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1.
The phases LaxSr2−xFeyRu1−yO4±δ (x=0.2-0.8; y=0.6-0.9) have been synthesized by solid-state techniques and yield tetragonal structures with I4/mmm symmetry. The oxygen stoichiometry and high-temperature structures have been examined using diffraction techniques and in situ Mössbauer spectroscopy at temperatures up to ∼600°C. Furthermore, new reduced phases that adopt structures with Immm symmetry have been discovered. Unusual coordination numbers have been determined for the most highly reduced samples with square planar coordination evident for the B site cations. The reduced orthorhombic Immm phases were found to readily reoxidize in air to the tetragonal I4/mmm structure at relatively low temperatures of only ∼500°C. 相似文献
2.
H. El Shinawi J. Hadermann B. Santos F.J. Berry 《Journal of solid state chemistry》2010,183(6):1347-7122
The n=2 Ruddlesden-Popper phases LaSr2CoMnO7 and La1.2Sr1.8CoMnO7 have been synthesized by a sol-gel method. The O6-type phases LaSr2CoMnO6 and La1.2Sr1.8CoMnO6 were produced by reduction of the O7 phases under a hydrogen atmosphere. The materials crystallize in the tetragonal I4/mmm space group with no evidence of long-range cation order in the neutron and electron diffraction data. Oxygen vacancies in the reduced materials are located primarily at the common apex of the double perovskite layers giving rise to square pyramidal coordination around cobalt and manganese ions. The oxidation states Co3+/Mn4+ and Co2+/Mn3+ predominate in the as-prepared and reduced materials, respectively. The materials are spin glasses at low temperature and the dominant magnetic interactions change from ferro- to antiferromagnetic following reduction. 相似文献
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Jakoah Brgoch Ruslan Prozorov Gordon J. Miller 《Journal of solid state chemistry》2010,183(12):2917-2924
Polycrystalline samples of two complex intermetallic borides Zr2Fe1−δRu5+δB2 and Zr2Fe1−δ(Ru1−xRhx)5+δB2 (δ=ca. 0.10; x=0.20) were synthesized by high-temperature methods and characterized by single-crystal X-ray diffraction, energy dispersive spectroscopy, and magnetization measurements. Both structures are variants of Sc2Fe(Ru1−xRhx)5B2 and crystallize in the space group P4/mbm (no. 127) with the Ti3Co5B2-type structure. These structures contain single-atom, Fe-rich Fe/Ru or Fe/Ru/Rh chains along the c-axis with an interatomic metal-metal distance of 3.078(1) Å, a feature which makes them viable for possible low-dimensional temperature-dependent magnetic behavior. Magnetization measurements indicated weak ferrimagnetic ordering with ordering temperatures ca. 230 K for both specimens. Tight-binding electronic structure calculations on a model “Zr2FeRu5B2” using LDA yielded a narrow peak at the Fermi level assigned to Fe-Fe antibonding interactions along the c-axis, a result that indicates an electronic instability toward ferromagnetic coupling along these chains. Spin-polarized calculations of various magnetic models were examined to identify possible magnetic ordering within and between the single-atom, Fe-rich chains. 相似文献
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Yttrium-doped uranium brannerite (U1−xYxTi2O6) and thorutite (Th1−xYxTi2O6−δ) phases were synthesized in air at 1400°C. Powder X-ray diffraction revealed that these phases crystallized to form monoclinic (C2/m) structures. Crystal structures of U0.54Y0.46Ti2O6 (1) (a=9.8008(2); b=3.7276(1); c=6.8745(1); β=118.38(1); V=220.97(1); Z=2; RP=7.3%; RB=4.6%) and Th0.91Y0.09Ti2O6−δ (2) (a=9.8002(7); b=3.7510(3); c=6.9990(5); β=118.37(4); V=226.40(3); Z=2; RP=4.5%; RB=2.9%) were refined from powder neutron diffraction data. These two phases were isostructural, revealing planes of corner and edge-sharing TiO6 octahedra separated by irregular eight-fold coordinate U/Y or Th/Y atoms. The oxygen sites within the structure of 1 were found to be fully occupied, confirming that the doping of lower valence Y atoms occurs in conjunction with the oxidation of U(IV) to U(V). Y doping of the thorutite phase 2 does not lead to oxidation but rather the formation of oxygen vacancies within the structure. 相似文献
5.
Shigenori Onuma Keiji Yashiro Kenichi Kawamura Tatsuya Kawada Natsuko Sakai 《Journal of solid state chemistry》2003,170(1):68-74
The chemical stability of perovskite-type La1−xCaxCrO3−δ (x=0.1, 0.2, 0.3) in high oxygen partial pressure, PO2, was investigated with three methods: thermogravimetry, XRD analysis, and thermodynamic calculation. The second phase, CaCrO4 was observed by XRD analysis on the powder equilibrated in high PO2. Thermogravimetry under fixed temperatures sensitively detected the segregation of the second phase in the form of oxygen incorporation, because oxidation of chromium ion accompanies the segregation. The second phase tended to appear in high PO2 and at low temperature. The single-phase regions of La1−xCaxCrO3−δ obtained from the two experimental methods well agreed with each other. The results of thermodynamic calculation on the assumption of ideality of the solid solution also agreed with the experimental results. These results suggested the sufficient chemical stability of La1−xCaxCrO3−δ in high PO2 concerning the application to an interconnector of high-temperature solid oxide fuel cells; for example, La0.7Ca0.3CrO3−δ is stable at 1273 K in air. 相似文献
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LaFe1−xNixO3−δ (x=0.1−1.0) perovskites were synthesized via citrate route. The p(O2)-stability of the perovskite phases LaFe1−xNixO3−δ has been evaluated at 1100 °C based on the results of XRD analysis of powder samples annealed at various p(O2) and quenched to room temperature. The isothermal LaFeO3−δ-“LaNiO3−δ” cross-section of the phase diagram of the La-Fe-Ni-O system has been proposed in the range of oxygen partial pressure −15<log p(O2)/atm≤0.68. The unit cell parameters of orthorhombic perovskites O-LaFe1−xNixO3−δ increase with decrease in p(O2) at fixed composition x. This behavior is explained on the basis of size factor. The decomposition temperatures of rhombohedral phases R-LaFe1−xNixO3−δ for x=0.7, 0.8, 0.9 and 1.0 in air were determined as 1137, 1086, 1060 and 995 °C, respectively. 相似文献
8.
Keikichi Nakamura 《Journal of solid state chemistry》2003,173(2):299-308
The effects of oxygen reduction treatments on the magnetic properties of La-deficient manganites, La1−ΔMnO3+δ and Sr- and Ca-doped manganites, La1−xMxMnO3+δ (M: Sr, Ca) have been investigated to confirm the contrasting oxygen reduction effects on the magnetization properties. It is found that oxygen reduction treatments in reduced oxygen pressures of 103- for La1−ΔMnO3+δ result in a continuous change in the magnetization but the reduction treatments for La1−xMxMnO3+δ result in a negligible change under the same reduction conditions. To interpret the contrasting behavior of the La-deficient manganites, several possible models have been discussed. Among the models, the most probable model is that vacancies generated by the La deficiency Δ are partially replaced by Δ2(=Δ−Δ1?Δ1) Mn ions to give both La and Mn site vacancies according to the formula La1−ΔVΔMnO3+δ→{La1−ΔMnΔ2VΔ1}{Mn1−Δ2VΔ2}O3+δ. Details of thermodynamic basis of this model have been presented. 相似文献
9.
Latifa Aneflous Sylvie Villain Houria Benyaich 《Journal of solid state chemistry》2004,177(3):856-865
Cerium dioxides doped or substituted by neodymium have been prepared using low- (320°C) and high-temperature (1600°C) processes. The Nd substituted ceria phase obtained at high temperature is a solid solution Ce1−xNdxO2−δ 0?x?0.30. Electrical impedance spectroscopy analyses have been performed in the temperature range 40-700°C. At temperatures above 400°C, Nyquist representations allow to separate three signals corresponding to bulk, grain boundary and electrode responses. Non-linear variations of the resistance and the capacitance as functions of temperature and composition x are observed. In the case of grain boundary and electrode interface signals, constant phase elements with non-integer exponent n have been used to represent the equivalent circuits. For each contribution, the conductance strongly increases then reaches a limit value, above x=0.10. When composition x increases, the condensation of Nd-vacancy defect clusters might be at the origin of the non-linear evolution of the conductance. Bulk and grain boundary conductions present different activation energies (0.7 and 1.3 eV). 相似文献
10.
The novel oxide Sr2Co2−xGaxO5 with brownmillerite-type structure has been synthesized in the compositional range 0.3?x?0.8. Rietveld refinements using neutron powder diffraction data have been performed for the end compositions, x=0.3 and 0.8. The structure is best described in the space-group Icmm (no. 74) with unit cell parameters a=5.5678(6), 5.6126(7) Å, b=15.749(2), 15.733(2) Å and c=5.4599(6), 5.4559(7) Å for the x=0.3 and 0.8 compositions, respectively. The compounds were found to be G-type antiferromagnetic with the magnetic moments parallel to the c-axis. High-temperature magnetic susceptibility measurements confirmed the samples to be antiferromagnetic with Néel temperatures TN=505, 468 and 423 K for the x=0.3, 0.5 and 0.8 samples, respectively. High-resolution transmission electron microscopy and electron diffraction studies confirmed the I-centred structure and revealed the presence of disorder. 相似文献
11.
La4BaCu5−xMxO13+δ: M=Fe, Co, Ni, Zn were prepared by the solid-state route at 1000°C. Solid solution limits of x=1.0(1) [Fe], x=1.1(1) [Co], x=1.56(7) [Ni] and x=0.47(1) [Zn] were determined from XRD and EPMA results. Rietveld refinement of combined XRD/neutron powder diffraction data was carried out on undoped La4BaCu5O13+δ and x=1 for M=Fe, Co, Ni. For La4BaCu5O13+δ, which is an oxygen-deficient perovskite, the presence of square planar CuO4 groups, disordered over the Cu(2) sites with CuO5 square pyramids, is indicated, together with, for δ<0, either square planar CuO4 or square pyramidal CuO5 and octahedral CuO6 groups disordered over the Cu(1) sites. For M=Fe, Ni, there was preferential substitution onto the one-fold octahedral site; for M=Co, substitution took place on both the one-fold octahedral and four-fold square pyramidal sites. 相似文献
12.
M.V. Patrakeev E.B. Mitberg V.L. Kozhevnikov K.R. Poeppelmeier 《Journal of solid state chemistry》2003,172(1):219-231
The conductivity of the entire solid solution La1−xSrxFeO3−δ, where x=0.2, 0.4, 0.5, 0.7 and 0.9, in the oxygen partial pressure range 10−19-0.5 atm and temperatures between 750°C and 950°C is reported. The partial contributions from different charge carriers and the energetic parameters governing transport of charged species reveal that the lanthanum-strontium ferrites can be characterized as mixed, ion-electron conductors in the low oxygen pressure/high oxygen deficiency limit. The partial contributions to conductivity from oxygen ions, electrons and holes increase with strontium content and attain maximal values at x=0.5. Further increase in doping results in development of oxygen vacancy ordering phenomena and deterioration of conducting properties. 相似文献
13.
Y.A. ShilovaM.V. Patrakeev E.B. MitbergI.A. Leonidov V.L. Kozhevnikov K.R. Poeppelmeier 《Journal of solid state chemistry》2002,168(1):275-283
The Ruddlesden-Popper ferrite Sr3Fe2O6+δ and its titania-doped derivatives Sr3Fe2−xTixO6+δ, where 0<x≤2, have been characterized by X-ray powder diffraction and thermogravimetry. The changes in oxygen content and crystal lattice parameters are consistent with titanium ions entering the solid solution in 4+ oxidation state with octahedral oxygen coordination. Electronic conductivity measurements on polycrystalline Sr3Fe2O6+δ and Sr3Fe0.8Ti1.2O6+δ in the temperature range 750-1000°C and oxygen partial pressures (pO2) varying between 10−20 and 0.5 atm revealed that the predominant partial conductivity of electrons is proportional to pO2−1/4 in the low pO2 region, while the predominant partial contribution of holes to the conductivity is proportional to pO2+1/4 in the high pO2 range. The pressure-independent oxygen ion conductivity is found to decrease with the increase in titanium content. A possible pathway for oxygen ion migration is discussed in relation to disorder in the oxygen sublattice and titanium doping. 相似文献
14.
Gabriel M. VeithRongji Chen Guerman PopovMark Croft Yuliya ShokhIsrael Nowik Martha Greenblatt 《Journal of solid state chemistry》2002,166(2):292-304
A series of oxygen-deficient n=2 Ruddlesden-Popper phases, Sr3Fe2−xCoxO7−δ (0.25≤x≤1.75), were prepared by solid-state reactions. Temperature-dependent susceptibility and field-dependent magnetization data indicate that for x≥0.25 the dominant magnetic interactions are ferromagnetic. The onset of strong ferromagnetic interactions is evident at ∼200 K, and a transition to a cluster-glass state is observed for all compositions below ∼45 K. The temperature variation of resistivity for all the compounds shows variable-range hopping behavior with two different localization energy scales: one for T<40 K and another for T>80 K. Large negative magnetoresistance (the largest MR ∼−65% for x=0.25) is observed for all phases. The magnetic susceptibility, Mössbauer and X-ray absorption near-edge spectroscopy data indicate that the formal oxidation state of Fe is close to 4+. The key role of d delocalization in the Sr3Fe2−xCoxO7−δ system is compared to the Sr3Fe2−xMnxO7−δ series, where d localization dominates the properties. 相似文献
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M.G. Zuev E.V. ArkhipovaL.A. Perelyaeva L.V. ZolotukhinaO.B. Lapina R.N. Pletnev 《Journal of solid state chemistry》2002,167(1):73-79
Phase equilibria in the LaVO4-Nb2O5-Ta2O5 system were analyzed. New solid solutions LaTa2−2xNb2xVO9−δ (x=0-0.1) and LaNb2−2xTa2xVO9−δ (x=0-0.4) were detected in this system. The structures of the vanadate-niobate LaNb2VO9 and vanadate-tantalate LaTa2VO9 are not known. The structures of the vanadate-tantalate LaTa2VO9 and LaTa2VO9-based solid solutions are similar to the structure of LaTa7O19, which refers to the hexagonal crystal system. The influence of the oxygen nonstoichiometry δ(x) on crystallochemical characteristics and spectral properties of these solid solutions were examined by the X-ray phase analysis, IR and radio spectroscopic methods. A correlation between the nonstoichiometry δ(x) and the volume of a unit cell V(x) of solid solutions LaTa2−2xNb2xVO9−δ was found. The IR spectrum of LaTa2VO9−δ transformed in going from δ=0 to δ≠0. Two types of VO4 tetrahedra were formed in solid solutions LaNb2−2xTa2xVO9−δ depending on δ(x). 相似文献
18.
A.Yu. Suntsov I.A. Leonidov M.V. Patrakeev V.L. Kozhevnikov 《Journal of solid state chemistry》2011,184(8):1951-1955
The oxygen content, conductivity and thermopower in the double perovskite-like cobaltite PrBaCo2O5+δ are reported in the oxygen partial pressure range 2×10−6-0.21 atm and temperatures between 650 and 950 °C. The electrical properties are shown to be continuous through the transition from δ>0.5 to δ<0.5. The variations of transport parameters with temperature and oxygen content reveal hole polaron hopping conduction within oxygen non-stoichiometry domain δ<0.5. 相似文献
19.
V.L. Kozhevnikov I.A. Leonidov M.V. Patrakeev K.R. Poeppelmeier 《Journal of solid state chemistry》2003,172(2):296-304
Measurements of the equilibrium oxygen content, electrical conductivity and thermopower in the perovskite-like solid solution La0.7Sr0.3Co1-zMnzO3−δ (z=0 and 0.25) as a function of the temperature and oxygen partial pressure are used to determine the temperature dependence of the conductivity and thermopower at different values of the oxygen deficiency. A model for a hopping conductor with screened charge disproportionation is applied for the data analysis in combination with trapping reactions of n- and p-type carriers on local oxygen vacancy clusters and manganese cations, respectively. Changes in the ratio of n-type to p-type mobility are due to variations in oxygen vacancy concentration and manganese content, while the energetic parameters governing charge disproportionation of the trivalent cobalt cations and formation of vacancy associates are shown to be essentially invariable. These calculated charge carrier site occupancies are used to model temperature variations of the electrical properties in La0.7Sr0.3Co1−zMnzO3−δ in favorable correspondence with experimental observations. 相似文献
20.
Clear evidence (in the form of structured diffuse scattering) is found for short range ordering of metal ions and associated induced structural relaxation in two members of the cubic BZN pyrochlore (Bi1.5−αZn0.5−β)(Zn0.5−γNb1.5−δ)O(7−1.5α−β−γ−2.5δ) solid solution. An average neutron powder diffraction structure refinement is carried out for one of these. Electron probe micro-analysis suggests that the primary mechanism for non-stoichiometry in cubic BZN is the removal of ZnO from the nominally fully occupied (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 end-member. A detailed bond valence sum analysis of a recently reported average crystal structure is used to suggest possible local cation ordering schemes along with the induced displacive relaxation that is likely to accompany such local cation ordering. The observed diffuse distribution is qualitatively interpreted via Monte Carlo modelling. 相似文献