Fe ^IV) was the only intermediate species during the decomposition of BaFeO₄. Nature of ferrite species, formed as final Fe(III) species, of thermal decomposition of K₂FeO₄ and BaFeO₄ under different conditions are evaluated. Steps of the mechanisms of thermal decomposition of ferrate(VI), which reasonably explained experimental observations of applied approaches in conjunction with thermal and surface techniques, are summarized.     

Spin state and magnetic interaction of cobalt ions in niobium-doped cobaltites

The magnetic properties and electrical conductivity of La_1?x Sr_xCo_1?x/2Nb _x/2O₃ solid solutions with trivalent cobalt ions are studied. These solid solutions are found to be spin glasses with T _f ～ 25 K. The ferromagnetic component is most pronounced in the composition with x = 0.15. The electrical conductivity decreases with increasing strontium content. The results obtained are interpreted within a model according to which cobalt ions located in the vicinity of strontium ions reside in an intermediate-spin state and the Co³⁺-O-Co³⁺ super-exchange interaction is ferromagnetic because of the local dynamic orbital correlations.     

Excited states and photochemistry of saturated molecules. The vibrational structure in the electronic spectrum of ethane

Ab initio wavefunctions including single excitation CI are used to analyze the origin of the vibrational structure in the low-energy band of the electronic spectrum of ethane. The calculations suggest that the vibrational structure appears to be due to the overlapping progressions from two vertical states (1E_u and 1A_2u), both of which are unstable to distortion to C_2h symmetry, wherein they are characterized by significantly lengthened CC bonds.     

Zn-substituted brownmillerite-type strontium ferrites

New La/Zn substituted strontium ferrites Sr_{(2 ?x)}La _x [Fe_{(2 ? x)}Zn _x ]O₅, (0 ≤ x ≤ 0.3) with brownmillerite- type structure are obtained and studied via X-ray phase analysis and Mössbauer spectroscopy. Depending on the conditions of synthesis, substituting Zn²⁺ cations can either mainly occupy tetrahedral positions in a brownmillerite structure, or be uniformly distributed between the tetrahedral and octahedral positions. It is shown that they are reversibly oxidized by atmospheric oxygen at elevated temperatures.     

Synthesis,crystal structure and characterization of chiral,three-dimensional anhydrous potassium tris(oxalato)ferrate(III)

The synthesis, crystal structure and physical properties of chiral, three-dimensional anhydrous potassium tris(oxalato)ferrate(III) [K₃F_e(C₂O₄)₃] are described. X-ray analysis reveals that the compound crystallized in the chiral space group P4₁32 of cubic system with a=b=c=13.5970(2), Z=4. The structure of the complex consists of infinite anionic [Fe(C₂O₄)₃]³⁻ units with distorted octahedral environment of iron surrounded by six oxygen atoms of three oxalato groups. The anionic units are interlinked through K⁺ ions of three different coordination environments of distorted octahedral, bicapped trigonal prismatic and trigonal prismatic yielding a three-dimensional motif. The two broad absorption bands at 644 and 924 nm from UV–vis–NIR transmittance spectra were ascribed to a ligand-to-metal charge transfer. The room temperature crystalline EPR spectra indicate the high-spin (S=5/2) of Fe(III) ion. The vibrating sample magnetometer measurement shows the paramagnetic nature at room temperature. Thermal studies of the compound confirm the absence of water molecule.     

Correction: Ab initio study of anisotropic mechanical and electronic properties of strained carbon-nitride nanosheet with interlayer bonding

The magnetic and electronic properties of strontium titanate with different carbon dopant configurations are explored using first-principles calculations with a generalized gradient approximation (GGA) and the GGA+U approach. Our results show that the structural stability, electronic properties and magnetic properties of C-doped SrTiCO₃ strongly depend on the distance between carbon dopants. In both GGA and GGA+U calculations, the doping structure is mostly stable with a nonmagnetic feature when the carbon dopants are nearest neighbors, which can be ascribed to the formation of a C-C dimer pair accompanied by stronger C-C and weaker C-Ti hybridizations as the C-C distance becomes smaller. As the C-C distance increases, C-doped SrTiCO₃ changes from an n-type nonmagnetic metal to ferromagnetic/antiferromagnetic half-metal and to an antiferromagnetic/ferromagnetic semiconductor in GGA calculations, while it changes from a nonmagnetic semiconductor to ferromagnetic half-metal and to an antiferromagnetic semiconductor using the GGA+U method. Our work demonstrates the possibility of tailoring the magnetic and electronic properties of C-doped SrTiO₃, which might provide some guidance to extend the applications of strontium titanate as a magnetic or optoelectronic material.

     

Structural,electric and pyroelectric properties of up and down graded PZT multilayers

Multi-layered structures, composed of thin films from materials with different compositions or physical properties, represents a way to obtain enhanced properties or even new functionalities. In this work, lead zirconate titanate PbZr_xTi_1-xO₃ (PZT; x = 0.20, 0.52, 0.80) multilayers were grown by pulsed laser deposition (PLD) on a single crystal strontium titanate (SrTiO₃, STO) substrate, using a strontium ruthenate (SrRuO₃, SRO) film as buffer layer for epitaxial growth, and also as back electrode.Up and down multi-layers were grown and their physical and structural properties were compared, up being the structure in which Zr concentration was varied from 20% near the STO substrate to 80% at the surface, while down is for the structure in which the Zr concentration starts with 80% near the substrate and ends with 20% at the surface. It was found that the electric and pyroelectric properties of the two graded structures are significantly different. The up structure presents electric properties that are comparable with those of single composition PZT films while the properties of the down structure are deteriorated, especially in terms of the leakage current magnitude. Pyroelectric signal could be measured only for the up structure. These differences were attributed to larger density of structural defects in the down structure compared to the up one. This is due to the different growth sequence: up structure starts with tetragonal PZT on cubic substrate (lower lattice mismatch, 1.1%) while down structure starts with rhombohedral PZT on cubic substrate (larger lattice mismatch, almost 5%).     

High-density strontium hydride: An experimental and theoretical study

Powder x-ray diffraction experiments and first-principles calculations have been carried out to investigate the possibility of a structural phase transition, characterized by a change from ionic to covalent bonding, in strontium hydride at pressures greater than 50 GPa. The powder x-ray diffraction results confirm a previously reported transition from the cotunnite structure to the Ni₂In structure at approximately 8 GPa. The Ni₂In phase remained stable up to the maximum experimental pressure of 113 GPa. The first-principles calculations, however, predict that under hydrostatic conditions a transition from the Ni₂In structure to the AlB₂ structure will occur at 115 GPa. A comparison of the pressure-dependent volume yielded by the respective experimental and theoretical studies suggests that in many cases the bulk modulus obtained from experiments carried out under non-hydrostatic conditions may be overestimated. Raman spectroscopy experiments corroborated the previously proposed Ni₂In structure, as the spectra obtained at pressures greater than 8 GPa exhibited two Raman-active modes, consistent with those expected from the Ni₂In structure.     

Magnetic properties and formation of Sr ferrite nanoparticle and Zn,Ti/Ir substituted phases

Strontium hexaferrite nanoparticles are prepared by the chemical sol–gel route. Specific saturation magnetization σ_s and coercive field strength H_c are determined depending on the heat treatment of the gel and iron/strontium ratio in the starting solution. These ultrafine powders with single-domain behavior have specific saturation magnetization σ_s=74 emu/g and coercive field strength H_c=6.4 kOe. Experimental results show that it is necessary to preheat the gel between 400 and 500°C for several hours . It can prevent the formation of intermediate γ-Fe₂O₃ and help to obtain ultrafine strontium ferrite single phase with narrow size distribution at a low annealing temperature. Additionally, the magnetic properties of sol–gel derived strontium ferrite with iron substituted by Zn²⁺, Ti⁴⁺ and Ir⁴⁺ are discussed. For an amount of substitution 0<x⩽0.6, the (Zn, Ti)_x substituted strontium ferrite shows higher values of both coercive field strength and saturation magnetization than the (Zn, Ir)_x substituted phase.     

Investigation of the electronic structure of La1−x(M2+)xCrO3, Cr2O3 and La2O3 by X-ray photoelectron spectroscopy

The electronic structures of lanthanum chromites, pure and doped with magnesium and strontium, have been studied in comparison with Cr₂O₃ and La₂O₃ through the use of X-ray photo-electron spectroscopy. The main peaks and satellites of inner and outer electrons are properly assigned. The band structure of LaCrO₃ is determined by using the XPS data and a calculation based on point charge model. The partially filled Cr(3d) band is localized. The conduction is mainly extrinsic. The measured Fermi-level is close to the valence band indicating a low activation energy in agreement with the results of conductivity measurements.     

Electronic structure of binary zirconium compounds: Cluster calculations using different molecular orbital methods

Cluster calculations of the electronic structure and charge distribution in the refractory compounds ZrX(X = C, N, O) and ZrO₂ have been performed using different molecular orbital methods; a semiempirical LCAO model based on the Mulliken-Wolfsberg-Helmholz approximation, and the Hartree-Fock-Slater model in both the Discrete Variational X_α and Multiple Scattering X_α versions. The main features of chemical bonding are discussed and illustrated by contour level diagrams of some bonding molecular orbitais of the valence band. Finally, the results are compared with X-ray emission and X-ray photoelectron spectra, and also with band structure calculations.     

First principles study of electronic structure and optical properties of LiMTe2 (M=Al, Ga, In) crystals

The results of first principles calculations of the electronic band structure, density of states and frequency dependent dielectric functions of LiAlTe₂, LiGaTe₂ and LiInTe₂ chalcopyrite crystals are reported. The calculations have been carried out within the density functional theory using norm-conserving pseudopotentials and a plane-wave basis. The peculiarities of the imaginary part ε₂(ω) of the complex permittivity are discussed and interpreted on the basis of the obtained band spectra. Our calculations show that the Ga-containing compound is characterized by the largest optical anisotropy compared to Al- and In-containing compounds and, therefore, is the most promising candidate for nonlinear optical applications among considered crystals.     

Specific features of the electrical resistance of half-metallic ferromagnetic alloys Co2CrAl and Co2CrGa

It has been shown by comparing the results of studying the electrical and magnetic properties of the half-metallic ferromagnetic Heusler alloys Co₂CrAl and Co₂CrGa with the calculations of their electronic structure that high values of the electrical resistivity ρ are caused by a disordered distribution of atoms over the sites of the L2₁ cubic structure, and the anomalous behavior of ρ(T) is associated with the transformation of the electronic spectrum due to the ferromagnetic-to-paramagnetic transition.     

High-spin structures of 86,87,88,89Y isotopes

In the present paper nuclear structure properties of ^86,87,88,89Y isotopes have been investigated using large-scale shell-model calculations within the full f _5/2pg9/2 model space. The calculations have been performed with JUN45 and jj44b effective interactions that have been proposed for use in the f _5/2, p _3/2, p _1/2, g _9/2 model space for both protons and neutrons. Reasonable agreement between experimental and calculated values are obtained. This work will add more information to the previous study by the projected shell model [Eur. Phys. J. A 48, 138 (2012)], where full-fledged shell-model calculations were proposed for these nuclei.