Incommensurate spin order in the metallic perovskite MnVO3

Incommensurate Mn spin order has been discovered in the perovskite MnVO3 containing localized 3d5 Mn2+ and itinerant 3d1 V4+ states. This phase has a distorted Pnma crystal structure (a = 5.2741(6) Å, b = 7.4100(11) Å, and c = 5.1184(8) Å at 300 K) and is metallic at temperatures of 2-300 K and at pressures of up to 67 kbar. Neutron scattering reveals a (0.29 0 0) magnetic vector below the 46 K spin ordering transition, and both helical and spin density wave orderings are consistent with the diffraction intensities. Electronic structure calculations show large exchange splittings of the Mn and V 3d bands, and (kx 0 0) crossings of the Fermi energy by spin up and down V 3d bands may give rise to Ruderman-Kittel-Kasuya-Yosida coupling of Mn moments, in addition to their superexchange interactions. © 2011 American Physical Society

The Crystal Chemistry of Ca_{10–<i>y</i>}(SiO₄)₃(SO₄)₃Cl_{2–<i>x</i>–2<i>y</i>}F_<i>x</i> Ellestadite

Fluor-chlorellestadite solid solutions Ca₁₀(SiO₄)₃(SO₄)₃Cl_2–<i>x</i>F_<i>x</i>, serving as prototype crystalline matrices for the fixation of hazardous fly ash, were synthesized and characterized by powder X-ray and neutron diffraction (PXRD and PND), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). The lattice parameters of the ellestadites vary linearly with composition and show the expected shrinkage of unit cell volume as fluorine (IR = 1.33 Å) displaces chlorine (IR = 1.81 Å). FTIR spectra indicate little or no OH^– in the solid solutions. All compositions conform to <i>P</i>6₃/<i>m</i> symmetry where F^– is located at the 2<i>a</i> (0, 0, ¹/₄) position, while Cl^– is displaced out of the 6<i>h</i> Ca(2) triangle plane and occupies 4<i>e</i> (0, 0, <i>z</i>) split positions with <i>z</i> ranging from 0.336(3) to 0.4315(3). Si/S randomly occupy the 6<i>h</i> tetrahedral site. Ellestadites rich in Cl (<i>x</i> ≤ 1.2) show an overall deficiency in halogens (<2 atom per formula unit), particularly Cl as a result of CaCl₂ volatilization, with charge balance achieved by the creation of Ca vacancies (Ca²⁺ + 2Cl^– →□_Ca + 2□_Cl) leading to the formula Ca_{10–<i>y</i>}(SiO₄)₃(SO₄)₃Cl_{2–<i>x</i>–2<i>y</i>}F_<i>x</i>. For F-rich compositions the vacancies are found at Ca(2), while for Cl-rich ellestadites, vacancies are at Ca(1). It is likely the loss of CaCl₂ which leads tunnel anion vacancies promotes intertunnel positional disorder, preventing the formation of a <i>P</i>2₁/<i>b</i> monoclinic dimorph, analogous to that reported for Ca₁₀(PO₄)₆Cl₂. Trends in structure with composition were analyzed using crystal-chemical parameters, whose systematic variations served to validate the quality of the Rietveld refinements

From T to T′-La₂CuO₄ via Oxygen Vacancy Ordered La₂CuO_3.5

T-La₂CuO₄ can be transformed into T′-La₂CuO₄ via a two-step topotactic reaction mechanism. First T-La₂CuO₄ is reduced by CaH₂ at 200 °C to La₂CuO_3.5 which can be subsequently reoxidized to T′-La₂CuO₄ at 300 °C in air. La₂CuO_3.5 does not adopt the Sr₂CuO₃ type “S-phase” structure, as taken for granted for almost 20 years, but constitutes an oxygen deficient T′-framework, with copper in 4-fold planar and 2-fold linear dumbbell coordination. Upon heating the T-La₂CuO₄ phase is reobtained from T′-La₂CuO₄ above 650 °C, confirming the “T-phase” to be the thermodynamically stable modification at least at higher temperature

Triangular Exchange Interaction Patterns in K₃Fe₆F₁₉: An Iron Potassium Fluoride with a Complex Tungsten Bronze Related Structure

The synthesis and structural and magnetic characterizations of K₃Fe₆F₁₉, a new iron potassium fluoride with a complex tungsten bronze related structure, are presented. This phase was found during the investigation of relatively low-temperature (600 °C) synthesis conditions of classical tetragonal tungsten bronze (TTB) fluorides and can be considered an intermediate that forms at this temperature owing to faster crystallization kinetics. The K₃Fe₆F₁₉ compound has an orthorhombic structure (space group <i>Cmcm</i> (63), <i>a</i> = 7.6975(3) Å, <i>b</i> = 18.2843(7) Å, <i>c</i> = 22.0603(9) Å) related to the TTB one, where the perovskite cage is substituted by a large S-shaped channel simultaneously occupied by two potassium atoms. The magnetic structure, characterized by magnetization measurements on an oriented single crystal and powder neutron diffraction, is dominated by the presence of interconnected double stripes of antiferromagnetic triangular exchange interaction patterns alternately rotated in clock- and anticlockwise fashion. The magnetic order takes place in a wide temperature range, by increasing progressively the interaction dimensionality

Triangular Exchange Interaction Patterns in K₃Fe₆F₁₉: An Iron Potassium Fluoride with a Complex Tungsten Bronze Related Structure

The synthesis and structural and magnetic characterizations of K₃Fe₆F₁₉, a new iron potassium fluoride with a complex tungsten bronze related structure, are presented. This phase was found during the investigation of relatively low-temperature (600 °C) synthesis conditions of classical tetragonal tungsten bronze (TTB) fluorides and can be considered an intermediate that forms at this temperature owing to faster crystallization kinetics. The K₃Fe₆F₁₉ compound has an orthorhombic structure (space group <i>Cmcm</i> (63), <i>a</i> = 7.6975(3) Å, <i>b</i> = 18.2843(7) Å, <i>c</i> = 22.0603(9) Å) related to the TTB one, where the perovskite cage is substituted by a large S-shaped channel simultaneously occupied by two potassium atoms. The magnetic structure, characterized by magnetization measurements on an oriented single crystal and powder neutron diffraction, is dominated by the presence of interconnected double stripes of antiferromagnetic triangular exchange interaction patterns alternately rotated in clock- and anticlockwise fashion. The magnetic order takes place in a wide temperature range, by increasing progressively the interaction dimensionality

Electrical and Structural Characterization of Ba₃Mo_1–<i>x</i>Nb_1+<i>x</i>O_{8.5–<i>x</i>/2}: The Relationship between Mixed Coordination, Polyhedral Distortion and the Ionic Conductivity of Ba₃MoNbO_8.5

The electrical and structural properties of the series Ba₃Mo_1–<i>x</i>Nb_1+<i>x</i>O_{8.5–<i>x</i>/2} (<i>x</i> = 0.0, 0.1, 0.2, 0.3) have been determined. Ba₃Mo_1–<i>x</i>Nb_1+<i>x</i>O_{8.5–<i>x</i>/2} crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures, in which (Mo/Nb)­O₄ and (Mo/Nb)­O₆ units coexist within the structure. Nb substitutes preferentially at the octahedral site so that the ratio of (Mo/Nb)­O₄ tetrahedra to (Mo/Nb)­O₆ octahedra decreases with increasing x resulting in a reduction in the magnitude of the ionic conductivity from 1.3 × 10^–6 S cm^–1 for x = 0.0 to 1.1 × 10^–7 S cm^–1 for <i>x</i> = 0.3 at 300 °C. However, upon heating the conductivities of the solid solution converge, which suggests that the unusual thermal structural rearrangement previously reported for Ba₃MoNbO₈ preserves the high temperature conductivity. The results demonstrate that the presence of (Mo/Nb)­O₄ tetrahedra with nonbridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba₃MoNbO_8.5 system

Structural, Magnetic, and Electronic Properties of CaBaCo_4–<i>x</i>M_<i>x</i>O₇ (M = Fe, Zn)

The effect of substituting iron and zinc for cobalt in CaBaCo₄O₇ was investigated using neutron diffraction and X-ray absorption spectroscopy techniques. The orthorhombic distortion present in the parent compound CaBaCo₄O₇ decreases with increasing the content of either Fe or Zn. The samples CaBaCo₃ZnO₇ and CaBaCo_4–<i>x</i>Fe_<i>x</i>O₇ with <i>x</i> ≥ 1.5 are metrically hexagonal, but much better refinements in the neutron diffraction patterns are obtained using an orthorhombic unit cell. The two types of substitution have opposite effects on the structural and magnetic properties. Fe atoms preferentially occupy the sites at the triangular layer. Thus, the replacement of Co by Fe suppresses the ferrimagnetic ordering of the parent compound, and CaBaCo_4–<i>x</i>Fe_<i>x</i>O₇ (0.5 ≤ <i>x</i> ≤ 2) samples are antiferromagnetically ordered following a new propagation vector <i>k</i> = (1/3,0,0). However, the Zn atoms prefer occupying the Kagome layer, which is very detrimental for the long-range magnetic interactions giving rise to a magnetic glass-like behavior in the CaBaCo₃ZnO₇ sample. The oxidation states of iron and zinc are found to be 3+ and 2+, respectively, independently of the content, as confirmed by X-ray absorption spectroscopy. Therefore, the average Co oxidation state changes accordingly with the Fe³⁺ or Zn²⁺ doping. Also, X-ray absorption spectroscopy data confirm the different preferential occupation for both Fe and Zn cations. The combined information obtained by neutron diffraction and X-ray absorption spectroscopy indicates that cobalt atoms can be either in a fluctuating Co²⁺/Co³⁺ valence state or, alternatively, Co²⁺ and Co³⁺ ions being randomly distributed in the lattice. These results explain the occurrence of local disorder in the CoO₄ tetrahedra obtained by EXAFS. An anomaly in the lattice parameters and an increase in the local disorder are observed only at the ferrimagnetic transition for CaBaCo₄O₇, revealing the occurrence of local magneto-elastic coupling

Cubic Sr₂ScGaO₅ Perovskite: Structural Stability, Oxygen Defect Structure, and Ion Conductivity Explored on Single Crystals

Oxygen-deficient Sr₂ScGaO₅ single crystals with a cubic perovskite structure were grown by the floating-zone technique. The transparent crystals of this pure 3D oxygen electrolyte are metastable at ambient temperature, showing one-sixth of all oxygen positions vacant. While neutron single-crystal diffraction, followed by maximum entropy analysis, revealed a strong anharmonic displacements for the oxygen atoms, a predominant formation of ScO₆ octahedra and GaO₄ tetrahedra is indicated by Raman spectroscopic studies, resulting in a complex oxygen defect structure with short-range order. Temperature-dependent X-ray powder diffraction (XPD) and neutron powder diffraction (NPD) studies reveal the cubic Sr₂ScGaO₅ to be thermodynamically stable only above 1400 °C, while the stable modification below this temperature shows the brownmillerite framework with orthorhombic symmetry. Cubic Sr₂ScGaO₅ remains surprisingly kinetically stable upon heating from ambient temperature to 1300 °C, indicating a huge inertia for the retransformation toward the thermodynamically stable brownmillerite phase. Ionic conductivity investigated by impedance spectroscopy was found to be 10^–4 S/cm at 600 °C, while oxygen ¹⁸O/¹⁶O isotope exchange indicates a free oxygen mobility to set in at around 500 °C

Investigation of the Relationship between the Structure and Conductivity of the Novel Oxide Ionic Conductor Ba₃MoNbO_8.5

A variable temperature neutron diffraction study of the novel oxide ion conductor Ba₃MoNbO_8.5 has been performed between 25 and 600 °C. Nonmonotonic behavior of the cell parameters, bond lengths, and angles are observed indicating a structural rearrangement above 300 °C. The oxygen/vacancy distribution changes as the temperature increases so that the ratio of (Mo/Nb)­O₄ tetrahedra to (Mo/Nb)­O₆ octahedra increases upon heating above 300 °C. A strong correlation between the oxide ionic conductivity and the number of (Mo/Nb)­O₄ tetrahedra within the average structure of Ba₃MoNbO_8.5 is observed. The increase in the number of (Mo/Nb)­O₄ tetrahedra upon heating from 300–600 °C most likely offers more low energy transition paths for transport of the O^2– ions enhancing the conductivity. The unusual structural rearrangement also results in relaxation of Mo(1)/Nb(1) and Ba(2) away from the mobile oxygen, increasing the ionic conductivity. The second order Jahn–Teller effect most likely further enhances the distortion of the MO₄/MO₆ polyhedra as distortions created by both electronic and structural effects are mutually supportive

A- and B‑Site Ordering in the A‑Cation-Deficient Perovskite Series La_2–<i>x</i>NiTiO_6−δ (0 ≤ <i>x</i> < 0.20) and Evaluation as Potential Cathodes for Solid Oxide Fuel Cells

The La_2–<i>x</i>NiTiO_6−δ (0 ≤ <i>x</i> < 0.2) series has been investigated in order to assess its possible use as a solid oxide fuel cell (SOFC) cathode material. These perovskite-like oxides exhibit monoclinic symmetry, as determined by a series of high-resolution structural techniques (X-ray diffraction (XRD), neuron powder diffraction (NPD), selected-area electron diffraction (SAED), and transmission electron microscopy (TEM)). Ni and Ti order over the B-site and, unusually, for <i>x</i> > 0, the A-site ions are also ordered along the <i>c</i>-axis in alternate La-rich and □-rich layers (where □ represents a vacancy). Structural determination combined with accurate compositional and magnetic characterization indicates a change in the predominant charge-compensating mechanism of A-site vacancies with composition. For <i>x</i> = 0.1, oxygen-vacancy formation seems to be the main-charge compensating mechanism, whereas, for <i>x</i> = 0.2, partial replacement of Ni by Ti in the B-substructure is dominant. In addition, a small amount of trivalent nickel is present in all samples. The composition dependence of the electrical conductivity of La_2–<i>x</i>NiTiO_6−δ (<i>x</i> = 0, 0.1, 0.2), investigated by impedance spectroscopy, as a function of temperature and oxygen partial pressure, is successfully interpreted on the basis of the relevant charge-compensating mechanisms and associated valence states. Thermal and chemical stability have also been studied in order to perform a preliminary electrochemical characterization as prospective cathode materials for SOFCs. The material La_1.80NiTiO_6‑δ exhibits excellent stability under oxidizing conditions and a polarization resistance of ∼0.5 Ω cm² at 1073 K with a yttria-stabilized zirconia (YSZ) electrolyte, slightly lower than that of the state-of-the-art La_1–<i>x</i>Sr_<i>x</i>MnO₃ (LSM)-based cathodes. A higher thermal stability and a better chemical compatibility of La_1.80NiTiO_6−δ with common electrolytes (e.g., YSZ), in comparison with LSM, suggests that this oxide warrants further study and optimization as a prospective improved cathode material for SOFCs