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The electrical conductivity (σ) of Ca1–xPrxMnO3–δ (х = 0–0.15) manganites with a perovskite-like structure has been measured by the four probe method in air at temperatures (T) from 320 to 1220 K. The results have been analyzed in terms of a model of small adiabatic polarons. The observed decrease in σ with increasing T is primarily due to the decrease in carrier concentration as a result of Mn3+ disproportionation into Mn2+ and Mn4+. The polaron mobility ranges from 0.2 to 1.2 cm2/(V s) and decreases with increasing praseodymium content.

The Seebeck coefficient (S) of Ca1–x Pr x MnO3–δ (х = 0, 0.05, 0.10, 0.15) manganites with a perovskite-like structure has been measured in air at temperatures (T) from 300 to 1200 K. The negative sign of their S indicates that all of the samples have n-type conductivity. The observed increase in the magnitude of the Seebeck coefficient with increasing T is interpreted in terms of small-polaron transport with allowance for the decrease in Mn3+ concentration as a result of the disproportionation reaction 2Mn3+ = Mn2+ + Mn4+. Based on a theoretical analysis of experimental S(T) data, we calculated equilibrium constants for the disproportionation reaction, carrier concentration, and the concentration of sites available for carrier migration as functions of temperature. It has been shown that, for an adequate analysis of electron hopping and calculation of the Seebeck coefficient of the electron-doped manganites, the spin state of the Mn4+ ions should be taken into account.

The chemical potential of oxygen ΔμO relative to its standard state in the gas phase was calculated from the experimental dependences of the oxygen content in various polymorphic modifications of Ca0.9Pr0.1MnO3–δ on the partial pressure and temperature. The partial molar enthalpy \(\Delta {\overline H _O}\) and entropy \(\Delta {\overline S _O}\) of oxygen were obtained from the linear temperature dependences of ΔμO. Based on the ideal solution approximation, \(\Delta {\overline H _O}\) and \(\Delta {\overline S _O}\) were correlated with the enthalpies and entropies of the defect formation reactions, the concentrations of manganese cations, and the oxygen nonstoichiometry. The thermal excitation of Mn4+ cations was shown to substantially affect the thermodynamic functions of oxygen.

The content of oxygen in Ca1−x Pr x MnO3−δ (x = 0.1) material with perovskite-like structure is determined at 750–950°C and oxygen partial pressures in the gas phase from 3 × 10−6 to 0.55 atm. The regions of existence of orthorhombic, tetragonal, and cubic structural modifications are determined. The equilibrium constants, enthalpies, and entropies of reactions of the formation of defects are determined, allowing us to describe the experimental δ ( $${p_{{O_2}}}$$ ,Т) dependences accurately and calculate the concentrations of manganese ions. It is shown that a change in Ca0.9Pr0.1MnO3–δ structure has a considerable effect on the thermodynamic functions of reactions of the formation of defects.

Manganites Ca1–х Pr х MnO3–δ, where 0 ≤ х ≤ 0.15 were studied. The compounds have orthorhombic structure (space group Pnma) at ambient temperature, which consecutively transforms on heating in air into phases with tetragonal I4/mcm and cubic 000000 structure. Increase in praseodymium concentration leads to higher phase transition temperatures, lower heat of structural transformations, more narrow homogeneity range of oxygen, and a wider temperature interval of tetragonal I4/mcm phase existence.

The perovskite-like electron doped manganites Ca 0.6− у Sr 0.4 La у MnO 3− δ (0 ⩽ y ⩽ 0.15) were synthesized in order to study oxygen content variations with composition, temperature and oxygen partial pressure. The samples are characterized with orthorhombic Pnma structure at 0 у I 4/ mcm structure at у ⩾ 0.13 according to room temperature powder X-ray diffraction (XRD). The high-temperature data of XRD in air show transition to cubic Pm 3 ¯ m structure at heating above 550 °С. The variations of oxygen content in Ca 0.6− у Sr 0.4 La у MnO 3− δ with temperature and oxygen pressure were measured using thermogravimetry and coulometric titration at temperatures between 650 and 950 °С in the oxygen pressure range 10 −4 –0.7 atm. The oxygen nonstoichiometry δ changes in Ca 0.6− у Sr 0.4 La у MnO 3− δ are observed to considerably decrease with the increase in lanthanum content. In order to explain the nonstoichiometry data, a thermodynamic model is developed based on defect formation reactions 2Mn 3+ + V O + ½O 2 = 2Mn 4+ + O 2− and 2Mn 4+ = Mn 3+ + Mn 5+ . The respective equilibrium constants, defect formation enthalpies and entropies are obtained. The concentrations of different defects are calculated as functions of temperature, oxygen pressure and lanthanum content. The high-temperature conductivity measurements are carried out for independent verification of the suggested model.

The incorporation of tantalum cations in mixed-conducting SrFe1-xTaxO3−δ (x = 0.03 − 0.10) results in the formation of single cubic perovskite-like phases in oxidizing atmospheres while under reducing conditions phase separation is observed, accompanied with an appearance of brownmillerite-type nanodomains on the background of the perovskite-like matrix. For SrFe0.97Ta0.03O3−δ after reduction, the x-ray and electron diffraction studies combined with transmission electron microscopy evidence the formation of approximately 30 vol.% brownmillerite phase with an average domain size of 20–40 nm. The oxygen partial pressure dependencies of the total conductivity in the \( {p}_{{\mathrm{O}}_2} \) range from 10−20 to 0.5 atm at 700–950 °C show that the electron transport parameters remain virtually independent on the dopant content and domain structure. Contrary to the materials with higher dopant content, however, the ion conduction in SrFe0.97Ta0.03O3−δ tends to substantially increase on redox cycling. This behavior was attributed to the brownmillerite domain disintegration and rearrangement, induced by cyclic formation and disappearance of oxygen vacancies.

The formation of fluorite-like solid solutions near composition Ln 5 Mo 3 O 16+ δ is observed in binary systems Ln 2 O 3 –MoO 3 , where Ln = Nd and Pr, at synthesis in the air. The oxide ratio Ln 2 O 3 :MoO 3 within homogeneous solubility range varies from 3:4 to 7:8 and from 9:11 to 7:8 for neodymium and praseodymium oxides, respectively. The oxygen pressure dependent isotherms in Ln 5 Mo 3 O 16+ δ demonstrate mixed, oxygen-ion and electron, character of conductivity. The ion transference numbers change from 0.16 to 0.92 at variations of oxygen pressure and temperature within 10 –5 –0.5 atm and 700–950 °С, respectively. The oxygen ion and electron components of conductivity are characterized by the activation energy of about 1.0 and 1.4 eV, respectively. It is argued that the ion contribution is related with jumps of off-stoichiometric oxygen over structural interstitials in the fluorite-like crystalline lattice.

Structural features, oxygen non-stoichiometry and transport properties are studied in the oxide series SrFe{sub 1-x}Ta{sub x}O{sub 3-{delta}}, where x=0.2, 0.3 and 0.4. X-ray diffraction and electron microscopy data evidence formation of the inhomogeneous materials at x=0.3 and 0.4, which include phase constituents with a cubic perovskite and a double perovskite structure types. The composition, the amount and the typical grain size of the phase inhomogeneities are shown to depend both on doping and oxygen content. The increased oxygen-ion conductivity is observed in oxygen depleted materials, which is explained by the increase in the amount of cubic perovskite-like phase and development of interfacial pathways favorable for enhanced oxygen ion transport. - Graphical abstract: The structural studies, oxygen content and conductivity measurements suggest that oxygen depletion from the double perovskite phase constituent of SrFe{sub 1-x}Ta{sub x}O{sub 3-{delta}} for x>0.2 is accompanied by formation of pathways for fast ion transport. Black-Small-Square Highlights: Black-Right-Pointing-Pointer The double perovskite type regions are shown to exist in SrFe{sub 1-x}Ta{sub x}O{sub 3-{delta}}. Black-Right-Pointing-Pointer The oxygen depletion is accompanied with phase separation. Black-Right-Pointing-Pointer The phase separation favors formation of pathways for enhanced oxygen ion transport.

The oxygen vacancies distribution in the rigid lattice and the thermally activated motion of oxygen atoms are studied in La{sub 1-x}Sr{sub x}Ga{sub 1-x}Mg{sub x}O{sub 3-x} (x=0.00; 0.05; 0.10; 0.15 and 0.20) compounds. For that {sup 71}Ga, {sup 25}Mg and {sup 17}O 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{sub 5/2}-{open_square}-GaO{sub 5/2}. These clusters decay at higher temperature into mobile constituents of the structural octahedra Ga(O{sub 5/6{open_square}1/6}){sub 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 {sup 17}O NMR spectra and the {sup 17}O 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-rangemore » 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

The influence of doping with niobium on the structure and oxygen nonstoichiometry of strontium ferrite SrFe1 − x Nb x O3 − δ (x = 0.05, 0.1, 0.2, 0.3, and 0.4) was studied. The content of oxygen in the doped derivatives was determined by coulometric titration as a function of temperature (650–950°C) and oxygen partial pressure in the gas phase (10−4−1 atm). The partial molar enthalpies Δ\( \bar H_O \)(x, δ) and entropies Δ\( \bar S_O \)(x, δ) of oxygen in SrFe1 − x Nb x O3 − δ were calculated. An analysis of Δ\( \bar S_O \)(x, δ) dependences showed that the model of a random distribution of ions and vacancies over accessible sites in the oxygen sublattice allowed the experimental data to be described satisfactorily. An increase in the partial enthalpy Δ\( \bar H_O \)(x, δ) as nonstoichiometry δ decreased was indicative of weak repulsive interactions between oxygen ions in the structure of SrFe1 − x Nb x O3 − δ.

Changes in the electronic structure and magnetic characteristics of Sr2FeMO6 double perovskites were studied by the FLAPW-GGA ab initio band structure method in relation to the type of the cation M = Sc, Ti, ..., Ni, Cu.

The complex oxides in the series SrFe 1 − x W x O 3 − δ were synthesized and attested to have cubic structure. The oxygen content was measured by coulometric titration technique, and electrical conductivity was measured by four-probe d.c. method in the oxygen partial pressure range 10 − 18 –0.5 atm and at temperatures 650–950 °C. Thermal expansion tests were performed in air within temperature interval 20–1000 °C. The partial molar thermodynamic functions of labile oxygen ions were calculated using coulometric titration data. Conductivity analysis was utilized to separate partial contributions from oxygen ions, n-and p-type electron charge carriers. The concentration and mobility of p-type charge carriers were calculated by combining data from oxygen content and conductivity measurements. It is shown that partial replacement of iron for tungsten results in a strong disordering of the oxygen sub-lattice that renders the perovskite to brownmillerite phase transition suppressed. The observed ion conductivity level in the sample x = 0.1 at T

The oxygen nonstoichiometry, electrical conductivity and Seebeck coefficient of perovskite-related SrFe1−xGaxO3−δ ( x = 0 – 0.2 ) and SrFe1−xAlxO3−δ ( x = 0.1 – 0.3 ) were studied in the temperature range 923–1223 K at oxygen partial pressures varying from 10−20 to 0.3 atm. The substitution of iron with gallium was found to promote long-range ordering of oxygen vacancies, resulting in the perovskite → brownmillerite transition at moderate oxygen pressures, and to decrease the mobility of the p-type electronic charge carriers. Opposite tendencies are observed for aluminum doping, which causes a higher oxygen nonstoichiometry in oxidizing atmospheres. As suggested by atomistic simulations of highly oxygen-deficient ferrite lattices, this behavior is associated with oxygen-vacancy trapping near Al3+ having energetically favorable tetrahedral coordination, whereas Ga3+ cations with preferential octahedral coordination in the perovskite lattice tend to repulse vacancies towards oxygen sites surrounded by iron and thus destabilize the structure. The relatively low hole mobility, with activation energies of 0.18–0.33 eV, indicates a small-polaron conduction mechanism. Under reducing conditions the oxygen content and ionic transport in brownmillerite-type polymorphs of SrFe1−xGaxO3−δ, exhibiting a relatively wide range of oxygen stoichiometry variation, are both lower compared to the Al-containing materials characterized by the co-existence of perovskite- and brownmillerite-like domains.

The parameters of the electronic structure and hyperfine interactions in perovskite SrFeO3 were determined for different types of spin and charge states by ab initio discrete variations within the cluster model. The best agreement with experiment was achieved when the charge disproportionation effect {[Fe4+O6]8− + [Fe4+O6]8−} → {[Fe3+O6]9− + [Fe5+O6]7−} was taken into account.

The total conductivity and oxygen deficiency of partially substituted strontium ferrite, SrFe0.9M0.1O3-δ (M=Cr, Ti, Al), at 700-950°C were measured depending on oxygen partial pressure varying in ranges 10-19-0.5 and 10-5-0.5 atm. The partial contributions of n- and p-type electronic charge carriers and oxygen ions to the electrical transport were determined analyzing the total conductivity vs. oxygen pressure dependencies. Additions of all dopants studied in this work are found to extend the cubic perovskite phase stability range and to improve oxygen transport in the intermediate-temperature range. The behavior of hole mobility suggests a polaron conduction mechanism. Doping with aluminum has a weak influence on the mobility level, while the incorporation of Cr and Ti cations into the ferrite lattice decreases hole mobility up to four times.

The oxygen content of the double cobaltite PrBaCo2O5 + δ was determined by coulometric titration in broad temperature and oxygen pressure ranges, and the partial thermodynamic functions of oxygen were evaluated. The results on the nonstoichiometry and structural properties of PrBaCo2O5 + δ demonstrate that, at 5 + δ < 5.3, this cobaltite has a high-temperature phase with lattice parameters a = 11.774 A, b = 11.932 A, and c = 7.609 A. The composition dependences of $$\Delta \bar H(O)$$ and $$\Delta \bar S(O)$$ indicate that the 3a p × 3a p × 2a p orthorhombic structure persists in the range 5.15 ≤ 5 + δ ≤ 5.30. Some of the Co3+ ions are shown to disproportionate according to the scheme 2Co3+=Co2++Co4+.

In order to identify the effects of minor titania additions and La:Sr cation ratio on the properties of lanthanum-strontium ferrites with high ionic conductivity, selected compositions in the La 1− x Sr x Fe 1− y Ti y O 3− δ system ( x = 0.5 and 0.7; y = 0 – 0.25 ) have been appraised employing dilatometry, oxygen permeation studies and total conductivity with Seebeck coefficient measurements in the oxygen partial pressure ( p (O 2 )) range, 10 −20 to 0.5 atm. The total conductivity is predominantly p -type electronic in the p (O 2 ) range 10 −8 –0.5 atm, with a relatively low, temperature-activated hole mobility indicating a hopping mechanism. Oxygen-ionic and n -type electronic transport is dominant for oxygen partial pressures below 10 −14 –10 −10 atm. Titanium addition lowers electronic and oxygen ionic conductivity in both regimes, but also substantially decreases thermal expansion coefficients (TECs). The average TEC values are ( 12.4 – 13.6 ) × 10 −6 K −1 at 300–900 K and ( 21.6 – 24.9 ) × 10 −6 K −1 at 900–1300 K in air. In oxidizing atmospheres the steady-state oxygen permeation fluxes through La 1− x Sr x FeO 3− δ ( x = 0.5 – 0.7 ) are similar, whereas under reducing conditions, La 0.5 Sr 0.5 FeO 3− δ exhibits the highest ionic conductivity, rendering this composition a most promising parent compound for mixed-conducting membranes operating under large p (O 2 ) gradients. The activation energies for ionic transport at low p (O 2 ), 68–75 kJ/mol, are essentially independent of dopant concentration.

Thermopower and conductivity are measured for the perovskite-like manganite La1−xBaxMnO3 (0.02⩽x⩽0.35) in the temperature range 200–950°C and oxygen pressure varying between 10−11 and 0.5 atm. The data for thermopower and electrical conductivity indicate a transport mechanism dominated by adiabatic hopes of small polarons. The high-temperature limit for the thermopower is mainly defined by the spin contribution while configurational contribution does not depend essentially on the doping level. Changes in the conductivity with doping are shown to reflect variations in the mobility of charge carriers.

The electrical properties of perovskite-related oxide La0.3Sr0.7Fe1 - xGaxO3 − δ (x = 0–0.5) are reported within the temperature range 750 to 950 °C and the oxygen partial pressure range between 10−19 and 0.5 atm. The maximum solid solubility of gallium in the iron sublattice of La0.3Sr0.7FeO3 − δ is close to 30%. At oxygen pressures about 10−4 atm, the materials undergo a transition from perovskite to oxygen vacancy ordered structures. The type of vacancy ordering depends on gallium content. Both p- and n-type electronic conductivities decrease when insulating gallium cations having stable oxidation state are incorporated into the iron sublattice of the ferrite. The observed temperature and oxygen pressure dependencies of the p-type electronic conductivity in the ordered phases suggest a transition from the intrinsic regime, when the electron–hole conduction is governed by the band gap, to the extrinsic regime, controlled by the amount of the oxygen excess in the vacancy ordered structure. The ordered strontium–lanthanum ferrite was shown to be a mixed conductor with oxygen ionic conductivity of about 0.1 S cm−1 at 900 °C. Ionic conduction in the vacancy ordered phases decreases with increasing gallium concentration.

The crystal structure of YBa2Cu3−xCoxO6+δ solid solutions, where x = 0.2, 0.4, 0.6, and 0.8, was studied by powder neutron diffraction. The charged states of the cations were calculated using the interatomic distances obtained in neutron diffraction measurements. It is shown that cobalt in the Cul position has valence 3+ and octahedral environment for x = 0.2 and 0.4. As x increases, the valence of some cobalt ions in the above positions changes to 4+, the environment changes to tetrahedral, and copper in the Cu2 position is partly substituted. Cobalt in the Cu2 positions has valence 2+ and does not change the square pyramidal arrangement of oxygen ions surrounding this position.

The equilibrium oxygen content of YBa2Cu3-xCoxO6+δ (0.2 ≤x ≤ 0.8) solid solutions was determined by coulometric titration at temperatures in the range 600–850°C and oxygen pressures 1 ≤po 2 ≤ 105 Pa. The partial enthalpy and entropy of oxygen dissolution were evaluated as a function of composition. The thermodynamic functions were found to change sharply near the high- and low-oxygen limits of the solid solution. The results are interpreted in terms of a quasi-chemical model considering three inequivalent oxygen sites in the basal plane.

The pressure-composition isotherms for oxygen in PrBa2Cu3O6 + x were measured by means of coulometric titration in the temperature range 550–825 °C and at oxygen pressures between 10−3 and 1 atm. The composition dependences of thermodynamic potentials reveal substantially non-ideal behavior of the ensemble of the oxygen ions in the basal plane. The oxygen ion conductivity of the PrBa2Cu3O6 + x compound was measured by a permeation technique in the temperature range 600–850 °C and at oxygen pressures between 0.08 and 1.00 atm. The data were employed for calculation of the oxygen chemical diffusion coefficient. The activation energy of the oxygen conductivity and oxygen diffusivity was found to depend linearly on the oxygen content, U = 2.46 − 1.57x (eV), in the integral 0.35 < x < 0.60 which was explained by interaction of movable oxygen ions.

The equilibrium oxygen content of (Nd2/3Ce1/3)4(Ba2/3 Nd1/3)4Cu6O16+x was measured by coulomeric titration in the temperature range 550-820°C and at oxygen pressures between 10-3 and 1 atm. From the results, the partial molar enthalpy and entropy of oxygen were calculated as functions of the oxygen content. The data were explained based on a point defects model and the equilibrium constants of the redox and charge disproportionation reactions involved were derived. A phase transition near the point x ≈ 1.83, which nominally corresponds to one trivalent copper cation per elementary unit, was observed.

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 to 0.5 atm and temperatures between 750 and 950 °C is reported. The analysis of the isothermal pressure dependences of the conductivity 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 contribution to conductivity from oxygen ions increases with strontium content and attains a maximal value at x=0.5. Further increase in doping results in the development of oxygen vacancy ordering phenomena and deterioration of the conducting properties.

The oxygen ionic conductivity of La 0.3 Sr 0.7 Fe 1-x Ga x O 3-δ (x = 0.2-0.4), determined by the oxygen permeation, faradaic efficiency, and total conductivity measurements at 1023-1223 K, is essentially independent of oxygen chemical potential and structural changes in the wide oxygen partial pressure range from 10 -14 to 0.21 kPa. At oxygen pressures close to atmospheric air, the ion transference numbers of perovskite-like La 0.3 Sr 0.7 Fe(Ga)O 3-δ phases vary from 4 × 10 -4 to 4 × 10 -2 , increasing with gallium content and temperature. Although there is a great difference between ionic and p-type electronic conductivities in oxidizing atmospheres, the electron-hole conduction was demonstrated to affect oxygen permeation and ambipolar conductivity. In oxidizing conditions, the oxygen permeability of La 0.3 Sr 0.7 Fe 1-x Ga x O 3-δ (x = 0-0.4) membranes increase with increasing x. The bulk ionic transport process and surface exchange kinetics both influence the permeation through La 0.3 Sr 0.7 FeO 3-δ ceramics, whereas the oxygen fluxes through Ga-containing materials are predominantly determined by the bulk ambipolar conduction. Thermal expansion coefficients of La 0.3 Sr 0.7 Fe 1-x Ga x O 3-δ (x = 0-0.4) in air vary in the range (11.7-14.9) × 10 6 K -1 at 300-800 K and increase up to (19.5-26.4) × 10 6 K -1 at 800-1170 K. Substitution of iron with gallium was found to suppress both thermal expansion and chemically induced expansion of La 0.3 Sr 0.7 Fe(Ga)O 3-δ materials, originating from oxygen nonstoichiometry variations under changing the oxygen partial pressure.