Crystal structure and electronic states of Co and Gd ions in a Gd<Subscript>0.4</Subscript>Sr<Subscript>0.6</Subscript>CoO<Subscript>2.85</Subscript> single crystal

X-ray diffraction and X-ray absorption near edge structure (XANES) spectra have been measured at the Со K-edge and Gd L ₃-edge in GdCoO₃ and Gd_0.4Sr_0.6CoO_2.85 cobaltites. The effect of Sr substitution on the crystal structure and electronic and magnetic states of Co³⁺ ions in a Gd_0.4Sr_0.6CoO_2.85 single crystal has been analyzed. The XANES measurements at the Co K-edge have not showed a noticeable shift of the absorption edge with an increase in the concentration of Sr. This indicates that the effective valence of cobalt does not change. An increase in the intensity of absorption at the Gd L ₃-edge is due to an increase in the degree of hybridization of the Gd(5d) and O(2p) states. The effect of hole doping on the magnetic properties results in the appearance of the ferromagnetic component and in a significant increase in the magnetic moment.     

X-ray emission study of the electronic structure of nanocrystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Valence states of metal ions and the phase composition of nanocrystalline Al₂O₃ (of the original oxide and the oxide irradiated by high-energy Fe⁺ ions) are studied by using x-ray emission Al L_{2, 3} and O Kα spectra. It is established that the shape of the Al L_{2, 3} spectra strongly changes as one goes from the original (bulk) Al₂O₃ to nanocrystalline oxide, while the O Kα spectra remain practically unchanged. Moreover, irradiation by high-energy Fe⁺ ions results in slight additional changes in the x-ray spectral characteristics of the aluminum oxides under study. The obtained experimental data are compared with the results of theoretical calculations of the electronic structure of α and γ phases of Al₂O₃ performed using the LDA formalism. Using the results of x-ray spectral studies, electronic structure calculations, and x-ray diffraction analysis, it is shown that the revealed spectral differences between the nanocrystalline state of aluminum oxide and the bulk material can be interpreted as a phase transition from the α phase to the γ phase of Al₂O₃ with an addition of bayerite.     

Structural features and the microscopic dynamics of the three-component Zr<Subscript>47</Subscript>Cu<Subscript>46</Subscript>Al<Subscript>7</Subscript> system: Equilibrium melt,supercooled melt,and amorphous alloy

The structural and dynamic properties of the three-component Zr₄₇Cu₄₆Al₇ system are subjected to a molecular dynamics simulation in the temperature range T = 250–3000 K at a pressure p = 1.0 bar. The temperature dependences of the Wendt–Abraham parameter and the translation order parameter are used to determine the glass transition temperature in the Zr₄₇Cu₄₆Al₇ system, which is found to be T_c ≈ 750 K. It is found that the bulk amorphous Zr₄₇Cu₄₆Al₇ alloy contains localized regions with an ordered atomic structures. Cluster analysis of configuration simulation data reveals the existence of quasi-icosahedral clusters in amorphous metallic Zr–Cu–Al alloys. The spectral densities of time radial distribution functions of the longitudinal (C?_L(k, ω)) and transverse (C?_T(k, ω)) fluxes are calculated in a wide wavenumber range in order to study the mechanisms of formation of atomic collective excitations in the Zr₄₇Cu₄₆Al₇ system. It was found that a linear combination of three Gaussian functions is sufficient to reproduce the (C?_L(k, ω)) spectra, whereas at least four Gaussian contributions are necessary to exactly describe the (C?_T(k, ω)) spectra of the supercooled melt and the amorphous metallic alloy. It is shown that the collective atomic excitations in the equilibrium melt at T = 3000 K and in the amorphous metallic alloy at T = 250 K are characterized by two dispersion acoustic-like branches related with longitudinal and transverse polarizations.     

Tunneling of 6<Emphasis Type="Italic">s</Emphasis><Superscript><Emphasis Type="Italic">2</Emphasis></Superscript> electrons in PbO and Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>: XANES spectroscopy and DFT calculations

The results of XANES spectroscopy and DFT calculations are evidence for the occurrence of anomalous 2p_2/3→6s_1/2 electronic transitions in lead(II) and bismuth(III) oxides. The observed transitions may be due to the tunneling of outer 6s² electrons.     

Ionic displacements and structure of disordered PbFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript>

A sequence of structural models of unit cells of complex oxides with perovskite-type structure has been constructed to refine the average structure of PbFe_1/2Nb_1/2O₃ (PFN) at temperatures above the ferroelectric Curie point (T _C ～ 110°C). Owing to the analysis of the probability theory considerations, each model is characterized by only two positional fitting parameters. With the use of the intensities of 95 symmetrically independent X-ray reflections from a PFN single crystal at 160°C, a model with the R factor below 3% (K = 2.44%) was chosen. The ravine method has been used to verify the existence of a single minimum for the R factor with respect to the fitting parameter.     

Atomic and Electronic Structures of Metal-Rich Noncentrosymmetric ZrO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The atomic and electronic structures of metal-rich noncentrosymmetric zirconium oxide synthesized by the ion beam sputtering of a metallic target in an oxygen atmosphere has been studied by X-ray photoelectron spectroscopy, Raman scattering, spectral ellipsometry, and quantum-chemical simulation. It has been established that ZrO_{x < 2} consists of ZrO₂, metallic Zr, and zirconium suboxides ZrO_y. The stoichiometry parameter of ZrO_y has been estimated. It has been shown that the optical properties of ZrO_{x < 2} are determined by metallic Zr. A model of fluctuation of the width of the band gap and a potential for electrons and holes in ZrO_{x < 2} based on spatial fluctuations of the chemical composition has been proposed.     

Superposition of M<Subscript>5</Subscript>X<Subscript>5</Subscript> superstructures and X-ray diffraction in TiO<Subscript>1.0</Subscript> titanium monoxide

The interpretation of diffraction spectra of ordered high-temperature phases of solid solutions and strongly nonstoichiometric compounds is discussed. It has been shown that variations of the intensities of superstructure reflections, which cannot be explained within simple ordering models, can be due to the superposition of superstructures with different symmetries in the matrix of the basis crystal structure. Using an example of atom–vacancy ordering in TiO_1.0 titanium monoxide, a model of the order–order transition state formed by the superposition of low-temperature monoclinic (space group A2/m (C2/m)) and high-temperature cubic (space group Pm3?m) M₅X₅ superstructures has been proposed. It has been shown that the transition state is thermodynamically equilibrium and should be implemented instead of the M₅X₅ cubic superstructure. The transition state model can be considered as an M_(5–i)X_(5–i) superstructure (i = 1, 14/18, 11/18) with the monoclinic symmetry (space group P1m1).     

Pressure-induced structural transformations,orbital order and antiferromagnetism in La<Subscript>0.75</Subscript>Ca<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript>

High pressure evolution of structural, vibrational and magnetic properties of La_0.75Ca_0.25MnO₃ was studied by means of X-ray diffraction and Raman spectroscopy up to 39 GPa, and neutron diffraction up to 7.5 GPa. The stability of different magnetic ground states, orbital configurations and structural modifications were investigated by LDA + U electronic structure calculations. A change of octahedral tilts corresponding to the transformation of orthorhombic crystal structure from the Pnma symmetry to the Immaone occurs above P ~ 6 GPa. At the same time, the evolution of the orthorhombic lattice distortion evidences an appearance of the e _g d _{x² ? z²} orbital polarization at high pressures. The magnetic order in La_0.75Ca_0.25MnO₃ undergoes a continuous transition from the ferromagnetic 3D metallic (FM) ground state to the A-type antiferromagnetic (AFM) state of assumedly 2D pseudo-metallic character under pressure, that starts at about 1 GPa and extends possibly to 20–30 GPa.     

Electronic structure of SiO<Subscript>2</Subscript>: An X-ray emission spectroscopic and density functional theoretical study

The electronic structure of SiO₂ is investigated by means of valence to core X-ray emission spectroscopy and quantum-mechanical calculations in the density functional theory approximation. Analysis of a complete set of SiK_α1, SiL_{2, 3}, and OKα X-ray emission and XPS spectra along with the calculated data provides comprehensive information on chemical interactions that occur in SiO₂.     

Temperature dependence of the kinetic energy in the Zr<Subscript>40</Subscript>Be<Subscript>60</Subscript> amorphous alloy

The average kinetic energy 〈E(T)〉 of the atomic nucleus for each element of the amorphous alloy Zr₄₀Be₆₀ in the temperature range 10–300 K has been measured for the first time using VESUVIO spectrometer (ISIS). The experimental values of 〈E(T)〉 have been compared to the partial ZrBe spectra refined by a recursion method based on the data obtained with thermal neutron scattering. The satisfactory agreement has been reached with the calculations using partial spectra based on thermal neutron spectra obtained with recursion method. In addition, the experimental data have been compared to the Debye model. The measurements at different temperatures (10, 200, and 300 K) will provide an opportunity to evaluate the significance of anharmonicity in the dynamics of metallic glasses.     

Experimental Study and Analysis of Absorption Spectra of Ni<Superscript>2+</Superscript> Ions in Nickel Orthoborate Ni<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>2</Subscript>

The results of studies of the absorption spectra of nickel orthoborate Ni₃(BO₃)₂ in the range of electronic d–d-transitions are reported. The obtained data are analyzed in the framework of the crystal field theory. The Ni²⁺ ions are located in two crystallographically nonequivalent positions 2a and 4f with point symmetry groups C_2h and C₂, respectively, surrounded by six oxygen ions forming deformed octahedra. The absorption spectra exhibit three intense bands corresponding to spin-resolved transitions from the ground state of nickel ion ³A_2g (³F) to the sublevels of the ³T_2g (³F), ³T_1g (³F) and ³T_1g (³P) triplets split by the spinorbit interaction and the rhombic component of the crystal field. At temperatures below 100 K, the spectra exhibit a thin structure, in which phonon-free lines can be distinguished. Comparison of the calculated frequencies of the zero-phonon transitions with the experimental data allows estimating parameters of the crystal field acting on the nickel ions in the 2a- and 4f-positions, as well as the parameters of electrostatic interaction between the 3d electrons and spin-orbit interaction constants.     

Suppression of superconductivity in layered Bi<Subscript>4</Subscript>O<Subscript>4</Subscript>S<Subscript>3</Subscript> by Ag doping

We report X-ray diffraction, magnetization and transport measurements for polycrystalline samples of the new layered superconductor Bi_4?x Ag _x O₄S₃(0 ≤ x ≤ 0.2). The superconducting transition temperature (T _C) decreases gradually and finally suppressed when x < 0.10. Accordingly, the resistivity changes from a metallic behavior for x < 0.1 to a semiconductor-like behavior for x > 0.1. The analysis of Seebeck coefficient shows there are two types of electron-like carriers dominate at different temperature regions, indicative of a multiband effect responsible for the transport properties. The suppression of superconductivity and the increased resistivity can be attributed to a shift of the Fermi level to the lower-energy side upon doping, which reduces the density of states at E _F. Further, our result indicates the superconductivity in Bi₄O₄S₃ is intrinsic and the dopant Ag prefers to enter the BiS₂ layers, which may essentially modify the electronic structure.     

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> and Ni<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript>: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co₃V₂O₈ and Co₃V₂O₈ single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co₃V₂O₈ is shifted toward low energies by 0.25–0.3 eV as compared to Co₃V₂O₈. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.     

Longitudinal structure function <Emphasis Type="Italic">F</Emphasis><Subscript>L</Subscript>: Perturbative QCD and <Emphasis Type="Italic">k</Emphasis><Subscript>T</Subscript>-factorization versus experimental data at fixed <Emphasis Type="Italic">W</Emphasis>

We use the results for the structure function F_L for a gluon target with a nonzero transverse momentum squared at the order α_s, obtained in our previous paper, for comparison with recent H1 experimental data for F_L at fixed W values and with collinear GRV predictions in the leading-order and next-to-leading-order approximations.     

Pressure dependence of the superconducting state parameters of a binary Ca<Subscript>70</Subscript>Mg<Subscript>30</Subscript> metallic glass superconductor

Theoretical computation of the pressure dependence of superconducting state parameters of a binary Ca₇₀Mg₃₀ metallic glass has been performed using the model potential formalism. Explicit expressions have been derived for the volume dependence of the electron-phonon coupling strength λ and the Coulomb pseudopotential μ*, considering the variation of the Fermi momentum k _F and Debye temperature θ_D with volume. Well-known Ashcroft’s empty core model pseudopotential and five different types of the local-field correction functions, namely, Hartree, Taylor, Ichimaru-Utsumi, Farid et al. and Sarkar et al. have been used for obtaining pressure dependence of transition temperature T _C and the logarithmic volume derivative Φ of the effective interaction strength N ₀ V for the metallic glass superconductor. It has been obtained that T _C of Ca₇₀Mg₃₀ metallic glass decreases rapidly with increasing pressure up to 60% decrease in the volume, for which the μ* and Φ curves show a linear nature. The superconducting phase disappears at about 60% decrease in the volume.     

X-Ray Spectral Studies of the Interface Interaction in CuO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/MWCNTs Nanocomposite

Results of a comprehensive study of the interface interaction of a nanostructured CuO_x and multiwalled carbon nanotubes (MWCNTs) in CuO_x/MWCNT nanocomposite by X-ray absorption spectroscopy (XANES, NEXAFS) and X-ray photoelectron spectroscopy (XPS) methods using a synchrotron radiation are presented. It is established that a nanostructured CuO_x in CuO_x/MWCNT nanocomposite is predominantly formed by CuO and has the form of flakelike particles 200–500 nm in size uniformly dispersed over an array of nanotubes. A chemical interaction of CuO_x and nanotubes with formation of covalent carbon–oxygen bonds, which does not lead to a significant destruction of the outer layers of carbon nanotubes, is observed at the interfaces of the nanocomposite.     

Magnetic circular dichroism and optical absorption in TmAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The polarized spectra of absorption and magnetic circular dichroism in a TmAl₃(BO₃)₄ single crystal are studied in the region of ³ H ₆ → ³ F ₄, ³ H ₆ → ³ F ₃, and ³ H ₆ → ³ F ₂ electronic transitions in the Tm³⁺ ion. The structure of the spectra is interpreted qualitatively. It is shown that the magnetic circular dichroism of the ³ H ₆ → ³ F ₄ transition is determined by the contribution from the splitting of the ground state, whereas the magnetic circular dichroism of the ³ H ₆ → ³ F ₃ transition is governed by the contribution from the splitting of an excited state in a trigonal crystal field.     

Structural transformations of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals with variations in temperature

Single crystals of the K₃H(SO₄)₂ compound are investigated using X-ray diffraction on Xcalibur S and Bruker diffractometers. The structure of the low-temperature monoclinic phase is refined (space group C2/c, z = 4, a = 14.698(1) Å, b = 5.683(1) Å, c = 9.783(1) Å, β = 103.01(1)°, T = 293 K, Bruker diffractometer), the structural phase transition is revealed, and the structure of the high-temperature trigonal phase is determined (space group R \(\bar 3\) m, z = 3, a = 5.73(1) Å,c = 21.51(1) Å,T = 458 K, Xcalibur diffractometer).     

High-Temperature Specific Heat of the TmBiGeO<Subscript>5</Subscript> and YbBiGeO<Subscript>5</Subscript> Compounds

The TmBiGeO₅ and YbBiGeO₅ compounds have been synthesized from Tm₂O₃ (Yb₂O₃), Bi₂O₃, and GeO₂ oxides by the solid-state synthesis with successive burning at 1003, 1073, 1123, 1143, 1173, and 1223 K. High-temperature specific heat of the oxide compounds has been measured by differential scanning calorimetry. Basing on the experimental dependences C_p = f(T), the thermodynamic properties of the oxide compounds, i.e., the enthalpy and entropy variations, have been calculated.     

Fine structure and mechanical properties of the shape-memory Ni<Subscript>50</Subscript>Ti<Subscript>32</Subscript>Hf<Subscript>18</Subscript> alloy rapidly quenched by spinning

We have reported the results of investigations of the structure and chemical and phase compositions of the amorphous Ni₅₀Ti₃₂Hf₁₈ alloy prepared by rapid quenching from melt by spinning and subjected to heat treatments. The specific features of the fine polycrystalline alloy structure formation depending on the heat-treatment mode have been studied by transmission and scanning electron microscopy, chemical microanalysis, electron diffraction, and X-ray diffraction analysis. According to the data on the temperature behavior of electrical resistivity, critical temperatures of devitrification and subsequent thermoelastic martensitic transformation B2 → B19′ have been determined. The mechanical properties in different heat-treatment modes have been investigated.