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We report the structural, magnetic, electrical and magentoresistance properties of (La 0.75 Ca 0.15 Sr 0.1 MnO 3 ) 1− x (YBa 2 Cu 3 O 7−δ ) x (with x =0, 0.025, 0.05, 0.075, 0.1, 0.2, and 0.3) composites synthesized through sol–gel method. The powder X-ray diffraction patterns indicate no evidence of reaction between La 0.75 Ca 0.15 Sr 0.1 MnO 3 (LCSMO) and YBa 2 Cu 3 O 7−δ (YBCO). The addition of YBCO induces a reduction of the total magnetization while the Curie temperature remains almost constant (∼312 K). The behavior of the electrical resistivity evolves differently depending on the doping level. Above the paramagnetic–insulating transition temperature the resistivity data were best-fitted by using the adiabatic small polaron and variable range hopping models. Ferromagnetic–metallic regime in the composites seems to emanate from the electron–phonon or/and electron–magnon scattering processes. With increasing the YBCO doping content (until x =0.1), the positive magnetoresistance (MR) of YBCO phase dominates the negative MR of LCSMO one, which gives rise to the decreasing of MR of the composites.

International audience; Under-doped La-214 cuprates show a charge-and spin-modulation known as " stripes " [1]. These stripe modulations are (quasi)-static close to 1/8 hole doping where superconductivity is suppressed. The pseudo-gap phase of other cuprate compounds recently also revealed charge modulation, but interpreted rather as a charge density wave (CDW) [2, 3, 4], that possibly competes with superconductivity. In this context, to better understand the interplay between the stripe phase and the superconductivity, we use angle-resolved photoemission spectroscopy to study the electronic band structure and gap in La-214 cuprates near 1/8 doping (La 2−x−y Nd y Sr x CuO 4 (x = 0.12; y = 0.0 & 0.4)) and compare with the previous results in the same system [5] and La 1.86 Ba 0.14 CuO 4 [6]. Our data shows a loss of spectral intensity towards the end of the Fermi arcs, that is possibly due to a strong renormalisation, as already pointed out elsewhere * matteo.dastuto@neel.cnrs.fr-Institut Néel CNRS-25, av des Martyrs-38042 Grenoble cedex 9; tel: (+33)(0)4 76 88 12 84 [5], with a noisy but still measurable gap. On the nodal direction no gap is observed within our statistics, but a sizeable decrease in intensity with temperature. Moreover, we do not see any shadow band, but our Fermi surface can be well modelled with a single electron band calculation in the tight binding approximation, even very close to the 1/8 doping La 2−x−y Nd y Sr x CuO 4 with and without Nd substitution.

Pure and doped high-quality single-crystals of the non-congruently melting chain compound SrCuO 2 were grown in an infra-red image furnace using the traveling solvent floating zone technique. The growth conditions, together with the characterization of the grown single crystals by optical microscopy, EDX, X-ray powder diffraction and magnetization in the 2–300 K temperature range are presented. Doping by Mg 2+ ( S =0) and Ni 2+ ( S =1) on the Cu 2+ ( S =½) sites leads to breaking of the chains of the structure in finite segments.

International audience; A new crystal growth device, in which a high static external electric voltage (up to 14 kV) is added to a floating zone method, is described. Our first experiments show that the application of such an electric field acts like an external force, introducing a pressure effect which is in direct competition with temperature in the solid/liquid thermodynamic equilibrium. High electric fields could therefore be an additional parameter in crystal growth, opening original routes to the synthesis of new materials.

The low energy magnetic excitations spectra of the pristine and doped quasi-one-dimensional spin chains cuprates $\mathrm{SrCu}{\mathrm{O}}_{2}$ have been investigated by inelastic neutron scattering. The momentum-integrated magnetic dynamical structure factor yields a constant integrated intensity with regard to energy in the pure compound, while it shows a strong decay, at low energies, in the compounds doped with nonmagnetic impurities, namely, $\mathrm{SrC}{\mathrm{u}}_{0.99}{\mathrm{M}}_{0.01}{\mathrm{O}}_{2}$ (with $M=\mathrm{Zn}$ or Mg) and $\mathrm{S}{\mathrm{r}}_{0.99}\mathrm{L}{\mathrm{a}}_{0.01}\mathrm{Cu}{\mathrm{O}}_{2}$ ($\mathrm{C}{\mathrm{u}}^{+}$ carrying $S=0$ created within the chains). These results evidence the opening of a spin pseudogap in the two-spinon continuum of $\mathrm{SrCu}{\mathrm{O}}_{2}$ upon doping, stemming from disruptions of the spin chains by quantum impurities.

A wet-chemistry technique, namely the citrate route, has been used to prepare high-quality polycrystalline samples of double perovskite Sr{sub 2}FeMoO{sub 6}. We report on the evolution of magnetic and magnetoresistive properties of the synthesized samples as a function of three parameters (i) the pH of the starting solution, (ii) the decomposition temperature of the citrate precursors and (iii) the sintering conditions. The low-field magnetoresistance (LFMR) value of our best samples is as high as 5% at room temperature for an applied magnetic field of 1 kOe. Additionally, the distinguishing feature of these samples is the persistence of LFMR, with a reasonably large value, up to 400 K which is a crucial parameter for any practical application. Our study indicates that the enhancement of LFMR observed is due to a good compromise between the grain size distribution and their magnetic polarization. -- Graphical abstract: The microstructure (left panel) and corresponding low-field magnetoresistance of one of the Sr{sub 2}FeMoO{sub 6} samples synthesized in the course of this work. Highlights: • Samples of Sr{sub 2}FeMoO{sub 6} are prepared using a citrate route under varying conditions. • Magnetoresistive properties are improved and optimized. • Low-field magnetoresitence values as large as 5% at 300 K/1more » kOe are reported. • Persistence of low-field magnetoresistance up to 400 K.« less

The preparation, structural, microstructural, dielectric, and low temperature magnetic properties of MgxNi1−xFe2O4 (x = 0, 0.17, 0.34, 0.50, 0.66, 0.83, 1) ferrites synthesized by using a self-combustion sol–gel method is presented. Good insulating properties were found for all the compositions at high frequencies (kHz and MHz range), which might drive the present ceramics as interesting for RF/microwaves applications. By increasing the Mg2+ concentration, the total resistivity strongly increases (from ~106 Ωm for the Ni ferrite to 109 Ωm for the Mg ferrite), corresponding to conductivities in the range 10−9–10−6 S/m at f = 1 Hz. Typical ferrimagnetic character with a small coercivity and saturation magnetization in the range (30–50) Am2/kg, which slightly decreases with increasing the Mg content, were found. On the basis of the combined results from the infrared spectroscopy and XRD analysis, it was shown that the magnetic properties depend on the Mg2+ ions distribution on the octahedral and tetrahedral sites and the experimental saturation magnetization allowed to compute the cation distribution for the MgxNi1−xFe2O4 ferrites.

The three polyoxotungstates [(NaOH

The glasses in the SrO-TiO2-P2O5 system were prepared by the conventional quenching route. The amorphous state of samples was verified by X-ray diffraction (XRD). Density, molar volume, micro-hardness, glass transition temperature (Tg), and crystallization temperature (Tc) parameters are determined for each glass. The results show that they depend strongly on the chemical compositions. The structure approach of the glasses is determined by using Infrared spectroscopy (IR). This investigation highlights that the glassy-matrix contains various phosphate structural units. The crystallization of the glasses by heat-treatments is performed and the crystallized phases Sr3P4O13, TiP2O7, Sr(PO3)2 are identified by XRD. The overall results are correlated to the glass structure and the nature of chemical bonds constituting the glass network.

By means of the conventional quenching route, the glass series 33Na2O–xSrO–xTiO2–(50 − 2x)B2O3–17P2O5 (x = 0–12.5 mol%) were prepared. The amorphous state of samples was verified by X-ray diffraction (XRD). Density, molar volume, micro-hardness, glass transition temperature (T g), and crystallization temperature (T c) parameters are determined for each glass. The results show that they depend strongly on the chemical compositions. The structure approach of the glasses is determined by Infrared spectroscopy (IR). This investigation highlights that the glassy-matrix contains various phosphate and borate structural units. The crystallization of the high-TiO2 glasses by heat-treatments favors the formation of titanate phosphate Na4TiO(PO4)2 or Sr0.5Ti2(PO4)3 along with Sr3(PO4)2 inside the glass-matrix.

We present the synthesis and structural characterization, assessed by various techniques (FTIR, TGA, UV-vis, elemental analysis, single-crystal X-ray diffraction for three compounds, magnetic susceptibility, and electrochemistry) of five manganese-containing Wells-Dawson sandwich-type (WDST) complexes. The dimanganese(II)-containing complex, [Na(2)(H(2)O)(2)Mn(II)(2)(As(2)W(15)O(56))(2)](18-) (1), was obtained by reaction of MnCl(2) with 1 equiv of [As(2)W(15)O(56)](12-) in acetate medium (pH 4.7). Oxidation of 1 by Na(2)S(2)O(8) in aqueous solution led to the dimanganese(III) complex [Na(2)(H(2)O)(2)Mn(III)(2)(As(2)W(15)O(56))(2)](16-) (2), while its trimanganese(II) homologue, [Na(H(2)O)(2)Mn(II)(H(2)O)Mn(II)(2)(As(2)W(15)O(56))(2)](17-) (3), was obtained by addition of ca. 1 equiv of MnCl(2) to a solution of 1 in 1 M NaCl. The trimanganese(III) and tetramanganese(III) counterparts, [Mn(III)(H(2)O)Mn(III)(2)(As(2)W(15)O(56))(2)](15-) (4) and [Mn(III)(2)(H(2)O)(2)Mn(III)(2)(As(2)W(15)O(56))(2)](12-) (6), are, respectively, obtained by oxidation of aqueous solutions of 3 and [Mn(II)(2)(H(2)O)(2)Mn(II)(2)(As(2)W(15)O(56))(2)](16-) (5) by Na(2)S(2)O(8). Single-crystal X-ray analyses were carried out on 2, 3, and 4. BVS calculations and XPS confirmed that the oxidation state of Mn centers is +II for complexes 1, 3, and 5 and +III for 2, 4, and 6. A complete comparative electrochemical study was carried out on the six compounds cited above, and it was possible to observe the distinct redox steps Mn(IV/III) and Mn(III/II). Magnetization measurements, as a function of temperature, confirm the presence of antiferromagnetic interactions between the Mn ions in these compounds in all cases with the exception of compound 2.

A novel and very effective wet chemical route for the preparation of metal dendritic nanostructures was developed by starting from VOSO4 and Ag2SO4 (or K2PdCl4) in aqueous solution at room temperature. No template or surfactant was used. VOSO4 serves as the reducing agent for the silver salt, and the resulting silver nanoparticles and nanocrystallites are then arranged into dendritic structures. The morphologies of the dendrites are found to be heavily influenced both by the molar ratio and the concentrations of the reactants. Observation of symmetric and asymmetric dendrites with variations of these parameters supports the proposal that diffusion and oriented attachment of silver nanoparticles or crystallites might compete, to various extents, and dictate the final morphologies of dendritic nanostructures. To the best of our knowledge, this is the first example of a surfactant-free, template-free, fully inorganic wet chemical route to metal dendritic structures. The method was also successful in the preparation of palladium nanodendrites.

The compound [Na 2 (H 2 O) 2 Ni 2 (As 2 W 15 O 56 ) 2 ] 18- , Ni2As4, was synthesized and structurally characterized for the first time by X-ray crystallography. This complex reacts with Fe III to form two new mixed-metal sandwich-type complexes, [Fe 2 (H 2 O) 2 Ni 2 (As 2 W 15 O 56 ) 2 ] 14- , Fe2_Ni2As4, and [Fe(H 2 O). Ni(H 2 O)Ni 2 (As 2 W 15 O 56 ) 2 ] 15- , FeNi-Ni2As4. In comparison, the synthesis of the previously reported complex [Na 2 (H 2 O) 2 -Fe 2 (As 2 W 15 O 56 ) 2 ] 16- , Fe2As4, was rationalized and improved, and the incorporation of Ni II into the latter compound leads to the new mixed-metal complex [Ni 2 (H 2 O) 2 Fe 2 (As 2 W 15 -O 56 ) 2 ] 14- , Ni2_Fe2As4. Complete characterization of these complexes by FTIR, UV/Vis, elemental analysis, magnetic measurements and X-ray crystallography, wherever possible, clearly defined their electronic and structural properties. These complexes undergo multiple electron-transfer processes and are effective in the electrocatalytic reduction of nitrite, dioxygen and hydrogen peroxide.

Magnetic properties of bulk SrTi 1− x Co x O 3− δ solid solutions (for x from 0 to 0.05), prepared in air as those partially reduced, were studied by following the influence of the dopant concentration and valence. A strong paramagnetic and/or diamagnetic contribution and an extremely weak ferromagnetic contribution maintained up to room temperature is observed for all the studied compositions, including the undoped samples. While the paramagnetic contribution shows a classical evolution with cobalt concentration and valence, the ferromagnetic part of the magnetization seems to be independent of the doping process. While some of the observations can support the assumption of an intrinsic property of the SrTiO 3 matrix, the hypothesis of a ferromagnetism associated to some contamination with external magnetic impurities cannot be completely discarded and will be retained here.

The influence of successive sintering treatments on the structural, magnetic and electric properties of high ordered Sr2FeMoO6 (SFMO) samples belonging to the same starting batch was investigated. The solid-state synthesis conditions influence the microstructure of the samples and the Fe/Mo order. Successive sintering treatments with intermediate grindings improve the saturation magnetization (3.8μB) which became close to the ideal value (4μB) and the low field magnetoresistance (LFMR) as a consequence of microstructural changes. Above the Curie temperature the evolution of effective magnetic moment values depicts the presence of mixed valency states: FeII/MoVI and FeIII/MoV where the contribution of FeII/MoVI configuration is favoured by the enhancement of Fe/Mo order during successive sintering treatments.

SrTi 1− x Co x O 3−δ solid solutions ( 0.0 5 ⩽ x ⩽ 0.5 ) have been prepared by solid-state reaction at high temperature and the achievement of their synthesis was checked by high-resolution X-ray diffraction. Magnetic data were fitted to a Curie–Weiss law with the addition of a temperature independent paramagnetism term (TIP). The samples with x ⩾ 0.3 5 exhibit paramagnetic to antiferromagnetic transitions. The resistivity data show an unexpected dependence ln ( ρ ) ∼ T - 1 / 2 for all the measured temperature range.

Magnetite synthesized via three different synthesis routes (coprecipitation process in aqueous media, electrochemical synthesis in presence of complexing agents and solid state reaction at high temperature) has been characterized by X-Ray diffraction, scanning electron microscopy, thermal analysis (TGA), FT-IR and Mossbauer spectroscopies. Although each procedure gave homogeneous magnetite powders, many differences could be seen in the physico-chemical properties of the samples mostly depending on the synthesis conditions. For instance, at least two factors seem to have a huge impact onto the Fe3O4 behaviour: the presence of hydration water molecules and the particle size of the powders since a superparamagnetic behaviour was observed with the thinnest particles, at room temperature, on the Mossbauer spectra via the appearance of line broadening and a pronounced central doublet.

The structural characterization and electrochemical performance of three nanosized iron oxyhydroxides (β-FeOOH, γ-FeOOH and δ-FeOOH) are reported in this work. The interest of having small particle sizes is to be able to circumvent the low intrinsic ionic conductivity of these materials through the increase of the active volume proportion of the operating grain, during the insertion process. Low temperature synthesis routes, in aqueous media, were thus used to obtain such crystallites with nanometer dimensions. The theoretical specific capacity of these materials is as high as 300 mA h g −1 which is actually very interesting for lithium batteries applications. However, the electrochemical behaviour greatly depends on the structure of the compounds. A phase transformation was, for example, evidenced in the case of γ-FeOOH first lithiation whereas the best performance were obtained with β-FeOOH, which develops a stable capacity of 180 mA h g −1 , after 10 cycles, at C/10.

The physicochemical and electrocatalytic behaviors of eight multi-iron Wells−Dawson sandwich-type polyoxometalates were studied with specific emphasis on the FeIII centers. Magnetization measurements were used to identify and quantify the antiferromagnetic interactions between the edge-sharing FeIII units. Electrochemical studies show the stepwise reduction of the FeIII centers, in complete agreement with magnetization conclusions. The location of the potential of each wave depends on the pH of the solution, as well as the concentration and composition of the electrolyte. In addition, ion-pairing studies show there is a positive shift of the FeIII centers with an increase in ion pairing (i.e. K+ > Na+ > Li+). The electrocatalytic reduction of dioxygen and hydrogen peroxide is efficient for all the complexes. However, there is a pronounced increase in efficiency as the number of FeIII centers in the complex increases (i.e. 4 FeIII > 3 FeIII > 2 FeIII). The mixed-metal complexes [αββα-Na14(MnIIOH2)2(FeIII)2(As2W15O56)2 and αββα-Na14(MnIIOH2)2(FeIII)2(P2W15O56)2] are also efficient catalysts for the reduction of NO and HNO2. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Ultrafine magnetite particles are prepared through an electrochemical process, at room temperature, from an iron-based electrode immersed in an alkaline aqueous medium containing complexing compounds. XRD and chemical analysis indicate that the product is pure magnetite, Fe3O4. The size and morphology of the particles are studied by SEM. The magnetite nanoparticles present a magnetoresistance of almost 3%, at 300 K, under a magnetic field of 1 T. A reactive mechanism for the electrochemical process is proposed.

The three novel, multi-nickel-substituted heteropolytungstates [Ni6As3W24O94(H2O)2]17- (1), [Ni3Na(H2O)2(AsW9O34)2]11- (2), and [Ni4Mn2P3W24O94(H2O)2]17- (3) have been synthesized and characterized by IR, elemental analysis, electrochemistry, and magnetic studies. Single-crystal X-ray analysis was carried out on Na16.5Ni0.25[Ni6As3W24O94(H2O)2]·54H2O, which crystallizes in the triclinic system, space group P1, with a = 17.450(4) A, b = 17.476(4) A, c = 22.232(4) A, α = 85.73(3)°, β = 89.74(3)°, γ = 84.33(3)°, and Z = 2, Na11[Ni3Na(H2O)2(AsW9O34)2]·30.5H2O, which crystallizes in the triclinic system, space group P1, with a = 12.228(2) A, b = 16.743(3) A, c = 23.342(5) A, α = 78.50(3)°, β = 80.69(3)°, γ = 78.66(3)°, and Z = 2, and Na17[Ni4Mn2P3W24O94(H2O)2]·50.5H2O, which crystallizes in the monoclinic system, space group P21/c, with a = 17.540(4) A, b = 22.303(5) A, c = 35.067(7) A, β = 95.87(3) A, and Z = 4. Polyanion 1 consists of two B-α-(Ni3AsW9O40) Keggin moieties linked via a unique AsW6O16 fragmen...

Reaction of the trivacant lacunary complex, α-Na12[As2W15O56], with an aqueous solution of Fe(NO3)3·9H2O yields the sandwich-type polyoxometalate, αββα-Na12(FeIIIOH2)2FeIII2(As2W15O56)2 (Na1). The structure of this complex, determined by single-crystal X-ray crystallography (a = 13.434(1) A, b = 13.763(1) A, c = 22.999(2) A, α = 90.246(2)°, β = 102.887(2)°, γ = 116.972(1)°, triclinic, P1, R1 = 5.5%, based on 25342 independent reflections), consists of an FeIII4 unit sandwiched between two trivacant α-As2W15O5612- moieties. UV−vis, infrared, cyclic voltammetry, and elemental analysis data are all consistent with the structure determined from X-ray analysis. Magnetization studies confirm that the four Fe(III) centers are antiferromagnetically coupled. A cyclic voltammogram of Na1 reveals that a three-wave W(VI) system replaces the two-wave W(VI) system found in the precursor α-As2W15O5612- complex. The observed modifications in the CV patterns of Na1 and α-As2W15O5612- are most likely due to subsequent cha...

We report new measurements of NMR parameters for 71 Ga in gallium bearing oxide reference compounds, ranging from perfectly ordered systems to disordered crystalline structures and their aluminate counterparts. Static, MAS, and QPASS spectra are obtained at magnetic fields ranging from 7.0 to 18.8 T. With these results we enhance the previously established correlation between isotropic chemical shifts of 71 Ga and 27 Al and propose a correlation between gallium and aluminum electric field gradients (EFG). This correlation shows that the EFG at 71 Ga sites are generally three times greater than those at equivalent 27 Al sites.

Superstructure reflections due to charge ordering have been observed in single crystals using X-ray diffraction. The critical temperatures for the ordering process are 150 K and 180 K in samples with Sr concentrations of and , respectively. For x =0.125 the charge order transition correlates with a drastic resistivity enhancement, while a much smaller change of the resistivity is found for x = 0.15. It is argued that the onset of charge ordering in the sample with x = 0.125 is related to the suppression of co-operative Jahn-Teller distortions which is initiated by rising ferromagnetism.

The conductance of a point contact between two crystals of metallic ${(\mathrm{L}\mathrm{a}}_{0.75}{\mathrm{Sr}}_{0.25}{)\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ shows steps as the contact is broken. A histogram of the conductance values observed in 60 breaks exhibits a series of sharp peaks at integer multiples of ${G}_{0}{=2e}^{2}/h.$ Quantum conductance in these ceramics is qualitatively different from that normally seen in metals and suggests that no neck formation occurs during fracture of a contact between these brittle materials.

Fault-filling calcite crystals sampled from the Eocene/Oligocene Gondrecourt graben, Paris Basin, Eastern France, have been studied in order to test the potentiality of calcite (U-Th)/He dating, based on recognized He retention behavior in crystal lattice at surface temperature (Copeland et al., 2007). The samples have been selected because of their relatively old Eocene to Oligocene precipitation age and cold thermal history (

La0.85Sr0.15MnO3 single crystals were grown by the floating zone method. They can be reversibly reduced under controlled oxygen partial pressure. Their defect chemistry is investigated together with the evolution of their cell parameters and that of their electric conductivity which is very sensitive to their oxygen content. On the other hand, stoichiometric crystals can be oxidized with a slow kinetics making possible the preparation of diphasic materials.

Single crystals of ${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}{\mathrm{MnO}}_{3\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ were prepared with various oxygen vacancies from \ensuremath{\delta}=0 to \ensuremath{\delta}=0.055. The $b$ and $c$ lattice parameters were found to increase with \ensuremath{\delta} whereas the $a$ parameter remains practically constant. The oxygen vacancies are expected to reduce the hole concentration, and therefore the Curie temperature and the electrical conductivity. However, these effects are somewhat different from those observed in the hole-doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ systems, owing to different lattice distortion. The lattice change of the sample is illustrated by a strong shift in the hyperfine field of ${\mathrm{Mn}}^{4+}$ ions, detected with the NMR method. The oxygen deficiency and all the above properties resulting from it were shown to be reversible.

The partial substitution of Pb for Bi made it possible to prepare single phase 2223 and iodide intercalated I-2223 BiSrCaCuO compounds. Both compounds were studied by TEM, HRTEM, X-ray diffraction, and electrical resistivity. A change in symmetry occurs upon intercalation, likely a Bbmb to Pbm2 transition, and the change in the stacking sequence of the perovskite blocks was directly observed by HRTEM. Low temperature electrical resistivity measurements showed a metallic type conduction with critical temperatures of 105 K and 83 K, respectively. The resistivity at room temperature was 1.6 × 10 −3 Ωcm for 2223 and 0.14 Ωcm for I-2223.

Alumina films were fabricated by the sol-gel route under various preparation conditions. The peptization rate and the structure of colloidal particles depend on these conditions. In particular, the peptization of a dried boehmite precipitate prepared from aluminum alkoxide was achieved within a few seconds without heating. The resulting sol is made of highly dispersed crystallites; this allows the formation of well-textured xerogel films as evidenced by the Weissenberg x-ray method.

We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, EPR, X-ray lattice constant measurements, conductivity and dielectric response, specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO3 nanotubes were also studied. The transition at T=140.3K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201K anomaly appears to signal the onset of glassy behaviour., submitted to PRB

The structure of glasses prepared in the Na2OGa2O3P2O5 system was investigated by vibrational (IR, Raman) and X-ray absorption spectroscopies. For the orthophosphate compositions, gallium is found in four-fold coordination, whereas it is found in six-fold coordination in a metaphosphate glass. For pyro-, tri and tetraphosphate glasses, gallium atoms are distributed on tetrahedral and octahedral sites. Gallium atoms found on octahedral sites are likely chelated by pyrophosphate groups. The ionic conductivity of the glasses depends mainly on their sodium content.

Seven monoclinic isotypic compounds with the general formula Na7M3 (X2O7)4 have been synthesized by crystallization in a flux of sodium phosphates or arsenates. They exhibit high ionic conductivity (σ≈10−3-10−2ω−1cm−1 at 300°C) and good cation exchange properties particularly with Ag+ ions. At temperatures varying from −20°C to 240°C, depending on the particular compound, they undergo a reversible phase transition, α⇌β, which is associated with a loss of long range ordering of the sodium ions and results in an increase of the conductivity. The crystal structures of β-Na7Fe3 (As2O7)4 and α-Na7Fe3 (P2O7)4 have recently been determined and a model of sodium diffusion paths has been proposed. This model indicates a two-dimensional conduction within planes parallel to (001). Conductivity measurements performed on single crystals confirm this two-dimensional character with a σ|/σ⊥ ratio of about 2×103.

Compounds (III)—(VI) and (VIII) are characterized by FTIR and UV/VIS spectroscopy, and magnetic susceptibility and electrochemical measurements.

Zero-field muon spin rotation and magnetization measurements were performed in La${}_{2}$Cu${}_{1\ensuremath{-}x}{M}_{x}$O${}_{4}$, for $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.12$, where Cu${}^{2+}$ is replaced either by $M={\text{Zn}}^{2+}$ or by $M={\text{Mg}}^{2+}$ spinless impurity. It is shown that while the doping dependence of the sublattice magnetization [$M(x)$] is nearly the same for both compounds, the N\'eel temperature [${T}_{N}(x)$] decreases unambiguously more rapidly in the Zn-doped compound. This difference, not taken into account within a simple dilution model, is associated with the frustration induced by the Zn${}^{2+}$ impurity onto the Cu${}^{2+}$ antiferromagnetic lattice. In fact, from ${T}_{N}(x)$ and $M(x)$ the spin stiffness is derived and found to be reduced by Zn doping more significantly than expected within a dilution model. The effect of the structural modifications induced by doping on the exchange coupling is also discussed.

Oriented lamellar eutectic composites of CoO and ZrO 2 (CaO) platelets were oxidized in oxygen at 880°C to form Co 3 O 4 between the zirconia plates. The relatively rapid longitudinal oxidation of CoO can be described by an electrochemical local cell model whereby oxygen ions are conducted from the external surface to the internal oxidation front through the zirconia electrolyte. The crystallographic relationship between the Co 3 O 4 and ZrO 2 (CaO) resulting phases is the same as that found for CoO and ZrO 2 (CaO) in the starting material.

The compound [Na 2 (H 2 O) 2 Ni 2 (As 2 W 15 O 56 ) 2 ] 18- , Ni2As4, was synthesized and structurally characterized for the first time by X-ray crystallography. This complex reacts with Fe III to form two new mixed-metal sandwich-type complexes, [Fe 2 (H 2 O) 2 Ni 2 (As 2 W 15 O 56 ) 2 ] 14- , Fe2_Ni2As4, and [Fe(H 2 O). Ni(H 2 O)Ni 2 (As 2 W 15 O 56 ) 2 ] 15- , FeNi-Ni2As4. In comparison, the synthesis of the previously reported complex [Na 2 (H 2 O) 2 -Fe 2 (As 2 W 15 O 56 ) 2 ] 16- , Fe2As4, was rationalized and improved, and the incorporation of Ni II into the latter compound leads to the new mixed-metal complex [Ni 2 (H 2 O) 2 Fe 2 (As 2 W 15 -O 56 ) 2 ] 14- , Ni2_Fe2As4. Complete characterization of these complexes by FTIR, UV/Vis, elemental analysis, magnetic measurements and X-ray crystallography, wherever possible, clearly defined their electronic and structural properties. These complexes undergo multiple electron-transfer processes and are effective in the electrocatalytic reduction of nitrite, dioxygen and hydrogen peroxide.

The metastable compound SrFe2O4 was prepared by crystallization at 630°C of a glass prepared by melt-spinning. It decomposes above 900°C. It has an orthorhombic unit cell at room temperature with a = 8.031(10) A , b = 18.208(18) A , c = 5.454(6) A , Z = 8, space group Pbc21.

We report a direct observation of a sharp Kohn-type anomaly in the doubly degenerate copper-oxygen bond-stretching phonon mode occurring at $\mathbf{q}=(0.3,0,0)$ in ${\text{La}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{4+\ensuremath{\delta}}$, with $x=0.14\ifmmode\pm\else\textpm\fi{}0.01$, thanks to the high $\mathbf{Q}$ resolution of inelastic x-ray scattering. This anomaly is clearly seen when the inelastic signal is analyzed using a single mode but is also consistent with a two-mode hypothesis, possibly due to a splitting of the degenerate modes due to symmetry-breaking stripes. Our observation shows that the effect persists at the stripe-propagation vector in a superconducting system close to optimal doping.

The phonon and crystal field excitations in several rare earth titanate pyrochlores are investigated. Magnetic measurements on single crystals of Gd2Ti2O7, Tb2Ti2O7, Dy2Ti2O7 and Ho2Ti2O7 are used for characterization, while Raman spectroscopy and terahertz time domain spectroscopy are employed to probe the excitations of the materials. The lattice excitations are found to be analogous across the compounds over the whole temperature range investigated (295-4 K). The resulting full phononic characterization of the R2Ti2O7 pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in Tb2Ti2O7 in Raman spectroscopy for the first time, among which all of the previously reported excitations. The presence of additional crystal field excitations, however, suggests the presence of two inequivalent Tb3+ sites in the low temperature structure. Furthermore, the crystal field level at approximately 13 cm-1 is found to be both Raman and dipole active, indicating broken inversion symmetry in the system and thus undermining its current symmetry interpretation. In addition, evidence is found for a significant crystal field-phonon coupling in Tb2Ti2O7. These findings call for a careful reassessment of the low temperature structure of Tb2Ti2O7, which may serve to improve its theoretical understanding., 13 pages, 7 figures

The recent discovery of spin ice is a spectacular example of the noncoplanar spin arrangements that can arise in the pyrochlore ${A}_{2}{B}_{2}{\mathrm{O}}_{7}$ structure. We present magnetic and thermodynamic studies on the metallic ferromagnet pyrochlore ${\mathrm{Sm}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$. Our studies, carried out on oriented crystals, suggest that the Sm spins have an ordered spin-ice ground state below about ${T}^{*}=15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The temperature and field evolution of the ordered spin-ice state are governed by an antiferromagnetic coupling between the Sm and Mo spins. We propose that as a consequence of a robust feature of this coupling, the tetrahedra aligned with the external field adopt a one-in, three-out spin structure as opposed to the three-in, one-out structure in dipolar spin ices, as the field exceeds a critical value.

Alumina and zirconia were joined to a Ti-6Al-4V alloy (TA6V) using an active braze of composition Cu-40Ag-5Ti. The interface region is shown to be very complex, consisting of numerous lavers of varying composition and thickness. The chemical interaction of the braze with the ceramic results in a Cu2 (Ti, Al)4O layer. Adjacent to the Ti-6Al-4V base metal, a series of Cu-Ti intermetallic phases is observed. The TA6V/Al2O3 and TA6V/ZrO2 interface regions exibit rather similar microstructures. However, the mechanical properties of the joints in these two systems are very different. The tensile strength of the joints in is significantly lower in the case of TA6V/Al2O3 (3MPa) than for TA6V/ZrO2 (150MPa).

A systematic study of ${(A,{A}^{\ensuremath{'}})}_{2}\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_{6}$ ($A,{A}^{\ensuremath{'}}=\mathrm{Ca}$, Sr, Ba) ferromagnetic oxides with double perovskite structure has been performed using $^{95,97}\mathrm{Mo}$ and $^{57}\mathrm{Fe}$ NMR spectroscopy. These oxides are isoelectronic but have substantially different Curie temperatures. The NMR analysis provides clear evidence that the magnetic moment at Mo sites is not constant but varies sensitively with the ionic size of the alkaline ions. The $^{95,97}\mathrm{Mo}$ frequency, and thus the electronic charge at Mo ions, is found to be smaller in Ba and Ca than in Sr-based oxides. The charge release from Mo sites is accompanied by an uptake at Fe sites, and thus a self-doping $\mathrm{Fe}\ensuremath{-}\mathrm{Mo}$ process is observed. This process is controlled by relevant structural parameters: the $\mathrm{Fe}\ensuremath{-}\mathrm{O}\ensuremath{-}\mathrm{Mo}$ bond length and bending. A clear relationship between the Curie temperature and the magnetic moment and thus electron density at Mo sites has been disclosed. The relevance of these findings for the understanding of ferromagnetic coupling in double perovskites is discussed.

[1] Samples from coral skeletons belonging to six species have been submitted to thermogravimetric analysis in the temperature range between 20° and 550°C. Weight losses are interpreted on the basis of simultaneously recorded infrared absorption spectra at specific wavelengths for water and CO2. The organic + water content of coral skeletons represents up to 2.5% of the total weight. Additionally, it is shown that during heating the main part of water remains linked to organic compounds up to 275°–330°C, suggesting organic materials associated with the skeleton are hydrated.

The three novel, multi-nickel-substituted heteropolytungstates [Ni(6)As(3)W(24)O(94)(H(2)O)(2)](17)(-) (1), [Ni(3)Na(H(2)O)(2)(AsW(9)O(34))(2)](11)(-) (2), and [Ni(4)Mn(2)P(3)W(24)O(94)(H(2)O)(2)](17)(-) (3) have been synthesized and characterized by IR, elemental analysis, electrochemistry, and magnetic studies. Single-crystal X-ray analysis was carried out on Na(16.5)Ni(0.25)[Ni(6)As(3)W(24)O(94)(H(2)O)(2)].54H(2)O, which crystallizes in the triclinic system, space group P1, with a = 17.450(4) A, b = 17.476(4) A, c = 22.232(4) A, alpha = 85.73(3) degrees, beta = 89.74(3) degrees, gamma = 84.33(3) degrees, and Z = 2, Na(11)[Ni(3)Na(H(2)O)(2)(AsW(9)O(34))(2)].30.5H(2)O, which crystallizes in the triclinic system, space group P1, with a = 12.228(2) A, b = 16.743(3) A, c = 23.342(5) A, alpha = 78.50(3) degrees, beta = 80.69(3) degrees, gamma = 78.66(3) degrees, and Z = 2, and Na(17)[Ni(4)Mn(2)P(3)W(24)O(94)(H(2)O)(2)].50.5H(2)O, which crystallizes in the monoclinic system, space group P2(1)/c, with a = 17.540(4) A, b = 22.303(5) A, c = 35.067(7) A, beta = 95.87(3) A, and Z = 4. Polyanion 1 consists of two B-alpha-(Ni(3)AsW(9)O(40)) Keggin moieties linked via a unique AsW(6)O(16) fragment, leading to a banana-shaped structure with C(2)(v)() symmetry. The mixed-metal tungstophosphate 3 is isostructural with 1. Polyanion 2 consists of two lacunary B-alpha-[AsW(9)O(34)](9)(-) Keggin moieties linked via three nickel(II) centers and a sodium ion. Electrochemical studies show that 1-3 exhibit a unique and reproducible voltammetric pattern and that all three compounds are stable in a large pH range. An investigation of the magnetic properties of 1-3 indicates that the exchange interactions within the trimetal clusters are ferromagnetic. However, for 1 and 3 intra- and intermolecular interactions between different trinuclear clusters are also present.

Reaction of the trivacant lacunary complex, alpha-Na(12)[As(2)W(15)O(56)], with an aqueous solution of Fe(NO(3))(3).9H(2)O yields the sandwich-type polyoxometalate, alphabetabetaalpha-Na(12)(Fe(III)OH(2))(2)Fe(III)(2)(As(2)W(15)O(56))(2) (Na1). The structure of this complex, determined by single-crystal X-ray crystallography (a = 13.434(1) A, b = 13.763(1) A, c = 22.999(2) A, alpha = 90.246(2) degrees, beta = 102.887(2) degrees, gamma = 116.972(1) degrees, triclinic, Ponemacr;, R1 = 5.5%, based on 25342 independent reflections), consists of an Fe(III)(4) unit sandwiched between two trivacant alpha-As(2)W(15)O(56)(12)(-) moieties. UV-vis, infrared, cyclic voltammetry, and elemental analysis data are all consistent with the structure determined from X-ray analysis. Magnetization studies confirm that the four Fe(III) centers are antiferromagnetically coupled. A cyclic voltammogram of Na1 reveals that a three-wave W(VI) system replaces the two-wave W(VI) system found in the precursor alpha-As(2)W(15)O(56)(12)(-) complex. The observed modifications in the CV patterns of Na1 and alpha-As(2)W(15)O(56)(12)(-) are most likely due to subsequent changes in the acid-base properties of two reduced POMs that occur as a result of Fe(III) incorporation. Na1 is shown to be more efficient than the monosubstituted complex alpha(2)-As(2)(Fe(III)OH(2))W(17)O(61)(7)(-) in the electrocatalytic reduction of dioxygen. This is attributed to cooperativity effects among the adjacent Fe(III) centers in Na1.

A tetranuclear manganous Wells–Dawson sandwich-type polyoxometalate has been synthesized by the reaction of α-Na12(As2W15O56) with an aqueous solution of MnCl2·4H2O. The structure of this complex, αββα-Na16(MnIIOH2)2MnII2(As2W15O56)2·55H2O (Na1), was determined by single-crystal X-ray crystallography (a = 14.5230(12) A, b = 14.7104(13) A, c = 19.8927(17) A, α = 84.326(2)°, β = 81.709(2)°, γ = 65.584(2)°, triclinic, R1 = 6.2%, based on 26721 independent reflections) and is similar to the phosphorus analogue, αββα-Na16(MnIIOH2)2MnII2(P2W15O56)2 (Na2). Magnetization studies confirm that both Na1 and Na2 show antiferromagnetic coupling of the four Mn(II) centers. Despite the structural similarities, electrochemical studies reveal that the presence of arsenic shifts the Mn waves to more positive potentials. Catalytic studies confirm that 1 is a significantly better catalyst than 2 for the H2O2-based epoxidation of cis-cyclooctene, cyclohexene, and 1-hexene.