Using CBED and crystallographic image processing to evidence a structural distortion in a new family of ionic conductor Sr1−xLa1+xAl1−xMgxO4 (0?x?0.7)

A previous study by Raman scattering of the Sr_1−xLa_1+xAl_1−xMg_xO₄ solid solution evidenced a distortion from ideal K₂NiF₄ structure. X-ray powder diffraction and selected area electron diffraction studies were carried out and no lowering of symmetry was observed. All the reflections could be indexed in the space group I4/mmm with a=b=0.38 nm and c=1.27 nm. A coupled study by convergent beam electron diffraction and crystallographic image processing was performed. These techniques have been used to determine a crystal distortion due to small atom displacements from mirrors or axes, which lower the structure symmetry. The mm2 point group symmetry was determined. A microdiffraction study leads to the Imm2 space group. This orthorhombic distortion allows a better understanding of the ionic conductivity behavior of these compounds.     

Synthesis and structural studies of Sr2Co2−xGaxO5, 0.3?x?0.8

The novel oxide Sr₂Co_2−xGa_xO₅ 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 T_N=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.     

Electron diffraction evidence for continuously variable, composition-dependent O/F ordering in the ReO3 type, Nb1−xNbxO2−xF1+x, 0?x?0.48, solid solution

A spread of compositions within the Nb^V_1−xNb^IV_xO_2−xF_1+x solid solution have been successfully synthesized in the range 0?x?0.48. In each case electron diffraction has revealed the presence of strong planar diffuse scattering perpendicular to each of the three 〈001〉 directions at the G±〈hkq〉^* regions of reciprocal space, where G represents a Bragg reflection of the ReO₃ type average structure, h and k are continuously variable and q is a fixed (at any one particular composition), composition-dependent parameter in the range from 1/3 to 1/2. Careful electron probe microanalysis indicates that the value of q is directly related to composition via the equation q=(x+1)/3 and hence acts as a “chemical ruler” for the composition. A plausible model for the implied one-dimensional O/F ordering as well as for the observed shift in the position of the diffuse versus composition is provided via a crenel type occupational atomic modulation function.     

Synthesis and characterization of La1+xSr2−xCoMnO7−δ (x=0,0.2; δ=0,1)

The n=2 Ruddlesden-Popper phases LaSr₂CoMnO₇ and La_1.2Sr_1.8CoMnO₇ have been synthesized by a sol-gel method. The O6-type phases LaSr₂CoMnO₆ and La_1.2Sr_1.8CoMnO₆ 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.     

Influence of Oxygen Nonstoichiometry on the Spectral Properties of Solid Solutions LaNb2−2xTa2xVO9−δ (x=0-0.4) and LaTa2−2xNb2xVO9−δ (x=0-0.1)

Phase equilibria in the LaVO₄-Nb₂O₅-Ta₂O₅ system were analyzed. New solid solutions LaTa_2−2xNb_2xVO_9−δ (x=0-0.1) and LaNb_2−2xTa_2xVO_9−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 LaTa_2−2xNb_2xVO_9−δ was found. The IR spectrum of LaTa₂VO_9−δ transformed in going from δ=0 to δ≠0. Two types of VO₄ tetrahedra were formed in solid solutions LaNb_2−2xTa_2xVO_9−δ depending on δ(x).     

Conductivity and carrier traps in La1−xSrxCo1−zMnzO3−δ (x=0.3; z=0 and 0.25)

Measurements of the equilibrium oxygen content, electrical conductivity and thermopower in the perovskite-like solid solution La_0.7Sr_0.3Co_1-zMn_zO_3−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 La_0.7Sr_0.3Co_1−zMn_zO_3−δ in favorable correspondence with experimental observations.     

Neutron powder diffraction study of TbBaCo2−xFexO5+γ layered oxides

The crystal and magnetic structures of layered perovskites TbBaCo_2−xFe_xO_5+γ (0.08?x?0.24) were studied by neutron powder diffraction. Increasing iron concentration up to the x values higher than 0.10 leads to the orthorhombic→tetragonal phase transition resulting from the transformation of 2×1×1-type superstructure, formed due to ordering of extra oxygen incorporated into the vacant sites in [TbO_γ] layers, into 3×3×1 superlattice. The concentration ranges, where the orthorhombic and tetragonal lattices exist, are separated with a narrow two-phase domain. For the tetragonal phases with 3×3×1 superstructure (space group P4/mmm), the Co/Fe ions are antiferromagnetically coupled, forming G-type spin-ordered configuration. The Co³⁺ cations located in square-pyramidal sites adopt intermediate spin state, whilst a relatively small magnetic moment of Co³⁺ ions in the octahedral sublattice indicates that a minor fraction of cobalt is in the low-spin state.     

Phase stability of La1−xCaxCrO3−δ in oxidizing atmosphere

The chemical stability of perovskite-type La_1−xCa_xCrO_3−δ (x=0.1, 0.2, 0.3) in high oxygen partial pressure, P_O2, 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 P_O2. 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 P_O2 and at low temperature. The single-phase regions of La_1−xCa_xCrO_3−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 La_1−xCa_xCrO_3−δ in high P_O2 concerning the application to an interconnector of high-temperature solid oxide fuel cells; for example, La_0.7Ca_0.3CrO_3−δ is stable at 1273 K in air.     

Electron/hole and ion transport in La1−xSrxFeO3−δ

The conductivity of the entire solid solution La_1−xSr_xFeO_3−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.     

Evolution of magnetic state in the La1−xCaxMnO3−γ (x=0.30, 0.50) manganites depending on the oxygen content

The crystal structure, magnetic and electrical properties of the La_1−xCa_xMnO_3−γ (x=0.30, 0.50; 0?γ?0.50) oxygen-deficient manganites have been studied. It is found that the compounds La_0.70Ca_0.30MnO_3−γ possess a long-range ferromagnetic order up to γ=0.06 and a cluster spin glass behavior at 0.06<γ?0.20. Antiferromagnetic state of La_0.50Ca_0.50MnO_3−γ (γ=0) composition transforms into inhomogeneous ferromagnetic one at γ=0.04. The system converts into cluster spin glass state at γ=0.10. As oxygen deficit reaches the value γ=0.25, a new type of ferromagnetic phase appears. The fraction of this ferromagnetic phase is the highest in the composition γ=0.30. It is supposed that the compounds with γ?0.35 represent an antiferromagnetic medium with inclusions of the ferromagnetic phase. The strongly reduced samples exhibit a large magnetoresistance below the temperature, at which the spontaneous magnetization develops. The magnetic phase diagrams of both La_0.70Ca_0.30MnO_3−γ and La_0.50Ca_0.50MnO_3−γ systems have been constructed. We argue, that the oxygen vacancies are disordered in the La_0.70Ca_0.30MnO_3−γ system in the studied region of oxygen vacancies concentration (0?γ<0.20) whereas for the La_0.50Ca_0.50MnO_3−γ they tend to order at γ>0.25 in a manner of Sr₂Fe₂O₅-type crystal structure. This study shows that Mn³⁺-O-Mn³⁺ ferromagnetic interaction may play an important role in the formation of magnetic state of manganites.     

Structural properties of LaxSr2−xFeO4±δ at high temperature and under reducing conditions

A series of materials, La_xSr_2−xFeO_4±δ (x=0.6-1.0), have been investigated structurally using neutron and X-ray diffraction techniques as well as Mössbauer spectroscopy as a function of temperature and composition. These materials adopt the I4/mmm K₂NiF₄-type structure over a wide range of temperatures (25-1200°C) and oxygen stoichiometries. In particular, it was observed that at a critical composition of x>0.8 there was a significant shift in the lattice parameters. This was attributed to changes in the Fe³⁺ content and the resultant effect of this on the d_z² orbitals giving a lengthening of the Fe-O bonds. On heating, completely linear behavior in both the a and c cell constants was observed, masking underlying bond length changes.     

The synthesis, crystal chemistry and structures of Y-doped brannerite (U1−xYxTi2O6) and thorutite (Th1−xYxTi2O6−δ) phases

Yttrium-doped uranium brannerite (U_1−xY_xTi₂O₆) and thorutite (Th_1−xY_xTi₂O_6−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 U_0.54Y_0.46Ti₂O₆ (1) (a=9.8008(2); b=3.7276(1); c=6.8745(1); β=118.38(1); V=220.97(1); Z=2; R_P=7.3%; R_B=4.6%) and Th_0.91Y_0.09Ti₂O_6−δ (2) (a=9.8002(7); b=3.7510(3); c=6.9990(5); β=118.37(4); V=226.40(3); Z=2; R_P=4.5%; R_B=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.     

LaxSr2−xFeyRu1−yO4±δ: a new family of K2NiF4 type oxides

The phases La_xSr_2−xFe_yRu_1−yO_4±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.     

Order-Disorder Enhanced Oxygen Conductivity and Electron Transport in Ruddlesden-Popper Ferrite-Titanate Sr3Fe2−xTixO6+δ

The Ruddlesden-Popper ferrite Sr₃Fe₂O_6+δ and its titania-doped derivatives Sr₃Fe_2−xTi_xO_6+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 Sr₃Fe₂O_6+δ and Sr₃Fe_0.8Ti_1.2O_6+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.     

Electronic, Magnetic, and Magnetoresistance Properties of the n=2 Ruddlesden-Popper Phases Sr3Fe2−xCoxO7−δ (0.25≤x≤1.75)

A series of oxygen-deficient n=2 Ruddlesden-Popper phases, Sr₃Fe_2−xCo_xO_7−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 Sr₃Fe_2−xCo_xO_7−δ system is compared to the Sr₃Fe_2−xMn_xO_7−δ series, where d localization dominates the properties.     

Effects of temperature and Nd composition on non-linear transport properties in substituted Ce1−xNdxO2−δ cerium dioxides

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 Ce_1−xNd_xO_2−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).     

Stability and oxygen ionic conductivity of zircon-type Ce1−xAxVO4+δ (A=Ca, Sr)

Zircon-type Ce_1−xA_xVO_4+δ (A=Ca, Sr; x=0-0.2) are stable in air up to approximately 1300 K, whilst further heating or reducing oxygen partial pressure leads to formation of A-site deficient zircon and CeO_2−δ phases. The stability boundaries of Ce_1−xA_xVO_4+δ are comparable to those of vanadium dioxide and calcium orthovanadate. At oxygen pressures lower than 10⁻¹⁵ atm, perovskite-type CeVO_3−δ is formed. The oxygen ion transference numbers of Ce_1−xA_xVO_4+δ, determined by faradaic efficiency measurements in air, vary in the range from 2×10⁻⁴ to 6×10⁻³ at 973-1223 K, increasing with temperature. The oxygen ionic conductivity has activation energy of 87-112 kJ/mol and is essentially independent of A-site dopant content. Contrary to the ionic transport, p-type electronic conductivity and Seebeck coefficient of Ce_1−xA_xVO_4+δ are influenced by the divalent cation concentration. The average thermal expansion coefficients of Ce_1−xA_xVO_4+δ, calculated from high-temperature XRD and dilatometric data in air, are (4.7-6.1)×10⁻⁶ K⁻¹.     

Oxygen Nonstoichiometry, Conductivity, and Seebeck Coefficient of La0.3Sr0.7Fe1−xGaxO2.65+δ Perovskites

The total electrical conductivity and the Seebeck coefficient of perovskite phases La_0.3Sr_0.7Fe_1−xGa_xO_2.65+δ (x=0-0.4) were determined as functions of oxygen nonstoichiometry in the temperature range 650-950°C at oxygen partial pressures varying from 10⁻⁴ to 0.5 atm. Doping with gallium was found to decrease oxygen content, p-type electronic conduction and mobility of electron holes. The results on the oxygen nonstoichiometry and electrical properties clearly show that the role of gallium cations in the lattice is not passive, as it could be expected from the constant oxidation state of Ga³⁺. The nonstoichiometry dependencies of the partial molar enthalpy and entropy of oxygen in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ are indicative of local inhomogeneities, such as local lattice distortions or defect clusters, induced by gallium incorporation. Due to B-site cation disorder, this effect may be responsible for suppressing long-range ordering of oxygen vacancies and for enhanced stability of the perovskite phases at low oxygen pressures, confirmed by high-temperature X-ray diffraction and Seebeck coefficient data. The values of the electron-hole mobility in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ, which increases with temperature, suggest a small-polaron conduction mechanism.