H. Ullmann

Oxides of the AMO3 and A2MO4-type: structural stability, electrical conductivity and thermal expansion

Abstract The structural and chemical stabilities, electrical conductivity, and thermal expansion of the A 2− a A a ′MO 4− x oxides (A=La; A′=Sr; M=Mn, Fe, Co, Ni) with the perovskite-related K 2 NiF 4 -type structure were investigated and compared with the characteristics of perovskite-type oxides AMO 3− x containing the same cations. The K 2 NiF 4 -type manganites, ferrites, cobaltites and nickelates are assumed to be reduction products of the corresponding perovskite-type oxides. The thermodynamic stabilities, in terms of reversible oxygen desorption, were higher than those of the corresponding perovskite-type oxides. Within the range of oxygen partial pressure ( p O 2 ) from air to argon/H 2 /H 2 O, the oxidation states of the M cations were determined. The comparison of the oxidation states of M in AMO 3− x and (AMO 3− x )·AO gives evidence on the stabilizing influence of the AO interlayer on the perovskite layer. The electrical conductivity of the A 2 MO 4 oxides was of p-type and reached values close to 100 S cm −1 at high oxygen partial pressures and 800 °C for nickelates and cobaltites. The thermal expansion of K 2 NiF 4 -type oxides is generally lower than that of the comparable perovskite-type oxides.

Transition metal doped lanthanum gallates

Abstract La0.9Sr0.1Ga0.8Mg0.2O3−x+δ (LSGM) was additionally doped by y=0.1–0.3 of the transition metals M=Cr, Mn, Fe, Co to form La0.9Sr0.1(Ga1−yMy)0.8Mg0.2O3−x+δ. After heating in air at 1400°C single phase materials of the cubic perovskite-type are obtained. Only the addition of Co y>0.1 in oxidizing conditions (air) leads to a hexagonal structure. The dopants occupied the Ga-site of the perovskite structure. The oxygen stoichiometry range δ between the reduced and oxidized state of the mixed oxides were determined by solid electrolyte coulometry. The δ-values increase with increasing transition metal doping and in the row Cr, Mn, Fe, Co. An approach is given for the calculation of mean ionic radii, Goldschmidt tolerance factors and free volumes of mixed and nonstoichiometric oxides taking into account the real mole fractions and oxidation states of the cations. At the dopant concentration y=0.1 the lattice parameter a increases from Co to Cr according to the mean ionic radii. The electrical conductivity of La0.9Sr0.1(Ga0.9M0.1)0.8Mg0.2O3−x+δ (M=Fe, Co) is of the ionic type. No break in the activation energy is observed, in opposition to the behaviour of LSGM near 600°C. The A-substoichiometric La0.85Sr0.1(Ga0.9Co0.1)0.8Mg0.2O3−x+δ, has the highest ionic conductivity, at 600°C twice as high as LSGM. At M-fractions y>0.1, the conductivity increases due to additional p-type conduction.

Composition and conductivity of some nickelates

Abstract Oxygen nonstoichiometry and electrical resistance of a series of La 2− x Sr x NiO 4± δ solid solutions with x =0.0–1.4 were investigated in argon/O 2 ( p O 2 =1.5–286 Pa) within a temperature range of 20–1050°C. The oxygen permeability of gas tight ceramics was determined. Increasing strontium content from 0 to 0.4 was found to decrease the amount of weak bonded oxygen, which can be reversibly exchanged with oxygen in the gaseous phase ( T =250–550°C). The equilibrium values of oxygen nonstoichiometry and specific resistance of the nickelates with 0.0≤ x ≤0.5 were determined as functions of temperature and oxygen partial pressure. The nickelates studied appear to be p-type conductors. Oxygen permeability of nickelates with x =0–0.4 increases in accordance with the increasing range of the oxygen nonstoichiometry ( T >800°C, the p O 2 gradient is (0.21×10 5 to 200) Pa).

Oxygen nonstoichiometry and electrical conductivity of the solid solutions La2−xSrxNiOy (0≤x≤0.5)

Abstract Oxygen nonstoichiometry and electrical resistance of a series La 2− x Sr x NiO y solid solutions, where x =0.0, 0.2 and 0.5 in argon flows at oxygen partial pressures 1.5, 10.2, 49.2, 100 and 286 Pa within the temperature range of 20–1050°C were studied. Nickelate oxygen desorption/sorption spectra when heating–cooling at constant rate demonstrated strong dependence of cation composition of the samples. Unlike La 1.5 Sr 0.5 NiO y compounds those of La 2 NiO y and La 1.8 Sr 0.2 NiO y have weakly bonded oxygen, capable to exchange reversibly with the gas phase at the temperatures higher than 250°C. The equilibrium values of oxygen nonstoichiometry and specific resistance for the these nickelates were determined at 300–1050°C and p O 2 =1.5–286 Pa as a functions of temperature versus oxygen partial pressure. All nickelate studied appear to be p-type conductors with metal electric conductivity at equilibrium states.

Composition, structure and transport properties of perovskite-type oxides

Abstract Perovskite-type oxides La 1− a A a M 1− b B b O 3− x with A=Sr 2+ , Ln 3+ , Ce 4+ , M=Fe, Co, Ga and B=Co, Fe, Mg were prepared in the concentration range a =0.1–1 mol and b =0.1–0.5 mol. Additionally, A-substoichiometric compositions were prepared. Preparation conditions for monophase materials and structure types of the perovskite were determined by X-ray investigation. The electrical conductivity as a function of p O 2 in the range 10 5 > p O 2 >10 −14 Pa and temperature (500–1000°C) was measured on ceramic shapes by a dc four-point technique in combination with solid electrolyte coulometry. The ionic part of the conductivity in mixed conductors was determined by oxygen permeation measurements. The II–III perovskites Sr(Co,Fe)O 3− x in their stabilized form are excellent mixed conductors (maximum 500 S cm −1 at 400°C) and have an up to two orders of magnitude higher oxygen ionic conductivity than the preferred III–III perovskite La(Sr)Mn(Co)O 3− x . The oxygen ionic conductivity of the electrolyte La(Sr)Ga(Mg)O 3− x could be increased by doping with 0.1 mol Co. By application of higher Co or Fe doping concentrations the lanthanum gallate becomes a mixed conductor.

Oxygen nonstoichiometry and some transport properties of LaSrNiO4−δ nickelate

Abstract Oxygen nonstoichiometry and electrical conductivity of LaSrNiO4−δ nickelate were investigated and oxygen permeability of LaSrNiO4−δ+40% Ag composite ceramics was estimated. The oxygen content in LaSrNiO4−δ changes from 3.99 to 3.45 in the temperature range 600–1000°C at oxygen partial pressures 100 to 2 Pa. The decrease of oxygen content in nickelate takes place by removing it from vertexes of NiO6 octahedrons which form the perovskite layers within the lattice of nickelate. This causes an increase of the c parameter and a decrease of the a parameter of the tetragonal cell of nickelate. The cell volume of the crystal remains constant. The nickelate studied appeared to be p-type conductor with a metallic-like temperature function of the electrical conductivity. At the same experimental conditions the oxygen permeability of LaSrNiO4−δ+40% Ag composite is lower by a factor of 100 in comparison with the permeability of La2NiO4±δ and La2NiO4±δ+40% Ag composite.

Oxygen stoichiometry and mixed ionic-electronic conductivity of Sr1−aCeaFe1−bCobO3−x perovskite-type oxides

Abstract The structure, oxygen stoichiometry, total and ionic electrical conductivities of Sr 1− a Ce a Fe 1− b Co b O 3− x ( a =0÷0.2, b =0÷1.0) were investigated. XRD patterns show the cubic perovskite structure of all air-treated samples. In reducing atmosphere the Co rich compositions were partially decomposed. The change of the lattice parameter a with Ce content could be explained on the basis of the oxygen stoichiometry values, determined by solid electrolyte investigations. The total conductivity (p-type, above 400–800°C changing to metallic-like behaviour) was enhanced by the Co content and reached 200 S cm −1 at 600°C. From permeation measurements an ionic conductivity higher than YSZ and for the compositions of lower ceria content as high as gadolinia doped ceria was observed. The results are compared with those of La 1− a Sr a Fe 1− b Co b O 3− x and of manganites.

Co-doped LSGM: composition–structure–conductivity relations

La0.9Sr0.1Ga0.8Mg0.2O3−x (LSGM) was additionally doped with 0.1 to 0.3 mol Co. Powders and pellets were prepared by a solid-state reaction in air. X-ray diffraction shows a cubic structure for y≤0.1 mol Co and an hexagonal structure for y≥0.2 mol Co. At pO2<10−6 Pa the structure of all compositions changed to cubic, independent of Co content. Oxygen stoichiometries of the reduced and oxidized states of the mixed oxides were determined by solid electrolyte coulometry. The Co2+/Co4+ concentrations were calculated from oxygen stoichiometries. At y=0.1 mol Co is stabilized at the Ga site as Co2+ up to high pO2. The electrical conductivity of La0.9Sr0.1(Ga0.9Co0.1)0.8Mg0.2O3−x+δ shows a wide ionic domain. With increasing y the conductivity increases due to additional p-type conduction. A defect model including electron holes and oxygen vacancies is discussed.

Estimation of effective ionic radii in highly defective perovskite-type oxides from experimental data

Abstract Effective ionic radii in crystals with higher concentrations of defects may considerably differ from tabulated values. For a number of perovskite-type oxides A 1− a A′ a B 1− b B′ b O 3± x (A, A′=rare earth, earth alkaline, B, B′=Al, Ga, In, Zr, Ce, Cr, Mn, Fe, Co, Mg) a calculation mode for average ionic radii of each sub-lattice is proposed on the basis of experimentally determined oxygen stoichiometries (vacancy concentrations) and unit cell volumes. The effect of the vacancies on the lattice expansion is considered. A two-dimensional radius diagram combined with Goldschmidt’s tolerance factors resulting from the effective radii represents the structure modifications of non-defective and cation vacant perovskite-type oxides. For anion vacant perovskite-type oxides a three-dimensional diagram with the axes r A – r B – r O was constructed. For more than 50 compositions of oxides the specific free volumes of the unit cell are correlated with the t -factors calculated from the effective ionic radii. Independently on structural modifications of the perovskite-type and of the defect-type a linear relation between V f,s and t was found. The approaching character of the calculation scheme is discussed, and the results are evaluated with regard to the gradually improved calculation modes of t -factors.

Oxygen non-stoichiometry and electrical conductivity of the binary strontium cobalt oxide SrCoOx

Abstract Strontium cobaltite was investigated using solid-electrolyte coulometry and resistivity measurements in the temperature range 20–1050°C and oxygen partial pressures 0.5–400 Pa. Two observed oxygen desorption/sorption maxima within the temperature range 500–950°C correlate with phase transitions of this compound as reported in the literature. An additional oxygen desorption/sorption maximum was found at a temperature of 965–1000°C, which is explained an order-disorder transition of the cubic high-temperature phase. The dependencies of equilibrium values of oxygen content as well as specific resistivity on temperature and oxygen partial pressure were founded for the cubic phase.