Cu5Sb2O8SiO4, an open framework structure with copper uncommon coordination modes: CuO6 and CuO8

Cu₅Sb₂O₈SiO₄ is orthorhombic, space group Pcca, with a = 19.031 (2), b = 9.3944 (6), c = 9.602 (2) Å, and z = 8. Its crystal structure was determined using single crystal X-ray diffraction (R₁ = 0.0432 and wR₂ = 0.1146). The compound presents a parwelite-like structure. Its anionic three-dimensional framework is built up of corner-sharing SbO₆ octahedra and SiO₄ tetrahedra, delimiting interconnected channels wherein Cu²⁺ with different coordination modes, are located. The χ_M and χ_MT product versus T plots, showed the Cu₅Sb₂O₈SiO₄ material to exhibit an anti-ferromagnetic character with a Neel temperature of about 27 K.     

Electronic properties of CuO2 planes

We study a 3-band model of CuO₂ with bare bones interactions: hopping from copper ions to nearest-neighbor oxygenst _pd only, a two-body interaction on the copper ionsU _dd only, and an overlap copper-oxygen parameter _pd. In the limitt _pd U _dd ^1/2, t^*=t _pd ²/2U _dd is the unit of energy with 8_pd ² as the only parameter of significance. If the two-body interaction is invariant under particle-hole interchange, the low-lying states (energy O(t^*)) can be described by conserved particles and can all be classified. They are quite distinct from the high-lying states (energies O(U _dd)). The dynamics of the conserved fermion-like elementary particles are well described by a modifiedt-J model with extended hopping and nearest-neighbor superexchange attraction. This is a scenario known to be favorable to high-temperature superconductivity, but it must be noted that both the hopping range and the exchange are functions of 8_pd ². Moreover, if the Hamiltonian isnot invariant under particle-hole interchange the dynamics becomes much more complex and possibly more inimical to high-temperature superconductivity. This may provide an explanation for the deleterious effect on superconductivity of very small concentrations of certain impurities.     

Superconductivity and Phase Diagram in a CuO2 Monolayer

We recall the van Hove scenario (J. Labbe and J. Bok, Europhys. Lett. 3, 1225 (1987); J. Bouvier and J. Bok, in The Gap Symmetry and Fluctuations in HTSC, J. Bok et al., eds. (Plenum, New York, 1998)) developed since 1987. It explains high T _c, anomalous isotope effect, gap anisotropy etc. We apply this scenario to the superconductive surface layer, obtained by field effect on CaCuO₂ by J. H. Schön et al. Preprint (private communication), to be published). We show that the variation of resistivity and Hall effect with temperature in the normal state can be understood by the presence of a van Hove singularity (v.H.s.) in the band structure. The doping by field effect changes the distance between the Fermi level and the v.H.s.     

SO2Cl2, SOCl2: energy dispersive X-ray diffraction, ab initio and molecular dynamics calculation

This work reports a new protocol for achieving microscopic structural information from theoretical (quantum-chemical (QC) and molecular dynamic (MD) calculations) and experimental methods (X-ray diffraction). The Lennard–Jones (L–J) parameters of the force field (FF) fit satisfactorily the X-ray diffraction results of the liquid samples. The study has been carried out on SOCl₂ and SO₂Cl₂ molecular liquids.     

Temperature-Dependent Local Distortions and the Inhomogeneous CuO2 Plane of La-Based Superconducting Oxides

Local structure of the La-based superconducting copper oxides has been studied by polarized Cu K-edge extended X-ray absorption fine structure (EXAFS) measurements. A study reveals anomalous temperature-dependent Cu–O displacements for the underdoped and optimally doped systems while no such anomaly has been found for the overdoped case. We find that the amplitude of temperature-dependent step-like increase in the correlated Debye–Waller factor of the Cu–O bonds decreases with increasing doping. The results also suggest substantial effect of substitutional disorder, shown by the large decrease in the anomalous temperature and decreased amplitude of the anomaly with increasing disorder.     

Microstructure and properties of (Sr, Ca)CuO2/(Sr, Ca)RuO3 multilayers

Epitaxial multilayer thin films of infinite-layer (Sr, Ca)CuO₂ and perovskite (Sr, Ca)RuO₃ have been prepared on (100) SrTiO₃ substrates by multitarget rf magnetron sputtering. X-ray diffraction analyses revealed that the multilayer structure of (Sr, Ca)CuO₃/(Sr, Ca)RuO₃ was successfully fabricated with a minimum layer thickness of 20 Å. Transmission electron microscopy measurements of the multilayers indicated that there was no dislocation which normally exists in single-layer films with an infinite-layer structure. Resistivities of multilayer films at room temperature ranged from 1 to 10 m cm and showed semiconductor-like dependence against the temperature.     

Self-Consistent Calculation of the Self-Trapping Barrier for a Hole in the CuO2 Plane

We study the behavior of an extra hole added to the ground state of the antiferromagnetically ordered CuO₂ plane. Both the Cu--O hybridization and the O--O transfer are taken into account. As has been shown previously (V. Hizhnyakov and E. Sigmund, Physica C 156, 655 [1988]), the energetically most favorable situation is given when a localized state is formed in which the antiferromagnetic order is locally destroyed and a small ferromagnetic cluster is built up. We show that due to the increase of the magnetic energy induced by the spin–flip process (E _s0.15 eV), the localized and the metastable free-hole states are separated by a barrier of energy (E _b0.05 eV), which can reveal itself in various kinetic phenomena.     

Facile synthesis of luminescent TiO2 nanorods using an anionic surfactant: Their photosensitization and photocatalytic efficiency

Mixed phase (rutile/anatase) TiO₂ nanorods have been synthesized using an anionic surfactant template. Nanorod synthesis has been achieved using mild reaction conditions and without using any rod-shaped template. The shape and crystallinity of the TiO₂ nanomaterials can be modulated by careful control of the surfactant concentration. The novelty of the present work is that the anatase/rutile mixed phase nanorods were formed without using the well-known layer by layer assembly method or the hydrothermal method. These mixed phase TiO₂ nanorods are highly luminescent, can be easily sensitized by fluorescent dyes, show significant dye adsorption ability and function as efficient photocatalysts.     

Antiferromagnetic, Neutral, and Superconducting Band in La2CuO4

The symmetry of the Bloch functions in the conduction band of tetragonal and orthorhombic La₂CuO₄ is examined for the existence of symmetry-adapted and optimally localizable (usual or spin-dependent) Wannier functions. It turns out that such Wannier functions do not exist in the tetragonal phase. In the orthorhombic phase, on the other hand, the Bloch functions can be unitarily transformed in three different ways into optimally localizable Wannier functions: they can be chosen to be adapted to each of the three phases observed in the pure or doped material, that is, to the antiferromagnetic phase, to the superconducting phase or to the phase evincing neither magnetism nor superconductivity. This group-theoretical result is proposed to be interpreted within a nonadiabatic extension of the Heisenberg model. Within this model, atomic-like states represented by these Wannier functions are responsible for the stability of each of the three phases. However, all the three atomic-like states cannot exist in the tetragonal phase, but are stabilized by the orthorhombic distortion of the crystal. A simple model is proposed which may explain the physical properties of La_2-x Sr _x CuO₄ as a function of the Sr concentration x.     

Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4

An extensive theoretical study is performed for wide bandgap crystalline oxides and nitrides, namely, SiO₂, GeO₂, Al₂O₃, Si₃N₄, and Ge₃N₄. Their important polymorphs are considered which are for SiO₂: α-quartz, α- and β-cristobalite and stishovite, for GeO₂: α-quartz, and rutile, for Al₂O₃: α-phase, for Si₃N₄ and Ge₃N₄: α- and β-phases. This work constitutes a comprehensive account of both electronic structure and the elastic properties of these important insulating oxides and nitrides obtained with high accuracy based on density functional theory within the local density approximation. Two different norm-conserving ab initio pseudopotentials have been tested which agree in all respects with the only exception arising for the elastic properties of rutile GeO₂. The agreement with experimental values, when available, are seen to be highly satisfactory. The uniformity and the well convergence of this approach enables an unbiased assessment of important physical parameters within each material and among different insulating oxide and nitrides. The computed static electric susceptibilities are observed to display a strong correlation with their mass densities. There is a marked discrepancy between the considered oxides and nitrides with the latter having sudden increase of density of states away from the respective band edges. This is expected to give rise to excessive carrier scattering which can practically preclude bulk impact ionization process in Si₃N₄ and Ge₃N₄.     

New phases in the systems EuF2:YF3 and EuF2:GdF3

Isothermal pseudo-binary phase studies of the systems EuF₂:YF₃ and EuF₂:GdF₃ have been completed at 1100°C. Three single phase regions corresponding to (1) EuF₃(ss); (a) hexagonal tysonite and (3) orthorhombic LnF₃(ss), Ln = Y, Gd, have been found. The single phase regions for both of these systems are similar and exist at 0–39, 73–82 and 91–100 mole % YF₃; 0–41, 70–77 and 85–100 mole % GdF₃.     

Spectroscopic ellipsometry study of Cu2ZnGeSe4 and Cu2ZnSiSe4 poly-crystals

We report the room temperature spectroscopic ellipsometry study of Cu₂ZnGeSe₄ and Cu₂ZnSiSe₄ crystals, grown by modified Bridgman technique. Optical measurements were performed in the range 1.2–4.6 eV. The spectral dependence of the complex pseudodielectric functions as well as pseudo- complex refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity of Cu₂ZnGeSe₄ and Cu₂ZnSiSe₄ crystals were derived. The observed structures in the optical spectra were analyzed by Adachi's model and attributed to the band edge transitions and higher lying interband transitions. The parameters such as strength, threshold energy, and broadening, corresponding to the E₀, E_1A and E_1B interband transitions, have been determined using the simulated annealing algorithm.     

K2(Rb2,Cs2,Tl2)TeBr6(I6) and Rb3(Cs3)Sb2(Bi2)Br9(I9) perovskite compounds

We systematize available experimental data on the crystal structure of the ternary halides K₂(Rb₂,Cs₂,Tl₂)TeBr₆(I₆) and Rb₃(Cs₃)Sb₂(Bi₂)Br₉(I₉), analyze the general trends in the properties of their single crystals, and examine the key features of the physicochemical interaction in related systems.     

Investigation on solar photocatalytic activity of TiO2 loaded composite: TiO2/Eggshell, TiO2/Clamshell and TiO2/CaCO3

The TiO₂/Eggshell, TiO₂/Clamshell and TiO₂/CaCO₃ loaded composites were prepared by sol-gel method and characterized by XRD and SEM. Their photocatalytic activities were measured through the degradation of Acid Red B under solar light irradiation. The influences of TiO₂ loaded content, heat-treated temperature and time on the photocatalytic activities were reviewed. The effects of irradiation time and dye initial concentration on the photocatalytic degradation were also investigated. The results showed that the photocatalytic activity can be greatly enhanced by appropriate TiO₂ loaded content.     

Natural and persistent superhydrophilicity of SiO2/TiO2 and TiO2/SiO2 bi-layer films

Sol-gel SiO₂/TiO₂ and TiO₂/SiO₂ bi-layer films have been deposited from a polymeric SiO₂ solution and either a polymeric TiO₂ mother solution (MS) or a derived TiO₂ crystalline suspension (CS). The chemical and structural properties of MS and CS bi-layer films heat-treated at 500 °C have been investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscospy. Water contact angle measurements show that MS SiO₂/TiO₂ and CS TiO₂/SiO₂ bi-layer films exhibit a natural superhydrophilicity, but cannot maintain a zero contact angle for a long time over film aging. In contrast, CS SiO₂/TiO₂ bi-layer films exhibit a natural, persistent, and regenerable superhydrophilicity without the need of UV light. Superhydrophilic properties of bi-layer films are discussed with respect to the nature of the TiO₂ single-layer component and arrangement of the bi-layer structure, i.e. TiO₂ underlayer or overlayer.     

The binary system SrF2-MgF2: Phase diagram and study of growth of SrMgF4

The binary phase diagram of the SrF₂-MgF₂ system below 1200°C has been constructed from DTA cooling data. The system shows an eutectic at 48.4 mole % MgF₂ and 871°C, and a peritectic at 50 mole % and 880°C, corresponding to the compound SrMgF₄, previously prepared here by solid state reaction. A solid-solid transition in SrMgF₄ occurs at about 809°C. Small single crystals of SrMgF₄ were obtained by extremely slow cooling of a stoichiometric melt.     

An anionic surfactant-templated synthesis and lamellar ordering of Co(OH)2 and Co3O4 nanodisks

Co(OH)₂ nanodisks were synthesized using a surfactant lamellar mesophase as a soft template. From the Co(OH)₂ nanodisks, porous Co₃O₄ nanodisks could also be obtained after a heat treatment. Co(OH)₂ nanodisks were successfully converted to porous Co₃O₄ nanodisks via thermal oxidation. Using the Co₃O₄ nanodisks as a basic building block and sodium dodecyl sulfate as a structure-directing agent, the lamellar mesostructures could be formed. The lamellar ordering is expected to originate through cooperative interaction-induced self-assembly and evaporation-induced self-assembly.     

Electron microscopic study of polymorphism and defects in AgBiSe2 and AgBiS2

The phase transformations of the ternary compounds AgBiSe₂ and AgBiS₂ are studied by means of electron microscopy and electron diffraction methods. In accordance with the results of other investigators two phase transitions are observed upon heating above room temperature. The transition from the high to the intermediate temperature phase is of the order- disorder type and is accompanied by the formation of a domain structure exhibiting orientation variants. A particular configuration of striations in each domain is attributed to ordering of APBs formed between translation variants.     

Al2O3/SiO2 and HfO2/SiO2 dichroic mirrors for UV solid-state lasers

HfO₂/SiO₂ and Al₂O₃/SiO₂ multilayers to be employed as high reflectance end mirrors in Cerium-doped fluoride solid-state lasers were produced by radio frequency sputtering. The components were designed to have high transmittance at the pumping wavelength and high reflectance in a wavelength band corresponding to the active medium emission. A photoacoustic beam deflection technique and inspection of the irradiated area under a microscope were used to measure the laser induced damage threshold of the mirrors at the pumping wavelength. These coatings were tested in a laser cavity.     

Syntheses and structures of Ta2(WO2)0.87H0.26(PO4)4 and Ta2(MoO2)(PO4)4

Tantalum hydrogen phosphate, β-TaH(PO₄)₂, has a three-dimensional structure that is stable to remarkably high temperature (∼600 °C) presumably due to the presence of strong hydrogen bonds. Impedance measurements indicate a low conductivity, 2.0 × 10⁻⁶ S/cm at 200 °C in 5% H₂. In further studies aimed at enhancing the conductivity by aliovalent doping, we have investigated systematically the synthesis of compounds in the TaH(PO₄)₂-W₂P₂O₁₁ system at 380 °C. As a result, a new phase, Ta₂(WO₂)_0.87H_0.26(PO₄)₄, was identified and subsequently the molybdenum analog Ta₂(MoO₂)(PO₄)₄ was also prepared. The structures were determined by single crystal X-ray diffraction techniques. The structures of Ta₂(WO₂)_0.87H_0.26(PO₄)₄ and Ta₂(MoO₂)(PO₄)₄ can be formally derived from the structure of β-TaH(PO₄)₂ by the replacement of two P-OH protons with an MO₂²⁺ (M = Mo and W) group together with a change in the orientation of some phosphate tetrahedra.