Structural aspects of Pr1−xSrxFeO3−w

Structural aspects of the distorted perovskite ABO₃ phase Pr_1−xSr_xFeO_3−w,x=0.00-0.80,w=0.000-0.332, were studied by powder X-ray diffraction, powder neutron diffraction, Mössbauer spectroscopy, and Fe K-, Sr K-, and Pr L_III-edge EXAFS techniques. The diffraction data revealed no indications for ordering of Pr and Sr at the A site, nor for oxygen vacancy ordering at O sites for heavily reduced samples. Mössbauer spectroscopy showed octahedral, square pyramidal, and tetrahedral Fe coordinations with relative amounts closely following the predictions for a binomial distribution of oxygen vacancies. In addition to Fe³⁺ and Fe⁴⁺, also Fe⁵⁺ appears at 77 K for (G-type) antiferromagnetic samples with high average Fe valence. This suggests dynamic 2 Fe⁴⁺↔Fe³⁺+Fe⁵⁺ fluctuations. At 296 K, a mixed valence Fe⁽³⁺ⁿ⁾⁺ component significantly improved the fit of Mössbauer spectra for the most oxidized paramagnetic samples. The qualitative EXAFS study shows that the local environments for Fe, Pr, and Sr strongly depend on x and w. The local Pr- and Sr-site geometries differ significantly from the cubic average structure for Pr_0.50Sr_0.50FeO_2.746.     

Vacancy ordering and oxygen dynamics in oxide ion conducting La1−xSrxGa1−xMgxO3−x ceramics: Ga, Mg and O NMR

The oxygen vacancies distribution in the rigid lattice and the thermally activated motion of oxygen atoms are studied in La_1−xSr_xGa_1−xMg_xO_3−x (x=0.00; 0.05; 0.10; 0.15 and 0.20) compounds. For that ⁷¹Ga, ²⁵Mg and ¹⁷O NMR was performed from 100 K up to 670 K, and ion conductivity measurements were carried out up to 1273 K. The comparison of the electric field gradients at the Ga- and Mg-sites evidences that oxygen vacancies appear exclusively near gallium cations as a species trapped below room temperature in local clusters, GaO_5/2-□-GaO_5/2. These clusters decay at higher temperature into mobile constituents of the structural octahedra Ga(O_5/6□_1/6)_6/2. At the same time, the nearest octahedral oxygen environment of magnesium cations persists at different doping levels. The case of two adjacent vacant anion sites is found highly unlikely within the studied doping range. The thermally activated oxygen motion starts to develop above room temperature as is observed from both the motional narrowing of ¹⁷O NMR spectra and the ¹⁷O nuclear spin-lattice relaxation rate. The obtained results show that two types of motion exist, a slow motion and a fast one. The former is a long-range diffusion whereas the latter is a local back and forth oxygen jumps between two adjacent anion sites. These sites are strongly differentiated by the probability of the vacancy formation, like the vacant apical site and the occupied equatorial site in the orthorhombic compositions x <0.15.     

Structural and magnetic properties of Nd18Li8Co4−xFexO39−y and Nd18Li8Co4−xTixO39−y

Nd₁₈Li₈Co₃FeO_39−y, Nd₁₈Li₈CoFe₃O_39−y and Nd₁₈Li₈Co₃TiO_39−y have been synthesised and characterised by neutron powder diffraction, magnetometry and Mössbauer spectroscopy. Their cubic structure (Pm3?n, a∼11.9 Å) is based on intersecting <1 1 1> chains comprised of alternating octahedral and trigonal-prismatic coordination sites. These chains lie within hexagonal-prismatic cavities formed by a Nd-O framework. Each compound has an incomplete oxide sublattice (y∼1), with vacancies located around the octahedral sites that lie at the points of chain intersection. These sites are fully occupied by a disordered arrangement of transition-metal cations but only 75% of the remaining octahedral sites are occupied. The trigonal-prismatic sites are fully occupied by lithium except in the case of Nd₁₈Li₈CoFe₃O_39−y where some iron is present. Antiferromagnetic interactions are present on the Nd sublattice in each composition, but a spin glass forms below 5 K when a high concentration of spins is also present on the octahedral sites.     

Polaron morphologies in SrFe1−xTixO3−δ

The morphologies of the charge carriers in the perovskite system SrFe_1−xTi_xO_3−δ are explored by transport and magnetic measurements. Oxygen vacancies are present in all samples, but they do not trap out the Fe³⁺ ions they introduce. The x=0.05 composition was prepared with three different values of δ. They all show small-polaron conduction above 225 K; but where there is a ratio c=Fe⁴⁺/Fe<0.5, the polaron morphology appears to change progressively with decreasing temperature below 225 K to two-Fe polarons that become ferromagnetically coupled in an applied magnetic field at lower temperatures; With an applied field of 2500 Oe, divergence of the paramagnetic susceptibility for zero-field-cooled and field-cooled samples manifests a greater stabilization of ferromagnetic pairs on cooling in the applied field. With a c>0.5, the data are consistent with a disproportionation reaction 2Fe⁴⁺=Fe³⁺+Fe(V)O_6/2 that inhibits formation of two-Fe polarons and, on lowering the temperature, creates Fe³⁺-Fe(V)-Fe³⁺ superparamagnetic clusters.     

Mixed conductivity and Mössbauer spectra of (La0.5Sr0.5)1−xFe1−yAlyO3−δ (x=0-0.05, y=0-0.30)

Aluminum incorporation in the rhombohedrally distorted perovskite lattice of (La_0.5Sr_0.5)_1−xFe_1−yAl_yO_3−δ (x=0-0.05, y=0-0.30) decreases the unit cell volume and partial ionic and p-type electronic conductivities, while the oxygen nonstoichiometry and thermal expansion at 900-1200 K increase on doping. The creation of A-site cation vacancies has an opposite effect on the transport properties of Al-substituted ceramics. The maximum A-site deficiency tolerated by the (La,Sr)(Fe,Al)O_3−δ structure is however limited, close to 3-4%. The Mössbauer spectroscopy revealed progressive localization of electron holes and a mixed charge-compensation mechanism, which results in higher average oxidation state of iron when Al³⁺ concentration increases. The average thermal expansion coefficients of (La_0.5Sr_0.5)_1−xFe_1−yAl_yO_3−δ are (12.2-13.0)×10⁻⁶ K⁻¹ at 300-900 K and (20.1-30.0)×10⁻⁶ K⁻¹ at 900-1200 K in air. The steady-state oxygen permeability (OP) of dense Al-containing membranes is determined mainly by the bulk ionic conductivity. The ion transference numbers at 973-1223 K in air, calculated from the oxygen permeation and faradaic efficiency (FE) data, vary in the range 1×10⁻⁴-3×10⁻³, increasing with temperature.     

Structural study by X-ray diffraction, magnetic susceptibility and Mössbauer spectroscopy of the Na2Mn2(1 − x)Cd2xFe(PO4)3 (0 ≤ x ≤ 1) solid solution

Na₂Mn_{2(1 − x)}Cd_2xFe(PO₄)₃ (0 ≤ x ≤ 1) phosphates were prepared by solid state reaction and characterized by powder X-ray diffraction, magnetic susceptibility and Mössbauer spectroscopy. The X-ray diffraction patterns indicated the formation of a continuous solid solution which crystallizes in the alluaudite structural type characterized by the general formula X(2)X(1)M(1)M(2)₂(PO₄)₃. The cation distribution, deduced from a structure refinement of the x = 0, 0.5 and 1 compositions, is ordered in the X(2) sites and disordered in the remaining X(1), M(1) and M(2) sites. The magnetic susceptibility study revealed an antiferromagnetic behaviour of the studied compounds. The ⁵⁷Fe Mössbauer spectroscopy confirmed the structural results and proved the exclusive presence of Fe³⁺ ions.     

Defect clusters and precipitation/oxidation of MgO-Co1−xO solid solution

MgO and Co_1−xO powders in 9:1 and 1:9 molar ratio (denoted as M₉C₁ and M₁C₉, respectively) were sintered and homogenized at 1600°C followed by annealing at 850°C and 800°C, respectively to form defect clusters and precipitates. Analytical electron microscopic observations indicated the protoxide remained as rock salt structure with complicated planar diffraction contrast for M₉C₁ sample, however with spinel paracrystal precipitated from the M₁C₉ sample due to the assembly of charge- and volume-compensating defects of the 4:1 type, i.e., four octahedral vacant sites surrounding one Co³⁺-filled tetrahedral interstitial site. The spacing of such defect clusters is 4.5 times the lattice spacing of the average spinel structure of Mg-doped Co_3−δO₄, indicating a higher defect cluster concentration than undoped Co_3−δO₄. The {111} faulting of Mg-doped Co_3−δO₄/Co_1−xO in the annealed M₁C₉ sample implies the possible presence of zinc blend-type defect clusters with cation vacancies assembled along oxygen close packed (111) plane.     

Oxygen-ion and electron conductivity in Sr2(Fe1−xGax)2O5

High-temperature electrical conductivity measurements, structural data from powder X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy were combined to study the interrelationship of oxygen ion transport and p- and n-type transport in Sr₂(Fe_1−xGa_x)₂O₅, where x=0, 0.1 and 0.2. Although gallium substitution generally decreases the total ion-electron transport, the transition of the orthorhombic brownmillerite structure to a cubic phase on heating results in the recurrence of the conductivity to the same high level as in the parent ferrite (x=0). The changes of the partial contributions to the total conductivity as a function of x are shown to reflect a complicated interplay of the disordering processes that develop in the oxygen sublattice on heating in response to replacement of iron with gallium.     

Structure and magnetic properties of the Al1−xGaxFeO3 family of oxides: A combined experimental and theoretical study

Magnetic properties of the Al_1−xGa_xFeO₃ family of oxides crystallizing in a non-centrosymmetric space group have been investigated in detail along with structural aspects by employing X-ray and neutron diffraction, Mössbauer spectroscopy and other techniques. The study has revealed the occurrence of several interesting features related to unit cell parameters, site disorder and ionic size. Using first-principles density functional theory based calculations, we have attempted to understand how magnetic ordering and related properties in these oxides depend sensitively on disorder at the cation site. The origin and tendency of cations to disorder and the associated properties are traced to the local structure and ionic sizes.     

The Crystal Structures, Microstructure and Ionic Conductivity of Ba2In2O5 and Ba(InxZr1−x)O3−x/2

This paper describes the results of electron microscopy, high-temperature powder neutron diffraction, and impedance spectroscopy studies of brownmillerite-structured Ba₂In₂O₅ and perovskite structured Ba(In_xZr_1−x)O_3−x/2. The ambient temperature structure of Ba₂In₂O₅ is found to adopt Icmm symmetry, with disorder of the tetrahedrally coordinated (In³⁺) ions of the type observed previously in Sr₂Fe₂O₅. Ba₂In₂O₅ undergoes a ∼6-fold increase in its ionic conductivity over the narrow temperature range from ∼1140 K to ∼1230 K, in broad agreement with previous studies. This transition corresponds to a change from the brownmillerite structure to a cubic perovskite arrangement with disordered anions. Electron microscopy investigations showed the presence of extended defects in all the crystals analyzed. Ba(In_xZr_1−x)O_3−x/2 samples with x=0.1 to 0.9 adopt the cubic perovskite structure, with the lattice parameter increasing with x.     

Preparation of ternary metal chalcogenide (M1-xFexS, M = Cd and Zn) nanocrystallites using single source precursors

M_1−xFe_xS (M = Cd, Zn) nanocrystallites were prepared by pyrolysis and solvothermal decomposition methods using [M(Aftscz)₂] and [M(AftsczH)₂Cl₂] (M = Cd, Zn and AftsczH = monoacetylferrocene thiosemicarbazone) as single source precursors. The M_1−xFe_xS nanocrystallites were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis and UV-Visible spectroscopy. XRD patterns show that the Cd_1−xFe_xS and Zn_1−xFe_xS nanocrystallites prepared by pyrolysis and solvothermal decomposition routes have hexagonal phase. TEM images show presence of spherical and spherical plate-like morphology of M_1−xFe_xS nanoparticles. M_1−xFe_xS nanoparticles obtained by solvothermal decomposition in ethylene glycol are found to be capped with ethylene glycol as evident from IR spectra.     

Magnetic properties related to structure and complete composition analyses of nanocrystalline La1−xMn1−yO3 powders

Structural and magnetic properties were studied on La_1−xMnO_3±δ nanocrystalline powders exhibiting different La/Mn ratios. These compounds were prepared using a gel combustion method based on a cation solution soaking by acrylamide polymerization. Structural properties were studied both by transmission electron microscopy and X-ray diffraction (XRD). Complete chemical composition analyses were performed by induced coupled plasma spectroscopy and by iodometric titration. Proportions of parasitic phases in samples, as La₂O₃ or Mn₃O₄, and actual compositions of La_1−xMnO_3±δ phases were then determined from refinements of XRD data and sample chemical compositions. As a result, perovskite structure is not any more stable for La/Mn<0.9 as it decomposes into a mixing of La_0.9MnO₃ and of Mn₃O₄ phases, in agreement with results on thermodynamic equilibrium in the La-Mn-O phase diagram. For La/Mn>0.9, a high oxygen excess is observed and leads to consider the creation of vacancies on both lanthanum and manganese sites, whose concentrations are evaluated. Magnetic properties agree well with the proposed structures and sample compositions since for La/Mn<0.9, for which a La_0.9MnO₃ phase is always found, the Curie temperature remains constant and equal to 295 K (the highest temperature never observed before in such series of compositions), while for La/Mn>0.9, there is a formation of Mn vacancies giving rise to a lowering of Curie temperature resulting of a frustration of ferromagnetic interactions.     

Structural and Transport Properties of Mg1−xMnxMn2O4±δ Spinels

The synthesis and structural properties of Mg_1−xMn_2+xO₄, for 0≤x≤1 are described. Complete miscibility in the solid state exists for this system. For the material with the correct stoichiometry, i.e. MgMn₂O₄, the effect of temperature on the cation distribution was investigated= above 600°C the inversion degree (m) starts increasing. The electrical conductivity shows a small dependence on P(O₂) which is consistent with the small oxygen non-stoichiometry determined by means of thermogravimetry. The main contribution to the transport properties arises from the inversion equilibrium. Two distinct conductivity regimes, below and above the inversion threshold, can be assumed to explain the electrical conductivity and thermoelectric power results.     

Effects of vacancy concentration on the magnetic and transport properties of (La1−xPbx)1−y□yMnO3

(La_1−xPb_x)_1−y□_yMnO₃ with x=0.05-0.5 and y=0, 0.05, 0.1 (where □ is a vacancy) was studied to evaluate the effects of A-site vacancies on the physical properties. In this system manganese perovskites form with tolerance factors close to 1 and low A-site cation size mismatch due to similarities in the effective ionic radii of La³⁺ and Pb²⁺. Increasing vacancy concentration indicates no significant effect on the lattice parameters or volume. However, the vacancies introduce a greater A-site cation size mismatch, which leads to a lowering of the ferromagnetic and metal-insulator transition temperatures, although the transitions are not broadened with increasing vacancy content. Due to the vacancies a distribution of Mn-O-Mn angles and Mn-O distances are created, and long range order in (La_1−xPb_x)_1−y□_yMnO₃ appears to be determined by Mn-O-Mn angles and Mn-O distances which most distort from 180° and are the longest, respectively, in the structure.     

Magnetic dilution of the iron sublattice in CoFe2−xScxO4 (0≤x≤1)

Substitution of Fe for Sc in CoFe₂O₄ spinel structure is presented. All CoFe_2−xSc_xO₄ compounds crystallize in the spinel type structure (space group Fd3?m). By using X-ray diffraction studies, magnetic measurements and in-field ⁵⁷Fe Mössbauer spectrometry, the limit of substitution has been determined to be equal to x=0.56. An increase in the cell parameter and the strains and a decrease in the apparent crystallites size are observed. For x>0.3, a partial oxidation of cobalt is evidenced and Co³⁺ is stabilized in the structure. A ferromagnetic behavior has been observed for all investigated compounds. As x increases, the Curie temperature and the hyperfine fields decrease. Following the Stephenson model, the diminution of T_C is ascribed to a decrease of the main J_AB interaction.     

Crystal Structure of Pseudorhombohedral InFe1−xTixO3+x/2 (x=2/3)

The structure of pseudorhombohedral-type InFe_1−xTi_xO_3−x/2 (x=2/3) was refined by Rietveld profile fitting. The crystal is a commensurate member of a series in a solution range on InFeO₃-In₂Ti₂O₇ including incommensurate structures. The structure with the unit cell of a=5.9188(1), b=10.1112(2), and c=6.3896(1) Å, β=108.018(2)°, and a space group P2₁/a is the alternate stacking of an edge-shared InO₆ octahedral layer and an Fe/Ti-O plane along c^*. Metal sites on the Fe/Ti-O plane are surrounded by four oxygen atoms on the Fe/Ti-O plane and two axial ones. Electric conductivities of the order 10⁻⁴ S/cm were observed for the samples at 1000 K, while the oxide ion transport number is almost zero as no electromotive force was detected by an oxygen concentration cell.     

Structural Characterization of the Complex Perovskites Ba1−xLaxTi1−xCrxO3

The series Ba_1−xLa_xTi_1−xCr_xO₃ (0≤x≤1) was synthesized at 1400°C for about 60 h. Their structure was carefully analyzed by the use of powder X-ray diffraction and Rietveld analysis software GSAS (General Structure Analysis System). Four solid solutions are found in this series: tetragonal solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0≤x≤0.029), cubic solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0.0365≤x≤0.600), rhombohedral solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0.700≤x≤0.873), and orthorhombic solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0.956≤x≤1). There are corresponding two-phase regions between the adjacent two solid solutions. The detailed lattice parameters are presented. The relationship between the lattice parameters and the composition of the solid solutions is developed.     

Etude par effet Mössbauer des composés de type “bronzes fluorés” MxFeF3 (M = K,Rb, Cs,NH4)

The hexagonal, tetragonal and pyrochlore-type nonstoichiometric iron fluorides M_xFeF₃ (M = K, Rb, Cs, NH₄) have been studied by Mössbauer spectroscopy over the temperature range 4.2 to 295 K. The magnetic transition temperatures have been determined. The ferrous and ferric ions remain in discrete oxidation states indicating the absence of charge hopping. The broadened lines of the spectra of the hexagonal and tetragonal phases are consistent with the disordering of Fe²⁺ and Fe³⁺ in the structure. By contrast, the narrow linewidths of the spectra of the pyrochlore-type phases characterize a structural ordering between the ferrous and ferric ions.     

Cation Distribution of the Transparent Conductor and Spinel Oxide Solution Cd1+xIn2−xSnxO4

The cation distribution in the transparent conducting oxide Cd_1+xIn_2−xSn_xO₄ was investigated to determine if there is a correlation between structure and electronic properties. Combined Rietveld refinements of neutron and X-ray diffraction data and ¹¹⁹Sn Mössbauer spectroscopic analysis were used to show that the cation distribution changed with x(0≤x≤0.7) from a primarily normal spinel (x=0) to an increasingly random spinel. CdIn₂O₄ quenched from 1175°C has an inversion parameter of 0.31 (i.e., (Cd_0.69In_0.31)^tet(In_1.69Cd_0.31)^octO₄). The inversion parameter decreases to 0.27 as the quench temperature is lowered from 1175°C to 1000°C. The decrease in inversion parameter with temperature correlates with an increase in optical gap from 3.0 eV to 3.3 eV for specimens prepared at 1175°C and 800°C, respectively. We show that this is a consequence of an increase in the fundamental band gap.