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 speciation of zirconium tetrafluoride complexes with phosphoryl-containing bases Ph<Subscript>3</Subscript>PO,Bu<Subscript>3</Subscript>PO and (Me<Subscript>2</Subscript>N)<Subscript>3</Subscript>PO in solutions

Zirconium fluoro complexes with phosphoryl-containing bases (L) have been synthesized by the reaction of a suspension of ZrF₄(dmso)₂ in toluene with an excess of Ph₃PO or Bu₃PO, as well as (Me₂N)₃PO in CH₂Cl₂. The composition and structure of the complexes in CH₂Cl₂ solutions have been studied by ¹⁹F NMR in the temperature range 293–203 K. Phosphine oxides are substituted for dmso to form mainly cis-tetrafluoro species, insignificant amounts of trans-tetrafluoro species, and mer- and fac-[ZrF₃L₃]⁺ complexes. In addition to these species, the reaction with Bu3PO yields the dimeric oxo complex (µ-O)[ZrF₃(Bu₃PO)₂]₂. Hydrolysis of fluoro complexes in CH₂Cl₂ with the use of an NBu₄OH solution in iso-PrOH does not lead to oxo and hydroxo complexes: first, the [ZrF₅L]^– complex is formed, and the final hydrolysis product is ZrF₆^2-. The fine structure of ¹⁹F NMR resonance lines for the zirconium fluoride compounds [ZrF5L]^–, cis- [ZrF₄L₂], dimeric oxo complex, and mer-[ZrF₃L₃]⁺ in organic solvents has been observed for the first time, which makes it possible to reliably identify the composition and structure of the zirconium coordination polyhedron in the complexes under consideration.     

Ionic Mobility in Glasses in the ZrF4-BiF3-MF Systems (M = Li,Na, K) as Probed by 7Li, 19F,and 23Na NMR

The ion mobility in new fluoride glasses (mol %) 45ZrF₄ · 25BiF₃ · 30MF (I) (M = Li, Na, K), (70 - x)ZrF₄ · xBiF₃ · 30LiF (II) (15 ≤ x ≤ 35), and 45ZrF₄ · (55-x)BiF₃ · xMF (III) (M = Li, Na; 10 ≤ x ≤ 30) has been studied by ⁷Li, ¹9F, and ²³Na NMR in the temperature range 250–500 K. The character of ion motion in bismuth fluorozirconate glasses I and III is determined by temperature and the nature and concentration of an alkali-metal cation. Major type of ion mobility in glasses I–III at temperature 400–440 K are local motions of fluorine-containing moieties and diffusion of lithium ions (except for the glass with x = 10). The factors responsible for diffusion in the fluoride sublattice of glasses I have been determined. Sodium ions in glasses I and III are not involved in ion transport.     

Catalytic properties of copper chromite ferrites in water gas shift reaction and hydrogen oxidation

The catalytic properties of a series of copper chromite ferrite samples with the composition CuCr_2–xFe_xO₄ (where x = 0–2) and a spinel-type structure in reactions with reducing (water gas shift reaction, WGSR) and oxidizing (the oxidation of hydrogen) reaction atmospheres were studied. The samples were obtained by the thermal decomposition of mixed hydroxo compounds. The distribution of Cu²⁺ ions in the tetrahedral and octahedral crystallographic positions of spinel, which depends on the Cr³⁺/Fe³⁺ ratio, affects the apparent activation energy (E_a) in both of the reactions. In WGSR, E_a is ～33 kJ/mol for CuCr₂O₄, in which Cu²⁺ ions mainly occupy tetrahedral positions, whereas E_a ≈ 100 kJ/mol for CuFe₂O₄, in which Cu²⁺ ions mainly occupy octahedral positions. In the reaction of hydrogen oxidation, E_a is ～71 kJ/mol for CuCr₂O₄ or ～42 kJ/mol for CuFe₂O₄. The value of E_a for the mixed chromite ferrites changes with the replacement of chromium ions by iron ions and, hence, with a ratio between the amounts of copper ions in the tetrahedral and octahedral oxygen positions of spinel.     

Ca10(CrVO4)6(CrVIO4), a disordered mixed‐valence chromium compound exhibiting inversion twinning

The structure analysis of so‐called 9CaO·4CrO₃·Cr₂O₃ proved it to be the title compound, decacalcium hexakis[chromate(V)] chromate(VI), with the simultaneous presence of unusual chromium oxidation states. The structure determination was carried out on a crystal that had inversion twinning. The Cr^VIO₄ tetrahedron is situated on a threefold axis and is disordered over two possible orientations that share three O atoms, while the Cr^VO₄ tetrahedra are in general positions and are ordered. The charge is balanced by Ca²⁺ cations, one of which is located on a threefold axis. The Ca²⁺ ions are coordinated by six, seven or eight O atoms. The compound is a significant phase in the CaO–CrO_x system and its formation reduces the refractoriness of calcium‐rich compositions in an oxidizing atmosphere.     

X-Ray CrKβ emission and K absorption spectra in mixed copper chromium oxides

The Kβ emission spectra of chromium in oxidic compounds were investigated by means of the fluorescent method. The origin of the main Kβ lines is discussed using MO theory and the dependence of the energy positions on the chemical bond is reported. The compounds studied include the oxides Cr₂O₃ CrO₂ and CrO₃, and the mixed copper chromium oxides CuCrO₂, CuCr₂O₄ and CuCrO₄. High-resolution chromium K-edge absorption spectra have been recorded using the synchroton radiation available at LURE (Orsay, France). The object of this investigation is to obtain further information about the chemical state analysis of chromium in catalysts: an evaluation of the atomic Cr(VI):Cr_total ratio in Cu_xCr_yO₄ catalysts is reported.     

Solid solution in the LiAlO2LiCrO2 ternary oxide system

The solid solution LiAl_xCr_1−xO₂ has been synthesized over the complete range 0 ⩽ x ⩽ 1. The syntheses at the high aluminum compositions (x ⩾ 0.60) have been achieved through stabilization of the α-LiAlO₂ structure with chromium ions in octahedral coordination.     

Zirconium titanate ceramic pigments: Crystal structure, optical spectroscopy and technological properties

Srilankite-type zirconium titanate, a promising structure for ceramic pigments, was synthesized at 1400 °C following three main doping strategies: (a) ZrTi_1−xA_xO₄, (b) ZrTi_1−x−yA_xB_yO₄ and (c) Zr_1−xC_xTiO₄ where A=Co, Cr, Fe, Mn, Ni or V (chromophores), B=Sb or W (counterions) and C=Pr (chromophore); x=y=0.05. Powders were characterized by XRD with Rietveld refinements and DRS in the UV-visible-NIR range; technological properties were appraised in several ceramic matrices (frits, glazes and body). Zirconium titanate can be usefully coloured with first row transition elements, giving green and greenish yellow (Co and Ni); orange-buff (Cr and V); tan-brown hues (Mn and Fe). In industrial-like synthesis conditions, a disordered structure as (Zr,Ti)O₂, with both Zr and Ti randomly distributed in the octahedral site, is achieved. Doping with chromophores and counterions induces unit cell dimensions variation and causes an oversaturation in zirconium oxide. Optical spectroscopy reveals the occurrence of Co²⁺, Cr³⁺, Fe³⁺, Mn²⁺, Mn³⁺, Ni²⁺, V³⁺ and V⁴⁺. The zirconium titanate pigments fulfil current technological requirements for low-temperature applications, but exhibit a limited chemico-physical stability for higher firing temperature and in chemically aggressive media.     

Synthesis,structure, ion mobility,phase transitions,and ion transport in rubidium ammonium hexafluorozirconates

Rubidium ammonium hexafluorozirconates Rb_2?x (NH₄)_x ZrF₆ (1.5 < x < 2.0) have been synthesized, and their structure, ion mobility (180–480 K), and electrophysical properties have been studied by X-ray crystallography, ¹H and ¹⁹F NMR, DTA, and impedance methods. Compounds with x > 1.5 are isostructural with (NH₄)₂ZrF₆. Rubidium cations are isomorphously substituted for the ammonium cations. The high-temperature modifications of the compounds, which form upon the phase transitions at 413–418 K, are characterized by translational diffusion of ions in the fluoride and ammonium sublattices. The ¹⁹F NMR spectra are characterized by uniaxial ¹⁹F magnetic shift anisotropy. The electrophysical properties of this series of compounds are studied in the temperature range 300–480 K.     

Structural properties and state of the surface layer of zirconium dioxide modified by tungstate anions

A tetragonal metastable phase of zirconium dioxide formed after the addition of tungstate anions (>13 mol %) to the hydroxide precursor by different methods with heating (600–700°C), as revealed by X-ray diffraction analysis and X-ray photoelectron and IR spectroscopy. The W⁶⁺ and W⁵⁺ cations formed a solid solution with ZrO₂. On the surface of the solid solution, the tungsten cations formed tungstate clusters (?WO _x ?)n. The formation of the WO₃ phase was observed at concentrations of tungstate anions higher than 17.6 mol % or at temperatures of 850–870°C.     

Studies regarding the formation from metal nitrates and diol of NiM 2 III O4 spinels, inside a silica matrix

The present study deals with preparation and characterization of spinel mixed oxide systems NiM ₂ ^III O₄, where M^III?=?Fe^III, Cr^III. In order to obtain 50% NiFe₂O₄/50% SiO₂ and 50% NiCr₂O₄/50% SiO₂ nanocomposite, we have used a versatile route based on the thermal decomposition inside the SiO₂ matrix, of some particular precursors, coordination compounds of the involved M^II and M^III cations with dicarboxylate ligands. The ligands form in the redox reaction between metal nitrates mixture and 1,3-propanediol at the heating around 140?°C of the gels (tetraethylorthosilicate?Cmetal nitrates?C1,3-propanediol?Cwater). The as-obtained precursors, embedded in silica gels, have been characterized by FT-IR spectrometry and thermal analysis. Both precursors thermally decompose up to 350?°C leading to the formation of the corresponding metal oxides inside the silica matrix. X-ray diffraction of the annealed powders have evidenced the formation of NiFe₂O₄ starting with 600?°C, and NiCr₂O₄ starting with 400?°C. This behavior can be explained by the fact that, by thermal decomposition of the Fe(III) carboxylate at 300?°C, the spinelic phase ??-Fe₂O₃ is formed, which interacts with the NiO, forming the ferrite nuclei. By thermal decomposition of chromium carboxylate, a nonstoichiometric chromium oxide (Cr₂O_3+x ) is formed. In the range 380?C400?°C, Cr₂O_3+x turns into Cr₂O₃ which immediately interacts with NiO leading to the formation of nickel chromites nuclei inside the pores of silica matrix. Both spinels have been obtained as nanocrystalites homogenously dispersed as resulted from XRD and TEM data.     

Der Einfluß von Sauerstoff,Wasser, Chlor und Brom auf den Transport von Kohlenstoff in fluorhaltigen Systemen

Quaternary System ZnSe - Cr₂Se₃ - In₂Se₃ The section Zn_1-xIn_0.667xCr₂Se₄ and ZnCr_2-yIn_ySe₄ as well as some samples of compositions outside these joins of the quaternary system ZnSe Cr₂Se₃ - In₂Se₃ were studied with the help of X-ray Guinier photographs of quenched samples. Whereas no detectable amounts of chromium can be incorporated into ZnIn₂Se₄ of the thiogallate structure (MnIn₂Te₄ type) in the case of the spinel ZnCr₂Se₄ (a = 1050.0 pm) up to 21 mol % of chromium and up to 20 mol % of zinc can be substituted by indium. However, spinel type solid solutions with larger indium content (up to a = 1076 pm) are formed by coincident substitution of both zinc and chromium corresponding to Zn_1-xIn_0.667xCr_2-yIn_ySe₄ (0 < x + y < 0.6) with indium in both tetrahedral and octahedral lattice sites.     

Study of the factors affecting the formation of copper–chromium/aluminum oxide compounds with a spinel structure

The effect of the Cr³⁺/Al³⁺ ratio on the crystallization temperature of mixed oxide compounds with a spinel structure and their structural features and morphological characteristics have been studied using a combination of physicochemical methods: thermal analysis, IR spectroscopy, X-ray powder diffraction, and electron microscopy. The role of temperature of synthesis and drying of Cu–Cr/Al hydroxy precursors in the formation of copper-containing spinels CuCr _x Al_2–x O₄, where x = 0–2, has been elucidated. The results are of interest for selection of the optimal composition and conditions of synthesis and formation of copper-containing spinels for their practical use.     

Structural characterization and physical properties of the system La1.33LixCrxTi2−xO6

New phases which arise from partial substitution of Ti⁴⁺ by Cr³⁺ and Li⁺ of the compound La_2/3TiO₃ have been obtained, giving rise to the series La_1.33Li_xCr_xTi_2−xO₆ (x=0.66, 0.55 and 0.44). These phases adopt a perovskite-type structure as deduced from their structural characterization. Rietveld's analyses of neutron diffraction data show that it is orthorhombic (S.G. Pbnm) with ordered domains. Conductivity has been examined by complex impedance spectroscopy and it increases with increasing lithium and chromium content. These materials behave as mixed conductors with low activation energies. Magnetic susceptibility variation with temperature shows antiferromagnetic interactions at the lowest temperatures.     

Influence of MgO/CeO2 ratio on CO2-oxidative transformation of C2H6 to C2H4 over highly active and stable Cr/MgO(x)–CeO2(100−x) nanocatalysts

Cr/MgO(x)–CeO₂(100?x) nanocatalysts were synthesized by a coprecipitation method and characterized by X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FESEM), energy-dispersive x-ray (EDX) spectroscopy, diffuse reflectance spectroscopy (DRS), and Brunauer–Emmett–Teller (BET) analysis. The effect of ceria addition on their physicochemical characteristics was investigated, and the results were correlated with their catalytic performance in oxidative dehydrogenation of ethane. A decrease in the size of the metal particles was found when adding a suitable content of ceria to the support. Crystalline Cr₂O₃ was not found in the calcined samples, indicating good dispersion of Cr species on the support. All samples showed nanosized particles with uniform morphology, with the best surface morphology for the Cr/MgO(50)–CeO₂(50) sample, on which the particle distribution mainly lay in the range of 40–60 nm. Variation of the amount of Ce in the support led to an enhancement of the Cr⁶⁺/Cr³⁺ ratio, with the highest value for the Cr/MgO(50)–CeO₂(50) sample. This catalyst effectively dehydrogenated ethane to ethylene with CO₂ at 700 °C even after 5 h on-stream, giving 42.76 % ethylene yield.

     

An X-ray diffraction, magnetic susceptibility and spectroscopic studies of Yb2−xCrxO3

Polycrystalline samples with general formula Yb_2−xCr_xO₃ (0<x<0.03), obtained by sol-gel method and analyzed by X-ray diffraction, formed solid solutions over all the mentioned range. Cr showed a maximum solubility of 2.8 mol% in Yb₂O₃ sesquioxide at 1000 °C. A preferential substitution of Cr³⁺ ions over two cationic sites, 8b and 24d in the space group Ia-3 was found. The lattice parameters a are found to vary linearly (10.4402(4) Å <a<10.4372(1) Å) with the composition x. The two independent atoms Yb/Cr have octahedral coordination; however, the degrees of distortion of their coordination polyhedron are different. Replacing Yb³⁺ by Cr³⁺ introduces slight changes in the atomic coordinates leading to an increase of the mean cation-anion distances. The ability of Raman spectroscopy to detect changes in local coordination is utilized. A pseudo-tetrahedral coordination for the Cr³⁺ in the 24d site was found. Magnetic susceptibility measurements of all samples were done in a temperature range of 2-50 K. For T<37 K, the inverse paramagnetic susceptibilities depend linearly on temperature. However, in the high-temperature region, for T>37 K, the inverse paramagnetic susceptibilities are non-linear versus temperature. This deviation from the Curie-Weiss behaviour was discussed.     

Magnetic Properties of Ti3O5 with Chromium Additions

Magnetic susceptibility measurements were used to study solid solutions Ti_3?xCr_xO₅ (0 ? × ? 0.17). Contributions to magnetic susceptibility for the α-phase were defined by the statistical analysis of the experimental data. A conclusion was made on the varing character of exchange interactions in α-Ti_3?xCr_xO₅ with increasing Cr content. In the present work, the information given indicates that Cr stabilizes the high-temperature α-Ti₃O₅ and decreases the temperature of semiconductor-metal phase transition. Magnetic properties of low-temperature α-Ti₃O₅ phase with chromium addition are shown to be explained by the existence of exchange-coupled pairs Ti³⁺? Cr³⁺.     

Consequence of doping mediated strain and the activation energy on the structural and optical properties of ZnO:Cr nanoparticles

We report on the sol-gel synthesis of Zn_1−xCr_xO (x=0.0, 0.05, 0.10, 0.15 and 0.20) nanoparticles. These nanoparticles were characterized by using thermogravimetry/differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman and Photoluminescence (PL). Electronegativity of Cr ions (Cr³⁺) reduces the final decomposition temperature by 40 °C and activation energy of the reaction when Cr is doped into ZnO. Doping of higher Cr concentration (x≥0.10) into ZnO shows formation of secondary spinel (ZnCr₂O₄) phase along with the hexagonal (ZnO) and is revealed by XRD. Formation of secondary phase changes the activation energy of the reaction and thus the strain in ZnO lattice. In Raman spectra, additional Raman modes have been observed for Zn_1−xCr_xO nanoparticles, which can be assigned to the modes generated due to Cr doping. The Cr doping into ZnO is also supported by PL, in which vacancies are formed with Cr ion incorporation and emission band shifts towards higher wavelength.