Crystal structure and electrical conductivity of lanthanum-calcium chromites-titanates La1−xCaxCr1−yTiyO3−δ (x=0-1, y=0-1)

Five series of perovskite-type compounds in the system La_1−xCa_xCr_1−yTi_yO₃ with the nominal compositions y=0, x=0-0.5; y=0.2, x=0.2-0.8; y=0.5, x=0.5-1.0; y=0.8, x=0.6-1.0 and y=1, x=0.8-1 were synthesized by a ceramic technique in air (final heating 1350 °C). On the basis of the X-ray analysis of the samples with (Ca/Ti)?1, the phase diagram of the CaTiO₃-LaCr^IIIO₃-CaCr^IVO₃ quasi-ternary system was constructed. Extended solid solution with a wide homogeneity range is formed in the quasi-ternary system CaCr^IVO₃-CaTiO₃-LaCr^IIIO₃. The solid solution La_{(1−x′−y)}Ca_(x′+y)Cr^IV_x′Cr^III_{(1−x′−y)}Ti_yO₃ exists by up to 0.6-0.7 mol fractions of CaCr^IVO₃ (x^′<0.6-0.7) at the experimental conditions. The crystal structure of the compounds is orthorhombic in the space group Pbnm at room temperature. The lattice parameters and the average interatomic distances of the samples within the solid solution ranges decrease uniformly with increasing Ca content. Outside the quasi-ternary system, the nominal compositions La_0.1Ca_0.9TiO₃, La_0.2Ca_0.8TiO₃, La_0.4Ca_0.6Cr_0.2Ti_0.8O₃ and La_0.3Ca_0.7Cr_0.2Ti_0.8O₃ in the system La_1−xCa_xCr_1−yTi_yO₃ were found as single phases with an orthorhombic structure. In the temperature range between 850 and 1000 °C, the synthesized single-phase compositions are stable at pO₂=6×10⁻¹⁶-0.21×10⁵ Pa. Oxygen stoichiometry and electrical conductivity of the separate compounds were investigated as functions of temperature and oxygen partial pressure. The chemical stability of these oxides with respect to oxygen release during thermal dissociation decreases with increasing Ca-content. At 900 °C and oxygen partial pressure 1×10⁻¹⁵-0.21×10⁵ Pa, the compounds with x>y (acceptor doped) are p-type semiconductors and those with x<y (donor doped) and x=y are n-type semiconductors. The type and level of electrical conductivity are functions of the concentration ratios of cations occupying the B-sites of the perovskite structures: [Cr³⁺]/[Cr⁴⁺] and [Ti⁴⁺]/[Ti³⁺]. The maximum electrical conductivity at 900 °C and pO₂=10⁻¹⁵ Pa was found for the composition La_0.1Ca_0.9TiO₃ (near 50 S/cm) and in air at 900 °C for La_0.5Ca_0.5CrO₃ (close to 100 S/cm).     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

Three-layer Aurivillius phases containing magnetic transition metal cations: Bi2−xSr2+x(Nb,Ta)2+xM1−xO12, M=Ru, Ir, Mn, x≈0.5

We report the synthesis of Aurivillius-type phases incorporating magnetic M⁴⁺ cations (M=Mn, Ru, Ir), based on the substitution of M⁴⁺ for Ti⁴⁺ in Bi₂Sr₂(Nb,Ta)₂TiO₁₂. The key to incorporating these magnetic transition metal cations appears to be the partial substitution of Sr²⁺ for Bi³⁺ in the α-PbO-type layer of the Aurivillius phase, leading to a concomitant decrease in the M⁴⁺ content; i.e., the composition of the prepared compounds was Bi_2−xSr_2+x(Nb,Ta)_2+xM_1−xO₁₂, x≈0.5. These compounds only exist over a narrow range of x, between an apparent minimum (x≈0.4) Sr²⁺ content in the α-PbO-type [Bi₂O₂] layer required for Aurivillius phases to form with magnetic M⁴⁺ cations, and an apparent maximum (x≈0.6) Sr²⁺ substitution in this [Bi₂O₂] layer. Rietveld-refinement of synchrotron X-ray powder diffraction data making use of anomalous dispersion at the Nb and Ru K edges show that the overwhelming majority of the incorporated M cations occupy the central of the three MO₆ octahedral layers in the perovskite-type block. Magnetic susceptibility measurements are presented and discussed in the context of the potential for multiferroic (magnetoelectric) properties in these materials.     

Percolative small-polarons conduction regime in Ce1−xGdxO2−x/2, probed by the EPR spectral intensity of Gd

EPR analysis is carried out with Ce_1−xGd_xO_(4−x)/2 (x=0.1; 0.2) nanopowders aiming at obtaining information about their oxidation and reduction properties. The EPR spectrum of these systems is composed of a single feature. The first derivative peak-to-peak spectral intensity decreases at higher temperatures, but this trend deviates from that of Curie's law with the x=0.1 sample, at difference with the x=0.2 sample. This unexpected result is related to oxygen deficiency, due to gas-solid equilibrium, present in the former sample only. As a consequence, some Ce³⁺ ions would form providing it with conduction electrons propagating as small polarons in a percolative way. This would result in a thinner skin depth at higher temperatures, able to explain the deviation of the spectral intensity from its expected value. Indeed, this deviation from Curie's law is reduced or disappears at all after thermal treatment of the x=0.1 sample with O₂.     

Zr1−xLnxW2O8−x/2 (Ln=Eu, Er, Yb): Solid solutions of negative thermal expansion-synthesis, characterization and limited solid solubility

Zr_1−xLn_xW₂O_8−x/2 solid solutions (Ln=Eu, Er, Yb) of different substitution fractions x have been synthesized. Their X-ray diffraction (XRD) patterns have been indexed and lattice parameters calculated based on the α-ZrW₂O₈ structure. The coefficients of thermal expansion (CTEs) of these solid solutions were estimated to be −10.3×10⁻⁶ K⁻¹ in temperature range of 30-100 °C. The solubility of lanthanide ions in these solid solutions decreases linearly with the increase in the radius of substituted lanthanide ions. Based on the concentration dependence of phase transition temperatures, a novel method for determination of solubility of the lanthanide ions in Zr_1−xLn_xW₂O_8−x/2 solid solutions has been developed. This method seems to be more sensitive as compared with that based on XRD technique.     

A structural study of the perovskite series Ca1−xNaxTi1−xTaxO3

Compounds in the solid solution series Ca_1−xNa_xTi_1−xTa_xO₃ were synthesized at 1300 °C, followed by annealing at 850 °C or 800 °C with quenching and/or slow cooling to room temperature. Rietveld refinement of their powder X-ray diffraction patterns show that all compounds are single-phase ternary perovskites which adopt the space group Pbnm (a≈b≈√2a_p; c≈2a_p; Z=4) at ambient conditions. The unit cell parameters and cell volumes of the compounds increase regularly with increasing values of x. The coordination of the A-site cations changes throughout the series from eight for CaTiO₃ to nine for NaTaO₃. Compounds with 0?x ?0.4 have A-site cations in eight fold coordination, whereas the coordination of those with 0.4<x<0.9 is ambiguous. Analysis of the crystal chemistry of the compounds shows that the change in coordination at x=0.4 is related to the departure of the B-site cations from the second coordination sphere of the A-site cations, as in compounds with x>0.4 the A-^IIO distances become less than the A-B intercation distances. Contemporaneous with these coordination changes, the tilt angles of the BO₆ polyhedra decrease with increasing values of x. This solid solution series is unusual in that these structural and coordination changes occur regardless that Goldschmidt tolerance factors remain essentially constant at approximately 0.89, and observed tolerance factors, assuming eight fold coordination of the A-site cations, range only from 0.91 to 0.93 (0?x?0.8).     

Synthesis and characterization of Ti1−2xNbxNixO2−x/2 solid solutions

Doped-rutile has been traditionally used in ceramic pigments for its intense optical properties. In this paper, we compare the classical ceramic synthesis of Ti_1−2xNb_xNi_xO_2−x/2 system with the sol-gel methodology, which allows a reduction of the anatase-rutile transformation temperature. The composition was optimised in order to obtain a unique rutile phase with the minimum amount of pollutant Ni(II) and enhanced chromatic coordinates. Incorporation of the doping ions in the rutile structure was corroborated by XRD and Rietveld refinements. The species responsible for the colour mechanism were studied by different techniques. UV-VIS spectroscopy showed the characteristic features of Ni²⁺ ions, whose existence was corroborated by EPR and magnetic measurements. From these results, (Ni,Nb)doped-TiO₂ powder samples can be now shaped as thin films, monoliths, etc. by using sol-gel methodology without modifying their properties. This study introduces new possibilities of coloured TiO₂-based solid solutions in new combined advanced applications (colouring agent and photocatalyst, etc.).     

Visible light-sensitive yellow TiO2−xNx and Fe-N co-doped Ti1−yFeyO2−xNx anatase photocatalysts

Nitrogen substituted yellow colored anatase TiO_2−xN_x and Fe-N co-doped Ti_1−yFe_yO_2−xN_x have been easily synthesized by novel hydrazine method. White anatase TiO_2−δ and N/Fe-N-doped samples are semiconducting and the presence of ESR signals at g ∼1.994-2.0025 supports the oxygen vacancy and g∼4.3 indicates Fe³⁺ in the lattice. TiO_2−xN_x has higher conductivity than TiO_2−x and Fe/Fe-N-doped anatase and the UV absorption edge of white TiO_2−x extends in the visible region in N, Fe and Fe-N co-doped TiO₂, which show, respectively, two band gaps at ∼3.25/2.63, ∼3.31/2.44 and 2.8/2.44 eV. An activation energy of ∼1.8 eV is observed in Arrhenius log resistivity vs. 1/T plots for all samples. All TiO₂ and Fe-doped TiO₂ show low 2-propanol photodegradation activity but have significant NO photodestruction capability, both in UV and visible regions, while standard Degussa P-25 is incapable in destroying NO in the visible region The mid-gap levels that these N and Fe-N-doped TiO₂ consist may cause this discrepancy in their photocatalytic activities.     

Synthesis and electrical conductivity of perovskite-type PrCo1−xMgxO3

Oxides in the system PrCo_1−xMg_xO₃ (x=0.0, 0.05, 0.10, 0.15, 0.20, 0.25) were synthesized by citrate technique and characterized by powder X-ray diffraction and scanning electron microscope. All compounds have a cubic perovskite structure (space group ). The maximum ratio of doped Mg in the system PrCo_1−xMg_xO₃ is x=0.2. Further doping leads to the segregation of Pr₆O₁₁ in PrCo_1−xMg_xO₃. The substitution of Mg for Co improves the performance of PrCoO₃ as compared to the electrical conductivity measured by a four-probe electrical conductivity analyzer in the temperature range from 298 to 1073 K. The substitution of Mg for Co on the B site may be compensated by the formations of Co⁴⁺ and oxygen vacancies. The electrical conductivity of PrCo_1−xMg_xO₃ oxides increases with increasing x in the range of 0.0-0.2. The increase in conductivity becomes considerable at the temperatures ?673 K especially for x?0.1; it reaches a maximum at x=0.2 and 1073 K. From x>0.2 the conductivity of PrCo_1−xMg_xO₃ starts getting lower. This is probably a result of the segregation of Pr₆O₁₁ in PrCo_1−xMg_xO₃ , which blocks oxygen transport, and association of oxygen vacancies. A change in activation energy for all PrCo_1−xMg_xO₃ compounds (x=0-0.25) was observed, with a higher activation energy above 573 K and a lower activation energy below 573 K. The reasons for such a change are probably due to the change of dominant charge carriers from Co⁴⁺ to Vö in PrCo_1−xMg_xO₃ oxides and a phase transition mainly starting at 573 K.     

Superstructure of hollandite KxMg(8+x)/3Sb(16−x)/3O16 (x≈1.76)

Single crystals of K_xMg_(8+x)/3Sb_(16−x)/3O₁₆ (x≈1.76) with a hollandite superstructure were grown. Ordering schemes for guest ions (K) and the host structure were confirmed by the structure refinement using X-ray diffraction intensities. The space group is I4/m and cell parameters are a=10.3256(6), c=9.2526(17)Å with Z=3. Superlattice formation is primarily attributed to the Mg/Sb occupational modulation in the host structure. Mg/Sb ratios at two nonequivalent metal sites are 0.8977/0.1023 and 0.1612/0.8388. Two types of the cavity are seen in the tunnel, where parts of K ions deviate from the cavity center along the tunnel direction. Probability densities for K ions in the two cavities are different from each other, which seems to have arisen from the Mg/Sb modulation.     

Structure and phase composition of nanocrystalline Ce1−xLuxO2−y

The microstructure and phase stability of nanocrystalline mixed oxide Lu_xCe_1−xO_2−y (x=0-1) are described. Nano-sized (3-4 nm) oxide particles were prepared by the reverse microemulsion method. Morphological and structural changes upon heat treatment in an oxidizing atmosphere were studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman and Yb³⁺ emission spectroscopy, the latter ion being present as an impurity in the Lu₂O₃ starting material. Up to 950 °C, the samples were single phase, with structure changing smoothly with Lu content from fluorite type (F) to bixbyite type (C). For the samples heated at 1100 °C phase separation into coexisting F- and C-type structures was observed for 0.35<x<0.7. It was also found that addition of Lu strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 °C.     

Cation ordering in the fluorite-like transparent conductors In4+xSn3−2xSbxO12 and In6TeO12

The cation ordering in the fluorite-like transparent conductors In_4+xSn_3−2xSb_xO₁₂ and In₆TeO₁₂, was investigated by Time of Flight Neutron Powder Diffraction and X-ray Powder Diffraction (tellurate). The structural results including atomic positions, cation distributions, metal-oxygen distances and metal-oxygen-metal angles point to a progressive cation ordering on both sites of the Tb₇O₁₂-type structure with a strong preference of the smaller 4d¹⁰ cations (Sn⁴⁺, Sb⁵⁺, Te⁶⁺) for the octahedral sites. The corresponding increase of the overall structure-bonding anisotropy is analyzed in terms of the crystal chemical properties of the OM₄ tetrahedral network of the antistructure. The relationships between the M₇O₁₂ and the M₂O₃ bixbyite-type structures are explored. Within the whole series of compositions In_4+xM_3−xO₁₂ (M=Sn, Sb, Te) there exists an increase of the symmetry gap between the more symmetrical bixbyite structure and the M₇O₁₂ type. This is tentatively correlated with the progressive weakening of thermal stability of these compositions from Sn to Te via Sb.     

Oxygen permeability, thermal expansion and mixed conductivity of GdxCe0.8−xPr0.2O2−δ, x=0, 0.15, 0.2

The non-linear thermal expansion behaviour observed in Ce_1−yPr_yO_2−δ materials can be substantially controlled by Gd substitution. Coulometric titration shows that the charge compensation mechanism changes with increasing x, in the system Gd_xCe_0.8−xPr_0.2O_2−δ. For x=0.15, charge compensation is by vacancy formation and destabilises the presence of Pr⁴⁺. At x=0.2, further Gd substitution is charge compensated by additionally raising the oxidation state of Pr rather than solely the creation of further oxygen ion vacancies. Oxygen concentration cell e.m.f. measurements in an oxygen/air potential gradient show that increasing Gd content decreases ionic and electronic conductivities. Ion transference numbers measured under these conditions show a positive temperature dependence, with typical values t_o=0.90,0.98 and 0.80 for x=0,0.15 and 0.2, respectively, at 950 °C. These observations are discussed in terms of defect association. Oxygen permeation fluxes are limited by both bulk ambipolar conductivity and surface exchange. However, the composition dependent trends in permeability are shown to be dominated by ambipolar conductivities, and limited by the level of electronic conductivity. At the highest temperatures, oxygen permeability of composition x=0.2 approaches that of composition x=0, Ce_0.8Pr_0.2O_2−δ, with specific oxygen permeability values approximately 2×10⁻⁹ mol s⁻¹ cm⁻¹ at 950 °C, but offering much better thermal expansion properties.     

Structural investigation of KxBa1−xGa2−xGe2+xO8 solid solutions using the X-ray Rietveld method

The K_xBa_1−xGa_2−xGe_2+xO₈ (x=0.6−1.0) solid solutions undergo a structural phase transition that has a significant effect on their sintering behavior and their microwave dielectric properties. The crystal structures of both phases within the solid-solution region were determined by the Rietveld method using powder X-ray diffraction data. We found that the low-temperature-stable phase is isostructural with the pseudo-orthorhombic KGaGe₃O₈ (space group P2₁/a), while the high-temperature-stable phase has a typical monoclinic feldspar structure (space group C2/m). Due to the topological differences between the two structures, the T-O bonds within the tetrahedra must be partially recombined to make a new framework, which causes an endothermic effect during the P2₁/a to C2/m phase transition. The correlation between the crystal structures, the microwave dielectric properties and the phase-transition behaviors were discussed in terms of the crystallographic features, the lattice parameters, and the strain-induced anisotropic peak-broadening.     

Synthesis, structure, and magnetic properties of SrMn1−xGaxO3−δ (x=0-0.5) perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga³⁺ for Mn in the cubic perovskite phase to x=0.5. In the cubic perovskite phase the maximum oxygen content is close to 3−x/2, which corresponds to 100% Mn⁴⁺. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x=0-0.4, with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr₂MnGaO_5.50 with nominally identical chemical composition.     

Structural and magnetic properties of pyrochlore solid solutions (Y,Lu)2Ti2−x(Nb,Ta)xO7±y

The synthesis and characterization of the pyrochlore solid solutions, Y₂Ti_2−xNb_xO_7−y, Lu₂Ti_2−xNb_xO_7−y, Y₂Ti_2−xTa_xO_7−y and Lu₂TiTaO_7−y (−0.4<y<0.5), is described. Synthesis at 1600 °C, and 10⁻⁵ Torr yields oxygen deficiency in all systems. All compounds are found to be paramagnetic and semiconducting, with the size of the local moments being less, in some cases substantially less, than the expected value for the number of nominally unpaired electrons present. Thermogravimetric analysis (TGA) shows that all compounds can be fully oxidized while retaining the pyrochlore structure, yielding oxygen rich pyrochlores as white powders. Powder neutron diffraction of Y₂TiNbO₇-based samples was done. Refinement of the data for oxygen deficient Y₂TiNbO_6.76 indicates the presence of a distribution of oxygen over the 8b and 48f sites. Refinement of the data for oxygen rich Y₂TiNbO_7.5 shows these sites to be completely filled, with an additional half filling of the 8a site. The magnetic and TGA data strongly suggest a preference for a Ti³⁺/(Nb,Ta)⁵⁺ combination, as opposed to Ti⁴⁺/(Nb,Ta)⁴⁺, in this pyrochlore family. In addition, the evidence clearly points to Ti³⁺ as the source of the localized moments, with no evidence for localized Nb⁴⁺ moments.     

Mn environment in layered Li[Mg0.5−xNixMn0.5]O2 oxides monitored by EPR spectroscopy

X-band and high-frequency EPR spectroscopy were used for studying the manganese environment in layered Li[Mg_xNi_0.5−xMn_0.5]O₂, 0?x?0.5. Both layered LiMg_0.5Mn_0.5O₂ and monoclinic Li[Li_1/3Mn_2/3]O₂ oxides (containing Mn⁴⁺ ions only) were used as EPR standards. The EPR study was extended to the Ni-substituted analogues, where both Ni²⁺ and Mn⁴⁺ are paramagnetic. For LiMg_0.5−xNi_xMn_0.5O₂ and Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂, an EPR response from Mn⁴⁺ ions only was detected, while the Ni²⁺ ions remained EPR silent in the frequency range of 9.23-285 GHz. For the diamagnetically diluted oxides, LiMg_0.25Ni_0.25Mn_0.5O₂ and Li[Li_0.10Ni_0.35Mn_0.55]O₂, two types of Mn⁴⁺ ions located in a mixed (Mn-Ni-Li)-environment and in a Ni-Mn environment, respectively, were registered by high-field experiments. In the X-band, comparative analysis of the EPR line width of Mn⁴⁺ ions permits to extract the composition of the first coordination sphere of Mn in layered LiMg_0.5−xNi_xMn_0.5O₂ (0?x?0.5) and Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ (x>0.2). It was shown that a fraction of Mn⁴⁺ are in an environment resembling the ordered “α,β”-type arrangement in Li_1−δ1Ni_δ1[Li_{(1−2x)/3+δ1}Ni_2x/3−δ1)_α(Mn_(2−x)/3Ni_x/3)_β]O₂ (where and δ₁=0.06 were calculated), while the rest of Mn⁴⁺ are in the Ni,Mn-environment corresponding to the Li_1−δ2Ni_δ2[Ni_1−yMn_y]O₂ () composition with a statistical Ni,Mn distribution. For Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ with x?0.2, IR spectroscopy indicated that the ordered α,β-type arrangement is retained upon Ni introduction into monoclinic Li[Li_1/3Mn_2/3]O₂.     

Structural and magnetic properties of Zn1−xSbxCr2−x/3Se4 (x=0.11, 0.16 and 0.20) single crystals

Single crystals of Zn_1−xSb_xCr_2−x/3Se₄ based on the ZnCr₂Se₄ spinel, which is known to exhibit interesting magnetic and electronic transport properties, have been prepared by solid state reaction from the appropriate selenides. Three compounds of different Sb content (x=0.11, 0.16, and 0.20) were studied by X-ray diffraction, X-ray photoelectron scattering technique and macroscopic magnetic measurements with the aim to determine (i) stability of the cubic symmetry and (ii) influence of the Sb admixture on the magnetic properties. The results show that the Sb³⁺ and Zn²⁺ ions share the tetrahedral sites in the spinel structure, while the Cr³⁺ions carrying magnetic moments, are located in the octahedral sites. The X-ray photoelectron spectroscopy (XPS) data indicate that in this series of compounds the chromium ions have a 3d³ electronic configuration. The three samples studied order antiferromagnetically at low temperatures, with the magnetic characteristics being hardly altered with respect to those reported for the parent ZnCr₂Se₄ compound.     

Structural and conductivity studies of Y10−xLaxW2O21

The aim of this work was to determine structural parameters of the Y_10−xLa_xW₂O₂₁ (x=0-10) solid solution series and investigate their electric properties. Crystallographic data shows a gradual increase in symmetry with increasing La content, as the structure evolves from orthorhombic, Y₁₀W₂O₂₁, towards the pseudo-cubic structure of Y₅La₅W₂O₂₁. The solubility limit of La₂O₃ was found to be 50% (x=5). Above this level two phases were observed, La₆W₂O₁₅ and (La,Y)_10+xW_2−xO_21−δ. The conductivity of Y rich samples was very low, with σ of the order 2×10⁻⁵-5×10⁻⁵ S cm⁻¹ at 1000 °C, whilst ionic conductivity was observed for most La rich doped samples. The highest conductivity was observed for La₁₀W₂O₂₁ and its doped analogues, at 1×10⁻³-5×10⁻³ S cm⁻¹ at 1000 °C. Unit cell parameters were determined as a function of temperature from 0 to 1000°C, and thermal expansion of these materials was determined from temperature studies carried out at the Australian Synchrotron facility in Melbourne, Victoria, Australia.     

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.