Potassium magnesium and potassium cobalt molybdates: Synthesis and ion conductivity

Potassium magnesium and potassium cobalt molybdates of composition K₂M ₂ ^II (MoO₄)₃ (M = Mg, Co) and the products of their heterovalent doping by scandium(III) and vanadium(V) ions have been studied by impedance spectroscopy, powder X-ray diffraction, and electron microscopy. The compounds have high ion conductivity. Partial heterovalent substitutions of scandium for magnesium or vanadium for molybdenum additionally increases ion conductivity. The high conductivity of potassium magnesium and potassium cobalt molybdates allows us to consider them as promising solid electrolytes with potassium conductivity.     

Synthesis and crystal structure of Ln2MGe4O12, Ln=rare-earth element or Y; M=Ca, Mn, Zn

The crystal structure of the promising optical materials Ln₂M²⁺Ge₄O₁₂, where Ln=rare-earth element or Y; M=Ca, Mn, Zn and their solid solutions has been studied in detail. The tendency of rare-earth elements to occupy six- or eight-coordinated sites upon iso- and heterovalent substitution has been studied for the Y_2−xEr_xCaGe₄O₁₂ (x=0-2), Y_2−2xCe_xCa_1+xGe₄O₁₂ (x=0-1), Y₂Ca_1−xMn_xGe₄O₁₂ (x=0-1) and Y_2−xPr_xMnGe₄O₁₂ (x=0-0.5) solid solutions. A complex heterovalent state of Eu and Mn in Eu₂MnGe₄O₁₂ has been found.     

Molecular relaxation processes in very dilute systems: A 13C NMR study of alcohols in alkanes

¹³C T₁ relaxation times have been determined for n-butanol in C₆D₁₂ in the concentration range 0.001 ? _x ? 1 (using ¹³C labelled alcohol) and for t-butanol in the range 0.01 ? _x ? 1. In the former case, this has allowed us to probe molecular mobility down to the region of monomeric alcohols. Comparison with previous results from measurements of ¹H chemical shifts, dielectric relaxation, and the picosecond dynamics of electron solvation allows us to build up a detailed picture of molecular clustering and liquid structure in alkane—alcohol mixtures. For n-butanol, the local liquid structure is established by x = 0.2 while t-butanol does not appear to form aggregates larger than trimers until x = 0.5.     

Etude structurale de seleniures,tellurures et selenio-tellurures de titane Ti3Se4−xTex (0 ⩽ x ⩽ 4)

The compounds Ti₃Se_4?xTe_x are prepared. Their structure closely depends upon thermal conditions of synthesis. Treatments at 600°C and quenching give pure monoclinic compounds isotypic with Ti₃Se₄ and Ti₃Te₄ in narrow ranges of composition (x ? 0.5 and x ? 3.5). At 800°C, polycrystalline pure products are obtained with an hexagonal B8 unit cell (a′, ?') when x ? 3. At 1000°C, for x ? 3.5, almost pure compounds crystallize with that same hexagonal unit cell; an orthorhombic lattice is observed for x = 4.Moreover, twinning is observed on single crystals obtained after quenching from 800 or 1000°C, with x = 1, 2, 3 and 4. These crystals seem to exhibit an hexagonal unit cell (2a′, 2?'), but in fact their lattice is orthorhombic or monoclinic with a real I unit cell: a ≈ a′√3, b ≈ a′, c ≈ 2c′. The explanation of the phenomenon allows us to correct some misinterpretations found in the literature on “TiTe” single crystals.     

Magnetic properties of iron atoms in the perovskite-like oxides

Magnetic and structural characteristics of magnetically diluted complex oxides La_2?x A _x MO₄ (M is Fe; A is Sr, Ba) were studied. An essential dependence of magnetic susceptibility on the composition of dia- and paramagnetic components of the structure and also the tendency of heterovalent iron atoms to form clusters were found.     

New anion-conducting fluorite-like solid solution Bi1 − x Te x (O,F)2 + δ (0.28 < x < 0.43) in the BiF3-BiOF-TeO2 system

A new fluorite-like solid solution, II-Bi_{1 ? x} Te _x (O,F)_{2 + δ}, was produced by solid-phase synthesis at 873 K with subsequent annealing, its concentration boundaries were determined, and a scheme of an isothermal (873 K) section of the BiF₃-BiOF-TeO₂ system was proposed. The new phase was characterized by X-ray powder diffraction, electron microscopy, and impedance spectroscopy. Making heterovalent substitutions simultaneously in the cation and anion sublattices, Te⁴⁺ ? Bi³⁺ and O^2? ? F^? allowed one to vary the tellurium cation content x (at constant anion nonstoichiometry δ) or the anion nonstoichiometry δ (at constant tellurium cation content x or constant fluoride ion content), which enabled one to describe the effect of these parameters on the properties of the solid solution. The anion excess δ was found to dominate the unit cell parameter of the solid solution and its ionic conductivity. The conduction within the studied temperature range was proven to be mainly by fluoride ions. It was assumed that the ordering of superstoichiometric anions, or clustering, can manifest itself as the structural modulations of the phase II-Bi_{1 ? x} Te _x (O,F)_{2 + δ} that were detected in this work.     

Electronic state and local surrounding of <Superscript>119</Superscript>Sn in calcium-substituted holmium orthochromites

The influence of Ca²⁺ doped into the holmium sublattice on the magnetically active surrounding of Sn⁴⁺ ions located in the chromium sublattice of Ho_1–x Ca _x Cr_0.997Sn_0.003O₃ (x = 0, 0.003, and 0.1) compounds was studied by ¹¹⁹Sn Mössbauer spectroscopy. At concentrations [Ca] = [Sn] = 0.3 mol %, an increase was observed in the spectral contribution of Sn⁴⁺ sites, having the full number of nearest-neighbor Cr³⁺cations (n = 6), where they perceived a magnetic field H(Sn)_{4.2 K} = 82 kOe, compared to the contribution of the relevant sites in the undoped chromite (x = 0). This observation was interpreted as resulting from a reduced probability of appearance of Cr³⁺ vacancies in the nearest surrounding of heterovalent Sn⁴⁺ ions. For x = 0.1, on the contrary, the ¹¹⁹Sn spectrum revealed a reduced contribution from the Sn⁴⁺ sites with n = 6. This evolution is shown not to be due neither to the appearance of Cr⁴⁺ nor Cr⁶⁺ ions in the nearest neighborhood of Sn⁴⁺ in the chromium sublattice to balance the charge deficiency of the Ca²⁺ ions doped into the holmium sublattice. This allowed us to suggest that the observed effect was due to the onset of Sn⁴⁺ segregations in the structure of Ho_0.9Ca_0.1Cr_0.997Sn_0.003O₃, which contained a far greater amount of Ca²⁺ ions whose charge deficiency was balanced mostly by Cr⁴⁺ formation. Studies of samples that were prepared under a hydrogen atmosphere revealed the reduction of Sn⁴⁺ to the oxidation state +2, with the concomitant stabilization of the formed Sn²⁺ ions on crystallite surfaces on sites having low coordination numbers.     

Synthesis and investigation of calcium-containing phosphatosilicates with NaZr2(PO4)3 structure

Calcium-containing phosphatosilicates of the type Ca_0.5(1+x)Zr₂(PO₄)_3?x (SiO₄) _x were obtained by sol-gel method using the salting out process. Phase formation was studied, and optimal temperature and time regimes of synthesis providing the formation of one-phase reaction products were established. It is found that phases of NZP-structure are formed. Boundaries of phase stability of phosphatosilicates at the heterovalent isomorphic substitutions P⁵⁺→Si⁴⁺: 0 ≤ x ≤ 0.5 are established. On the basis of powder X-ray analysis data the crystalline structure of Ca_0.75Zr₂(PO₄)_2.5(SiO₄)_0.5 was refined by the full-profile analysis (Rietveld method). Hydrolytic stability of phosphatosilicates was studied.     

Relationships between electronic spectroscopy and electrochemistry. A probe of reorganisation energies

The electronic spectra and electrochemical potentials of the species [Ru(bipy)_x(NN)_3-x]²⁺ (bipy = 2,2′-bipyridine. NN = unsaturated diimine, x = 1,2) are correlated via fundamental relationships involving reorganisation and solvation energies. It is argued that the presence of two distinct Ru → ligand charge transfer transitions allows comparison of the reorganisation energies therefore.     

Complex phosphates with the NASICON structure (M x A2(PO4)3)

Data on the synthesis and properties of NASICON complex phosphates M _x A₂(PO₄)₃ are summarized. The crystal chemistry and homology of these compounds are surveyed. The existence of large channels and numerous voids in the structure and various occupations of these voids by mobile cations generate many valuable physical and chemical properties of the M _x A₂(PO₄)₃ complex phosphates. The details of phase transitions, ionic conductivity, thermal expansion, ion-exchange properties, and sensor properties of the NASICON materials are considered, as well as the possibility of purposefully changing these properties through partial heterovalent substitution for the metal or phosphorus atoms.     

Synthesis and antioxidant activity of biocompatible maltodextrin-stabilized aqueous sols of nanocrystalline ceria

A novel method is proposed for preparing biocompatible aqueous CeO₂ sols, which allows purposefully adjusting particle sizes and biochemical properties of nanocrystalline ceria (NC). The usefulness of this method for preparing aqueous sols of Ce_{1 ? x} Gd _x O_{2 ? x/2} solid solutions is demonstrated. Nanocrystalline ceria has been demonstrated to have a size-dependent activity in hydroxyl radical inactivation and in protecting mouse fibroblast cells (L929) from oxidative stress induced by hydrogen peroxide administration.     

Direct observation of the evolving metal–support interaction of individual cobalt nanoparticles at the titania and silica interface

Understanding the metal–support interaction (MSI) is crucial to comprehend how the catalyst support affects performance and whether this interaction can be exploited in order to design new catalysts with enhanced properties. Spatially resolved soft X-ray absorption spectroscopy (XAS) in combination with Atomic Force Microscopy (AFM) and Scanning Helium Ion-Milling Microscopy (SHIM) has been applied to visualise and characterise the behaviour of individual cobalt nanoparticles (CoNPs) supported on two-dimensional substrates (SiO_xSi(100) (x < 2) and rutile TiO₂(110)) after undergoing reduction–oxidation–reduction (ROR). The behaviour of the Co species is observed to be strongly dependent on the type of support. For SiO_xSi a weaker MSI between Co and the support allows a complete reduction of CoNPs although they migrate and agglomerate. In contrast, a stronger MSI of CoNPs on TiO₂ leads to only a partial reduction under H₂ at 773 K (as observed from Co L₃-edge XAS data) due to enhanced TiO₂ binding of surface-exposed cobalt. SHIM data revealed that the interaction of the CoNPs is so strong on TiO₂, that they are seen to spread at and below the surface and even to migrate up to ∼40 nm away. These results allow us to better understand deactivation phenomena and additionally demonstrate a new understanding concerning the nature of the MSI for Co/TiO₂ and suggest that there is scope for careful control of the post-synthetic thermal treatment for the tuning of this interaction and ultimately the catalytic performance.

Understanding the metal–support interaction (MSI) is crucial to comprehend how the catalyst support affects performance and whether this interaction can be exploited in order to design new catalysts with enhanced properties.     

The impact of surface Cu2+ of ZnO/(Cu1−xZnx)O heterostructured nanowires on the adsorption and chemical transformation of carbonyl compounds

The surface cation composition of nanoscale metal oxides critically determines the properties of various functional chemical processes including inhomogeneous catalysts and molecular sensors. Here we employ a gradual modulation of cation composition on a ZnO/(Cu_1−xZn_x)O heterostructured nanowire surface to study the effect of surface cation composition (Cu/Zn) on the adsorption and chemical transformation behaviors of volatile carbonyl compounds (nonanal: biomarker). Controlling cation diffusion at the ZnO(core)/CuO(shell) nanowire interface allows us to continuously manipulate the surface Cu/Zn ratio of ZnO/(Cu_1−xZn_x)O heterostructured nanowires, while keeping the nanowire morphology. We found that surface exposed copper significantly suppresses the adsorption of nonanal, which is not consistent with our initial expectation since the Lewis acidity of Cu²⁺ is strong enough and comparable to that of Zn²⁺. In addition, an increase of the Cu/Zn ratio on the nanowire surface suppresses the aldol condensation reaction of nonanal. Surface spectroscopic analysis and theoretical simulations reveal that the nonanal molecules adsorbed at surface Cu²⁺ sites are not activated, and a coordination-saturated in-plane square geometry of surface Cu²⁺ is responsible for the observed weak molecular adsorption behaviors. This inactive surface Cu²⁺ well explains the mechanism of suppressed surface aldol condensation reactions by preventing the neighboring of activated nonanal molecules. We apply this tailored cation composition surface for electrical molecular sensing of nonanal and successfully demonstrate the improvements of durability and recovery time as a consequence of controlled surface molecular behaviors.

Unexpected features of surface Cu²⁺ on ZnO/(Cu1−xZnx)O nanowires for molecular transformation and electrical sensing of carbonyl compounds were found.     

Mechanism of formation of the complex oxide Gd<Subscript>2</Subscript>SrFe<Subscript>2</Subscript>O<Subscript>7</Subscript>

The mechanism of formation of the perovskite-like layered structure of the oxide Gd₂SrFe₂O₇ was studied. The limiting stages are those of formation of phases with perovskite (GdFeO₃, SrFeO_3?x ) and K₂NiF₄ (GdSrFeO₄) structures. The Mössbauer study has shown that iron atoms exist in a heterovalent state (Fe³⁺ and Fe⁴⁺) only in the structure of SrFeO_3?x ).     

Pyrochlore solid solutions (LaxY1−x)2Mo2O7, x = 0.0 to 0.5. Spin-glass-like behavior

The (La_xY_1−x)₂Mo₂O₇ system was investigated in the range x = 0.0 to x = 0.5. Single-phase materials exist up to x = 0.4; the x = 0.5 composition has a small impurity contamination. The lattice constants are linear with x and range from 10.224 Å (x = 0.0) to 10.461 Å (x = 0.5). These lattice constants span the same range as the R₂Mo₂O₇ series from R = Y to R = Nd. In this series, there is a discontinuous change from ferromagnetic long-range order to short-range spin-glass-like order between R = Gd and R = Tb. Yet, the solid solutions all show spin-glass-like properties with maxima in the susceptibility in the 20–25 K range and sample-history-dependent effects at lower temperatures. Deviations from the Curie-Weiss Law occur well above the susceptibility maxima. The Weiss constants change from −61 to +41 K for x = 0.0 and x = 0.5, respectively, indicating a competition between antiferromagnetic and ferromagnetic exchange interactions. This competition, coupled with the inherent frustration of the Mo⁴⁺ lattice in space group Fd3m is a possible origin of the spin-glass properties.     

Vibrational spectroscopic and X-ray diffraction study of crystalline phases in the Li2O-SiO2 system

xLi₂O · (100 ? x)SiO₂ phases, where x = 20, 25, 33, 40, 50, 55, and 60 mol % (hereinafter, Li20, Li25, Li33, Li40, Li50, Li55, and Li60) were crystallized from melts, and their qualitative composition was determined by X-ray diffraction. The Li₆Si₂O₇ phase was established to precipitate during the crystallization of the melt containing 60 mol % Li₂O, thus enabling us to locate characteristic bands in the IR and Raman spectra of lithium pyrosilicate.     

CS families derived from the ReO3 structure type: An electron microscope study of reduced WO3 and related pseudobinary systems

A broad survey of some ReO₃-related CS structures has been carried out by means of an electron microscopy/diffraction study of the systems WO_x and (W⁶⁺, M⁵⁺O_x with M = V, Nb, Ta and 2.90 ? x < 3.00; MoO_x and (W, Mo)O_x; Nb(O,F)_x with 2.75 ? x ? 2.96; as well as V₂MoO₈. The observations give a broader view of the field than was previously possible. (i) They reveal the expected “swinging” of the CS planes when the compositions and/or the component ions are varied. (ii) They provide information on the degree of long-range order and the types of defect present. (iii) They throw further light on the likely mechanism by which the composition changes when a CS structure is reduced or oxidized. These topics are discussed, and an attempt is made to systematize all the observations into a coherent picture incorporating previously available information. This necessitates changing some earlier, dubious conclusions from X-ray diffraction studies.     

Synthesis,optical properties,and photocatalytic activity of lanthanide-doped anatase

Quasi-one-dimensional (1D) Ti_{1 ? x} Ln _x O_{2 ? x/2} anatase solid solutions were prepared by heating Ti_{1 ? x} Ln _x (OCH₂CH₂O)_{2 ? x/2} precursors, where Ln = Nd, Eu, Tb, Er (x = 0.025), or Sm (0.005 ≤ x ≤ 0.025), in air. These solid solution were found to have a photocatalytic activity in hydroquinone oxidation in aqueous solution under exposure to UV radiation. UV-Vis absorption spectra were recorded for Ti_{1 ? x} Ln _x O_{2 ? x/2} (Nd, Sm, Eu, Tb, or Er). The electronic structure and optical absorption spectra were calculated for anatase doped with neodymium, samarium, or terbium.     

Synthesis of Ba(V1−xTix)S3 (0 ≦ x ≦ 1.0) compounds and their structural transitions

The two-layer hexagonal perovskites Ba(V_1?xTi_x)S₃ (0 ≦ x ≦ 1.0) are prepared in a H₂S stream. A in a H₂S stream. A structural phase transition from a hexagonal to an orthorhombic form takes place for the powder samples with 0 ≦ x ≦ 0.4 and their transition temperatures are determined to be 250 K for x = 0, 240 K for x = 0.1, 222 K for x = 0.2, 195 K for x = 0.3, and 160 K for x = 0.4, respectively. The phase transformation does not occur down to 90 K for the materials above x = 0.5.     

Etude par résonance magnétique des phénomènes diffusifs dans les phases du système NaFBiF3

¹⁹F and ²³Na wide-line NMR studies are reported on NaBiF₄ and the solid solution Na_1?xBi_xF_1+2x for x = 0.62 and x = 0.68 in the ?100° to +200°C temperature range. No sodium mobility could be detected in NaBiF₄, but the mobility is significant in the solid solution. The fluorine motions are weakly activated (0.28 eV) in NaBiF₄ and result only from short-range motion (correlated motions or reorientations). In the solid solution the concentration of mobile fluoride ions increases with temperature. Compared to the values given by complex impedance measurements, the activation energies in this thermal range (0.18 eV for x = 0.62 and 0.20 eV for x = 0.68) may be correlated with phenomena preceding the long range conductivity which appears at higher temperature.