Spin-glass-like behaviour in IrSr2RECu2O8 (RE=Sm and Eu)

We report the results of magnetic and specific heat measurements on the 1212-type compounds IrSr₂RECu₂O₈ with RE=Sm and Eu, prepared by high-pressure and high-temperature synthesis. The magnetic susceptibility of these compounds shows a large difference in the temperature dependence of the magnetization measured under zero-field-cooled and field-cooled conditions below 87 and 71 K, respectively, and upon further cooling below ∼10 K substantial maxima are observed too. Further AC susceptibility measurements support a glassy behaviour in lower magnetic transitions whereas the specific heat measurements do not show the typical long-range ordering commonly displayed in ferro, ferri or antiferromagnetic transitions. Hysteresis loops suggest the presence of magnetic clusters in the otherwise paramagnetic zone, indicating that these compounds probably display a reentrant spin-glass transition. Results are presented and discussed.     

A novel ferrimagnetic irido-cuprate: IrSr2GdCu2O8

We have performed an investigation of the structural, microstructural and magnetic properties of the new compound IrSr₂GdCu₂O₈. The sample was prepared under high temperature (∼1393 K) and high-pressure conditions (∼60 Kbars) in a Belt type apparatus. X-ray diffraction (XRD) analysis shows that this irido-cuprate is isostructural with the corresponding Ru-1212 phase. Structurally, this material shows an interesting hierarchy of ordering phenomena, whose observation actually depends on the technique used to analyze the material: from a “simple” cell a_p×a_p×3a_p which is supported by XRD, through a “diagonal” one, as seen by SAED, to a microdomain texture of this last one cell supported by HREM. A ferrimagnetic Ir^IV-Gd^III spin ordering is observed below 15 K. The iridium oxidation state seems to be +4.     

Crystallization and microstructure of Li2O-Al2O3-SiO2 glass containing complex nucleating agent

The crystallization and microstructure of Li₂O-Al₂O₃-SiO₂ (LAS) glass ceramic with complex nucleating agents (TiO₂ + ZrO₂ + P₂O₅ +/or F⁻) are investigated by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the effects of P₂O₅ and F⁻ on the crystallization of LAS glass are also analyzed. The introduction of both P₂O₅ and F⁻ promotes the crystallization of LAS glass by decreasing the crystallization temperature and adjusting the crystallization kinetic parameters, allows a direct formation of β-spodumene without the transformation of LiAl(SiO₃)₂ into β-spodumene and as a result, increases the crystal size and crystallinity of LAS glass ceramic.     

Microstructural and transport properties in substituted Bi2Sr2CaCu2O8+δ-modulated compounds

X-ray powder diffraction and resistivity measurements were performed on Bi₂Sr₂CaCu₂O_8+δ ceramics substituted by Y and Zn for Ca and Cu sites, respectively. X-ray diffraction patterns show an incommensurate modulated structure along the b-axis. The structural refinements were carried out using the four-dimensional space group Bbmb(0β1)0 0 0. From the X-ray peak profiles analysis, an anisotropic line-shape broadening was observed. The use of the “Williamson and Hall” method allows distinguishing the origin of broadening as mainly due to microstrains. A large transition from a metallic to semiconductor behaviour is observed on the resistivity curves at x≈0.4 for Bi₂Sr₂Ca_1−xY_xCu₂O_8+δ and at x^′≈0.36 for Bi₂Sr₂Ca_1−xY_xCu_1.94Zn_0.06O_8+δ, which can be also correlated to the defects. Oppositely to the metallic behaviour, which satisfies the Mathiessen's rule, the semiconducting one can be modelled by a variable range hopping process.     

The Crystal Structure of Sr2TiSi2O8

Sr₂TiSi₂O₈ single crystals were grown by Czochralski pulling and from a high-temperature solution. X-ray diffractometry revealed the modulated crystal structure of Sr₂TiSi₂O₈ to belong to the 5D superspace group P4bm (−α, α, 1/2; α, α, 1/2) with α=0.3. Atomic positions, anisotropic displacement factors and positional modulation parameters for Sr₂TiSi₂O₈ are determined and discussed. The positional modulation is further investigated by electron diffraction and high-resolution transmission electron microscopy. In the latter experiments, the 2D modulation appears to be superimposed by some 1D modulation waves. This effect is discussed in terms of growth conditions.     

Structure and magnetic properties of AgFeP2O7

AgFeP₂O₇ has been synthesized by flux crystallization and characterized by single crystal and powder X-ray diffraction (sp. gr. P2₁/c, a=7.3298(2), b=7.9702(2), c=9.5653(2) Å, β=111.842(1)°, V=518.68(2) Å³) and FTIR-spectroscopy. The structure is composed of isolated iron octahedra and phosphate tetrahedra interconnected into 3D network with hexagonal channels, where silver counter-ions are located. The magnetic behavior of the compound approaches the Curie-Weiss equation with a Weiss constant θ=−165.9 K indicating strong antiferromagnetic interaction between iron(III) ions.     

Synthesis, band structure, and optical properties of Ba2ZnV2O8

A novel compound Ba₂ZnV₂O₈ has been synthesized in high temperature solution reaction and its crystal structure has been characterized by means of single crystal X-ray diffraction analysis. It crystallizes in monoclinic system and belongs to space group P2₁/c with a=7.9050(16), b=16.149(3), , β=90.49(3). It builds up from 1-D branchy chains of [ZnV₂O₈⁴⁻]_∞, and the Ba²⁺ cations are located in the space among these chains. The IR spectrum, ultraviolet-visible diffuse reflection integral spectrum and fluorescent spectra of this compound have been investigated. The calculated results of energy band structure by the density functional theory method show that the solid-state compound of Ba₂ZnV₂O₈ is an insulator with direct band gap of 3.48 eV. The calculated total and partial density of states indicate that the top valence bands are contributions from the mixings of O-2p, V-3d, and Zn-3d states and low conduction bands mostly originate from unoccupied antibonding states between the V-3d and O-2p states. The V-O bonds are mostly covalence characters and Zn-O bonds are mostly ionic interactions, and the ionic interaction strength is stronger between the Ba-O than between the Zn-O. The refractive index of n_x, n_y, and n_z is estimated to be 1.7453, 1.7469, and 1.7126, respectively, at wavelength of 1060 nm for Ba₂ZnV₂O₈ crystal.     

High-pressure transitions in MgAl2O4 and a new high-pressure phase of Mg2Al2O5

Phase transitions in MgAl₂O₄ were examined at 21-27 GPa and 1400-2500 °C using a multianvil apparatus. A mixture of MgO and Al₂O₃ corundum that are high-pressure dissociation products of MgAl₂O₄ spinel combines into calcium-ferrite type MgAl₂O₄ at 26-27 GPa and 1400-2000 °C. At temperature above 2000 °C at pressure below 25.5 GPa, a mixture of Al₂O₃ corundum and a new phase with Mg₂Al₂O₅ composition is stable. The transition boundary between the two fields has a strongly negative pressure-temperature slope. Structure analysis and Rietveld refinement on the basis of the powder X-ray diffraction profile of the Mg₂Al₂O₅ phase indicated that the phase represented a new structure type with orthorhombic symmetry (Pbam), and the lattice parameters were determined as a=9.3710(6) Å, b=12.1952(6) Å, c=2.7916(2) Å, V=319.03(3) Å³, Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O₆ octahedra, and contains chains of edge-sharing octahedra running along the c-axis. A part of Mg atoms are accommodated in six-coordinated trigonal prism sites in tunnels surrounded by the chains of edge-sharing (Mg, Al)O₆ octahedra. The structure is related with that of ludwigite (Mg, Fe²⁺)₂(Fe³⁺, Al)(BO₃)O₂. The molar volume of the Mg₂Al₂O₅ phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al₂O₃ corundum. High-pressure dissociation to the mixture of corundum-type phase and the phase with ludwigite-related structure has been found only in MgAl₂O₄ among various A²⁺B³⁺₂O₄ compounds.     

Synthesis, structure and magnetic properties of new vanadate PbCo2V2O8

A new vanadate PbCo₂V₂O₈ was obtained through the study of PbO-CoO-V₂O₅ ternary system. The crystal structure was determined by Rietveld method, indicating that PbCo₂V₂O₈ has a tetragonal structure of space group I41cd with a spiral chain along the c-axis. Magnetic properties of the titled compound were investigated by means of susceptibility, magnetization, and heat capacity measurements. The results show that PbCo₂V₂O₈ is a quasi-one-dimensional canted antiferromagnet with Neel temperature of ∼4 K, being consistent with its crystal structure.     

Effect of the synthesis conditions on the magnetic and electrical properties of the BaFeO3−x oxide: A metamagnetic behavior

The BaFeO_2.95 oxide has been obtained from thermal decomposition of the [BaFe(C₃H₂O₄)₂(H₂O)₄] metallo-organic precursor at 800 °C under atmospheric oxygen pressure as small and homogeneous particles. From electronic paramagnetic resonance data, a metallic behavior in the 230-130 K temperature range has been observed. Magnetic measurements confirm the existence of a ferro-antiferromagnetic transition at 178 K. The magnetic properties of the BaFeO_2.95 oxide are strongly dependent on both temperature and magnetic field with a metamagnetic behavior. The synthesis conditions play an important role on the morphology and the electrical and magnetic properties. The syntherization of the sample produces a dramatic change in the transport properties and the existence of conductivity disappears.     

Hydrothermal synthesis of antimony oxychloride and oxide nanocrystals: Sb4O5Cl2, Sb8O11Cl2, and Sb2O3

We described herein a facile solution-phase route to three nanocrystals of antimony oxychlorides and oxides (Sb₄O₅Cl₂, Sb₈O₁₁Cl₂, and Sb₂O₃), whose morphologies and phases were varied with the pH value of a reaction mixture or composition of a mixed solvent. In particular, the solvent composition controlled the selective preparation of cubic Sb₂O₃ (senarmontite) and orthorhombic Sb₂O₃ (valentinite). Both cubic and orthorhombic Sb₂O₃ samples exhibited strong emission properties.     

XANES Study on the Generation and Distribution of Holes via Ca Substitution and O Doping in Cu(Ba0.8Sr0.2)2(Yb1−xCax)Cu2O6+z

Generation of holes is facilitated in the Cu(Ba_0.8Sr_0.2)₂ (Yb_1−xCa_x)Cu₂O_6+z (Cu-1212) system by two independent ways, i.e., by Ca substitution (0≤x≤0.35) and O doping (0<z<1). The distribution of holes between the CuO₂-(Yb_1−xCa_x)-CuO₂ block containing two identical superconductive CuO₂ planes and the “charge-reservoir” block consisting of a single CuO_z chain has been quantitatively investigated by means of O K-edge and Cu L_2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy. The resultant values for the CuO₂-plane hole concentration are compared with those calculated employing the bond-valence-sum (BVS) method from the neutron powder diffraction (NPD) data previously reported for the same samples. The results of the two methods are in good agreement. The two independent hole-doping ways are found to result in different distributions of holes over the crystal, i.e., different ratios of hole numbers at the CuO₂ plane and the CuO_z chain. With Ca substitution holes are directed efficiently into the CuO₂ plane, while for O doping holes are more homogeneously distributed between the CuO₂ plane and the CuO_z chain. Moreover, the value of T_c at a fixed CuO₂-plane hole concentration is shown to be higher for Ca-substituted than for O-doped samples.     

Structural studies on a high-pressure polymorph of NaYSi2O6

High-pressure synthesis experiments in the system Na₂O-Y₂O₃-SiO₂ revealed the existence of a previously unknown polymorph of NaYSi₂O₆ or Na₃Y₃[Si₃O₉]₂ which was quenched from 3.0 GPa and 1000 °C. Structural investigations on this modification have been performed using single-crystal X-ray diffraction data collected at ambient conditions. Furthermore, unpolarized micro-Raman spectra have been obtained from single-crystal material. The high-P modification of NaYSi₂O₆ crystallizes in the centrosymmetric space group C2/c with 12 formula units per cell (a=8.2131(9) Å, b=10.3983(14) Å, c=17.6542(21) Å, β=100.804(9)°, V=1481.0(3) Å³, R(|F|)=0.033 for 1142 independent observed reflections) and belongs to the group of cyclo-silicates. Basic building units are isolated three-membered [Si₃O₉] rings located in layers parallel to (010). Within a single layer the rings are concentrated in strings parallel to [100]. The sequence of directedness of up (U) or down (D) pointing tetrahedra of a single ring is UUU or DDD, respectively. Stacking of the layers parallel to b results in the formation of a three-dimensional structure in which yttrium and sodium cations are incorporated for charge compensation. In more detail, four non-tetrahedral cation positions can be differentiated which are coordinated by 6 and 8 oxygen ligands. Refinements of the site occupancies did not reveal any indication for mixed Na-Y populations on these positions. Finally, several geometrical parameters of rings occurring in cyclo-trisilicate structures have been compiled and are discussed.     

Structure and properties of the CaFe2O4-type cobalt oxide CaCo2O4

The calcium cobalt oxide CaCo₂O₄ was synthesized for the first time and characterized from a powder X-ray diffraction study, measuring magnetic susceptibility, specific heat, electrical resistivity, and thermoelectric power. CaCo₂O₄ crystallizes in the CaFe₂O₄ (calcium ferrite)-type structure, consisting of an edge- and corner-shared CoO₆ octahedral network. The structure of CaCo₂O₄ belongs to an orthorhombic system (space group: Pnma) with lattice parameters, a=8.789(2) Å, b=2.9006(7) Å and c=10.282(3) Å. Curie-Weiss-like behavior in magnetic susceptibility with the nearly trivalent cobalt low-spin state (Co³⁺, 3d, S=0), semiconductor-like temperature dependence of resistivity (ρ=3×10⁻¹ Ω cm at 380 K) with dominant hopping conduction at low temperature, metallic-temperature-dependent large thermoelectric power (Seebeck coefficient: S=+147 μV/K at 380 K), and Schottky-type specific heat with a small Sommerfeld constant (γ=4.48(7) mJ/Co mol K²), were observed. These results suggest that the compound possesses a metallic electronic state with a small density of states at the Fermi level. The doped holes are localized at low temperatures due to disorder in the crystal. The carriers probably originate from slight off-stoichiometry of the phase. It was also found that S tends to increase even more beyond 380 K. The large S is possibly attributed to residual spin entropy and orbital degeneracy coupled with charges by strong electron correlation in the cobalt oxides.     

Two mixed valent molybdenophosphates with a tunnel structure closely related to K0.17MoP2O7: Pb2(PbO)2Mo8(P2O7)8 and PbK2Mo8(P2O7)8

Two new mixed valent Mo(III)/Mo(IV) diphosphates containing lead Pb₂(PbO)₂Mo₈(P₂O₇)₈ and PbK₂Mo₈(P₂O₇)₈ have been synthesized. The [Mo₈P₁₆O₅₆]∞ frameworks of these phosphates are closely related to that of K_0.17MoP₂O₇: the MoO₆ octahedra and P₂O₇ groups form two sorts of large eight-sided tunnels. They are occupied in an ordered way by PbO chains and Pb²⁺ cations in Pb₂(PbO)₂Mo₈(P₂O₇)₈ and by K⁺ and Pb²⁺ cations in PbK₂Mo₈(P₂O₇)₈. It results in different symmetries of these two structures, which are tetragonal and monoclinic, respectively, showing the great flexibility of these mixed frameworks, susceptible to accommodate various species with different sizes.     

Structure and antiferroelectric properties of cesium niobate, Cs2Nb4O11

The compound cesium niobate, Cs₂Nb₄O₁₁, is an antiferroelectric, as demonstrated by double hysteresis loops in the electric field versus polarization plot. The crystal structure refinement by X-ray diffraction at both 100 and 297 K shows it to have a centrosymmetric structure in point group mmm and orthorhombic space group Pnna, which is consistent with its antiferroelectric behavior. The 100-K structure data is reported herein. The lattice is comprised of niobium-centered tetrahedra and octahedra connected through shared vertices and edges; cesium atoms occupy channels afforded by the three-dimensional polyhedral network. Antiferroelectricity is produced by antiparallel displacements of niobium atoms along the c-axis at the phase transition temperature of 165 °C. The critical field for onset of ferroelectric behavior in a single-crystal sample is 9.5 kV/cm at room temperature.     

Cristallisation de LiFe5O8 dans un verre 0,9 Li2B2O4 - 0,1 LiFe5O8

X-Ray diffraction, transmission electron microscopy, and magnetic measurements are used to study the crystallization of an amorphous compound: Li₂B₂O₄ (90 mole%)-LiFe₅O₈ (10 mole%). The crystalline phase which first appears in the amorphous matrix is LiFe₅O₈. The average particle size (50 to 300 Å) may be controlled by varying the temperature of annealing and/or the time of annealing. The crystallization kinetics are similar to those of metallic glasses. The fraction transformed, x, as a function of time, satisfies the Johnson-Mehl-Avrami equation with an exponent n of 0.75. The activation energy for the crystallization process is approximately 0.6 eV. Both these values characterize a primary crystallization.     

Synthesis, structure and properties of new chain cuprates, Na3Cu2O4 and Na8Cu5O10

Na₃Cu₂O₄ and Na₈Cu₅O₁₀ were prepared via the azide/nitrate route from stoichiometric mixtures of the precursors CuO, NaN₃ and NaNO₃. Single crystals have been grown by subsequent annealing of the as prepared powders at 500 °C for 2000 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structures (Na₃Cu₂O₄: P2₁/n, Z=4, a=5.7046(2), b=11.0591(4), c=8.0261(3) Å, β=108.389(1)°, 2516 independent reflections, R₁(all)=0.0813, wR₂ (all)=0.1223; Na₈Cu₅O₁₀: Cm, Z=2, a=8.228(1), b=13.929(2), , β=111.718(2)°, 2949 independent reflections, R₁(all)=0.0349, wR₂ (all)=0.0850), the main feature of both crystal structures are CuO₂ chains built up from planar, edge-sharing CuO₄ squares. From the analysis of the Cu-O bond lengths, the valence states of either +2 or +3 can be unambiguously assigned to each copper atom. In Na₃Cu₂O₄ these ions alternate in the chains, in Na₈Cu₅O₁₀ the periodically repeated part consists of five atoms according to Cu^II-Cu^II-Cu^III-Cu^II-Cu^III. The magnetic susceptibilities show the dominance of antiferromagnetic interactions. At high temperatures the compounds exhibit Curie-Weiss behaviour (Na₃Cu₂O₄: , , Na₈Cu₅O₁₀: , , magnetic moments per divalent copper ion). Antiferromagmetic ordering is observed to occur in these compounds below 13 K (Na₃Cu₂O₄) and 24 K (Na₈Cu₅O₁₀).     

Electronic properties and rare-earth ions photoluminescence behaviors in borosilicate: SrB2Si2O8

Undoped and RE ions doped SrB₂Si₂O₈ were successfully synthesized. After the application of UV and VUV spectroscopy measurements, we made a novel discovery that the emission of SrB₂Si₂O₈:Eu prepared in air can be switched between red and blue by the different excitations. The information is that quite a part of Eu³⁺ was spontaneously reduced to Eu²⁺ in air. The PL properties of Eu²⁺ in VUV and Eu³⁺, Ce³⁺ and Tb³⁺ in UV-VUV region in SrB₂Si₂O₈ were evaluated for the first time. The excitation mechanisms of the O²⁻-Eu³⁺ CT, Ce³⁺f-d and Tb³⁺f-d transitions in UV region as well as the Eu³⁺f-d, O²⁻-Ce³⁺ CT, O²⁻-Tb³⁺ CT transitions and the host lattice absorption in VUV region were established. In addition, first principles calculation within the LDA of the DFT was applied to calculate the electronic structure and linear optical properties of SrB₂Si₂O₈ and the results were compared with the experimental data.     

Solvothermal synthesis and luminescence properties of monodisperse Gd2O3:Eu and Gd2O3:Eu@SiO2 nanospheres

A series of uniform, monodispersed Gd(OH)³:Eu³⁺ nanospheres less than 100 nm were successfully synthesized with iron ions as catalyst and DMF as solvent under the solvothermal condition. Cetyltrimethyl ammonium bromide (CTAB) and Polyvinylpyrrolidone (PVP) were performed as co-surfactant during this facile procedure should be changed as A series of uniform, monodisperse Gd(OH)³:Eu³⁺ nanospheres less than 100 nm in diameter were successfully synthesized with solvothermal method. Iron ion was used as catalyst and Dimethylformamide (DMF) as solvent, Cetyltrimethyl Ammonium Bromide (CTAB) and Polyvinylpyrrolidone (PVP) were performed as surfactants. Further calcination process was applied to prepare Gd₂O₃:Eu³⁺ nanoshpheres during this facile procedure.