Oxygen deficiency, vacancy clustering and ionic transport in (La,Sr)CoO3 − δ

The equilibrium p(O₂)-T-δ diagrams of perovskite-type La_1 − xSr_xCoO_3 − δ (x = 0.3-0.7), collected at 873-1223 K in the oxygen partial pressure range 10^− 5-1 atm by coulometric titration and thermogravimetric analysis, were analyzed in order to appraise the effects of the point-defect interactions. The nonstoichiometry variations were adequately described combining the rigid-band approach for delocalized holes and the pair-cluster formation reaction involving oxygen vacancies and Co²⁺ cations, whilst coulombic repulsion between the positively charged vacancies can be neglected. The resultant relationships between the oxygen chemical potential and mobile vacancy concentration were used for numerical regression analysis of the steady-state oxygen permeation through dense La_1 − xSr_xCoO_3 − δ membranes, affected by the surface exchange kinetics when Sr²⁺ content is higher than 40-50%. The calculated ionic conductivity is strongly influenced by the defect association processes, and decreases with decreasing concentration of the mobile vacancies as clustering starts to prevail on reduction. The Mössbauer spectroscopy studies of La_1 − xSr_xCoO_3 − δ, doped with 1 mol% ⁵⁷Fe isotope and moderately reduced at p(O₂) ≈ 10⁻⁵ atm, show no long-range vacancy ordering at x ≤ 0.5.     

Non-universal dielectric relaxation in SrFeO3 − δ

SrFeO_3 − δ compound is prepared by the thermal decomposition method followed by ball milling. Analysis of Mössbauer spectrum and X-ray diffraction study proves the presence of multi-phase nature, i.e., Sr₈Fe₈O₂₃ and Sr₄Fe₄O₁₁ phases at room temperature. Furthermore, the Mössbauer spectrum at room temperature evidenced the presence of major Fe^3.5+ which is the resultant of equal contributions of Fe⁴⁺ and Fe³⁺. The Nyquist plot at all measured temperatures (80–230 K) suggests that the dielectric response is well associated with single relaxation time (exponential parameter, n∼1$n\sim 1$) i.e., the Debye-type. Modulus analysis exhibits the non-universal dielectric behaviour (stretched exponential parameter, β>1$\beta >1$) below 230 K and the Debye-type responds (β∼1$\beta \sim 1$) at and above 230 K. The Debye-type behaviour exhibited by SrFeO_2.81 at around room temperature in its defect state offers a new opening for this material for multifunctional applications.     

Defect structure and oxide ion transport in Sr- and Cr-doped LaCoO3 − δ

The results of oxygen nonstoichiometry, δ, measured by means of coulometric technique as a function of oxygen partial pressure, p_o2, in temperature range 1223 ≤ T, K ≤ 1323 are presented for the perovskite-type doped with chromium solely LaCo_0.7Cr_0.3O_3 − δ and simultaneously doped both with strontium and chromium La_0.7Sr_0.3Co_0.7Cr_0.3O_3 − δ cobaltites. The limit stability of the latter was found to exceed that of undoped cobaltite LaCoO_3 − δ on six orders of magnitude of p_o2 at a given temperature. The modeling of the defect structure of these perovskites was carried out and its adequate model was found. Chemical and self-diffusion coefficients of oxygen vacancies and oxygen ionic conductivity and ionic transport numbers were measured for the first time for La_0.7Sr_0.3Co_0.7Cr_0.3O_3 − δ as a function of oxygen partial pressure p_o2and temperature in the ranges − 4 ≤ log(p_o2, atm) ≤ 0 and 1223 ≤ T, K ≤ 1323, respectively. The additional substitution of Sr for La in LaCo_0.7Cr_0.3O_3 − δ was shown to lead to noticeable increase of ionic conductivity and oxygen chemical diffusion coefficient at given values of oxygen partial pressure and temperature as compared to lanthanum cobaltite doped with chromium solely. Self-diffusion coefficient of oxygen vacancies and their mobility in La_0.7Sr_0.3Co_0.7Cr_0.3O_3 − δ were found to be dependent on oxygen partial pressure and nonstoichiometry unlike undoped and doped with chromium lanthanum cobaltites.     

Oxygen nonstoichiometry, defect structure and electrical properties of LaFe0.7Ni0.3O3 − δ

Oxygen nonstoichiometry (δ), total conductivity (σ) and thermoelectric power (S) of the LaFe_0.7Ni_0.3O_3 − δ sample have been studied as functions of temperature and oxygen partial pressure. Based on the results of the direct reduction of the sample in hydrogen flow at 1100 °C the absolute oxygen content (3 − δ) has been found to vary from 2.999 to 2.974 in the range of 1273-1373 K and 10^− 3-0.21 atm. The point defect equilibrium models have been proposed and fitted to the set of experimental data in the form of log p(O₂) = f(δ)_T dependences. The values of standard thermodynamic quantities of defect formation reactions have been assessed. The joint analysis of oxygen nonstoichiometry, total conductivity and thermoelectric power has been performed using a small-polaron approach. The values of partial conductivity, partial thermopower and mobilities of electronic charge carriers have been calculated. The p-type semiconducting behavior of LaFe_0.7Ni_0.3O_3 − δ has been explained by the higher mobility values of electron holes than those of electrons in the whole range of thermodynamic parameters studied.     

Oxygen nonstoichiometry, thermo-chemical stability and lattice expansion of La0.6Sr0.4FeO3 − δ

The oxygen nonstoichiometry of La_0.6Sr_0.4FeO_3 − δ was measured at intermediate temperatures (773 to 1173 K) between 1 bar and the decomposition oxygen partial pressure by thermogravimetry and coulometric titration. The decomposition of the ABO₃ perovskite phase was found to occur at low oxygen partial pressures (below 10^− 20 bar). Using an atmosphere-controlled high-temperature XRD setup, the rhombohedral lattice parameters were obtained between 10^− 4 and 1 bar at 773 to 1173 K. A phase transition from rhombohedral to cubic might be expected to occur at high temperatures and for δ near the plateau at δ = [Sr] / 2. The lattice expansion was separated into “pure” thermal and chemically induced expansion by combining the lattice parameters with the oxygen nonstoichiometry data. The linear thermal expansion was formulated with a “pure” thermal expansion coefficient of α_th = 11.052 · 10^− 6 K^− 1 and a chemical expansion coefficient of α_chem = 1.994 · 10^− 2.The results were compared with previous data obtained for La_0.6Sr_0.4Co_1 − yFe_yO_3 − δ with y = 0.2-0.8. La_0.6Sr_0.4FeO_3 − δ was confirmed to show the highest thermo-chemical stability. While the chemical expansion of La_0.6Sr_0.4Co_1 − yFe_yO_3 − δ seems little affected by the iron content, the thermal expansion coefficient was the lowest for La_0.6Sr_0.4FeO_3 − δ.     

Co-L3 X-ray absorption near-edge structure analysis of Pr1−xCaxCoO3−δ and Pr1−xSrxCoO3−δ

The valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ has been investigated by an analysis of the Co-L₃ X-ray absorption near-edge structure (XANES) profile. The observed intensity distributions of Co-L₃ XANES change continuously with increasing concentration of alkaline-earth ions. To investigate the origin of this change in the XANES profile, charge transfer multiplet calculations were carried out, which could successfully explain the change in the spectral profile; they also suggest that the valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ is between 3+ and 4+ and increases gradually with the concentration of alkaline-earth ions.     

High-temperature magnetic properties of noninteracting randomly oriented single-domain Fe3 − δO4 nanoparticles

Magnetic measurements have been performed on 40-nm sphere-like Fe_3 − δO₄ (δ=0.043) nanoparticles using a Quantum Design vibrating sample magnetometer. Coating Fe_3 − δO₄ nanoparticles with SiO₂ effectively eliminates magnetic interparticle interactions so that the coercive field HC in the high-temperature range between 300 K and the Curie temperature (855 K) can be well fitted by an expression for noninteracting randomly oriented single-domain particles. From the fitting parameters, the effective anisotropy constant K is found to be (1.38±0.11)×¹⁰⁵ erg/cm³, which is very close to the bulk magnetocrystalline anisotropy constant of 1.35×¹⁰⁵ erg/cm³. Moreover, the inferred mean particle diameter from the fitting parameters is in quantitative agreement with that determined from transmission electron microscope. Such a quantitative agreement between data and theory suggests that the ensemble of our SiO₂-coated sphere-like Fe_3 − δO₄ nanoparticles represents a good system of noninteracting randomly-oriented single-domain particles.     

Fabrication, sintering and electrical properties of cobalt oxide doped Gd0.1Ce0.9O2 − δ

Ceria-based electrolytes have been widely researched in intermediate-temperature solid oxide fuel cell (SOFC), which might be operated at 500~600 °C. Gd0.1Ce0.9O1.95(GDC10) powders were prepared by a modified chemical co-precipitation process with Gd(NO3)3 and Ce(NO3)3 as precursors, and ammonia and hydrogen peroxide as precipitants. The precursors of GDC10 were fired at 350 °C for 2 h, then the fluorite structure cerias were identified by X-ray diffraction. The powders are well crystallized, with the size about 5 nm and surface area of 148.3 m2/g. Loading 1mol% cobalt oxide as additive, the GDC10 were succeeded to densify at 950 °C by liquid phase sintering mechanism. The grain size of 1CoGDC10 is small, about 100 nm. The electrical conductivity of samples sintered at 950 °C is about 0.01S/cm at 600 °C. The existence of cobalt oxide and smaller grain size of 1CoGDC10 don't affect the electrical conductivity.     

Structure and magnetism in Ho1−xSrxCoO3−δ

We have magnetically and structurally characterized the Ho_1−xSr_xCoO_3−δ family of materials where 0.67≤x≤0.95. The solid solution range and evolution of the structure as a function of x is established and correlated with the broad range of magnetic behavior observed. The structure is shown to be tetragonal I4/mmm although is possibly cubic when x=0.95. For 0.67≤x≤0.9 the material shows antiferromagnetic long range order and ferromagnetic clusters. At x=0.95 the magnetic transition is at 120 K and the imaginary susceptibility becomes non-zero and the temperature of the cusp in the ac susceptibility shows a frequency dependence indicative of glassiness.     

Antiferroelectric phase transition in pyrochlore-like (Dy1− xCax)2Ti2O7 − δ (x = 0, 0.1) high temperature conductors

Low-temperature ordering transitions in polycrystalline high temperature conductors (Dy_1 − xCa_x)₂Ti₂O_7 − δ (x = 0, 0.1) prepared using co-precipitation, mechanical activation and solid-state reactions at 1400 or 1600 °C have been studied by impedance spectroscopy at low frequencies and thermal mechanical analysis (TMA). The dielectric permittivity and loss tangent of the ceramics obtained have been measured as a function of temperature at low frequencies (0.5−500 Hz). The results provide evidence for the relaxation of point defects, most likely oxygen vacancies, at 500−600 °C and an antiferroelectric low-temperature phase transition of the second order, associated with re-arrangement process in the oxygen sublattice of pyrochlore structure. The temperature of the antiferroelectric transition is 700 to 800 °C, depending on the synthesis procedure and ceramic composition. Calcium doping of Dy₂Ti₂O₇ leads to the formation of additional oxygen vacancies and, in the case of the samples prepared via co-precipitation, increases the peaks in permittivity due to the relaxation process and ordering transition by three or six times, respectively.     

Surface and redox chemistry of Sm0.95Ce0.05Fe1 − xNixO3 − δ perovskites

A new series of perovskite materials with formula Sm_0.95Ce_0.05Fe_1 − xNi_xO_3 − δ (0 ≤ x ≤ 0.10) has been prepared by sol-gel combustion via a citrate precursor route. X-ray diffraction data showed that materials prepared by this method had a single orthorhombic phase belonging to the Pnma (62) space group. The study of powders sintered in air and in reducing atmospheres reveals that these materials do not show phase separation in air (up to 1350 °C) nor under 5% v/v H₂/N₂ (up to 700 °C), but a phase separation of Sm₂O₃ does occur at and above 800 °C under 5% v/v H₂/N₂ without deterioration of the perovskite phase. The surfaces of all the powders (fresh, in-situ reduced and ex-situ reduced) were Sm rich, and multiple oxidation states for Fe were observed. XP analysis of in-situ reduced samples (800 °C and above) shows that metallic Fe forms in all nickel doped materials except x = 0.07. The surface oxygen vacancies and percentages of lattice and adsorbed oxygen for this series of Ni doped materials were determined and the oxygen recapturing ability is explained in terms of the multiple oxidation states of Fe.     

Electrical conductivity and structural stability of SrCo1−xFexO3−δ

Perovskite compounds in the system of SrCo_1−xFe_xO_3−δ (x=0.2, 0.4 and 0.6) were synthesized by solid state reaction. SrCo_1−xFe_xO_3−δ shows the p-type small polaron conduction behavior. Electrical conductivity and oxygen vacancy content decrease with increase in Fe content. The incorporation of Fe increases the structural stability of SrCo_1−xFe_xO_3−δ at low temperatures, while decreasing the structural stability at high temperatures. Oxygen partial pressure has a strong influence on electrical conductivity. At low oxygen partial pressure, SrCo_0.8Fe_0.2O_3−δ will transform from cubic to orthorhombic structure. This structure can remain in 5%H₂/Ar only for a short time and then dissociates into Sr₃Fe₂O_6.64 and Co due to the reduction of B-site elements.     

Oxygen relaxation and phase transition in GdBaCo2O5 + δ oxide

Electrical conductivity, internal friction techniques and dilatometer have been used to investigate the oxygen relaxation, phase transition and thermal expansion behavior of GdBaCo₂O_5 + δ. The main electronic charge carriers in GdBaCo₂O_5 + δ are electronic holes, which could be assigned to the formation of Co⁴⁺. The oxygen exchange kinetics intensely depends on oxygen partial pressure and is also closely related to temperature. Both electrical conductivity and internal friction give rise to an abnormal at about 75 °C, which are related to the insulator-metal transition occurring in GdBaCo₂O_5 + δ. One large relaxation internal friction peak, due to the motion of oxygen within Gd-O plane, is also found in the oxide. The average thermal expansion coefficient (TEC) of GdBaCo₂O_5 + δ is about 21.4 × 10⁻⁶ K⁻¹ between 500 °C and 900 °C.     

Fast epitaxial growth of a-axis- and c-axis-oriented YBa2Cu3O7−δ films on (1 0 0) LaAlO3 substrate by laser chemical vapor deposition

a-axis- and c-axis-oriented YBa₂Cu₃O_7−δ (YBCO) films were epitaxially grown on (1 0 0) LaAlO₃ substrates by laser chemical vapor deposition. The preferred orientation in the YBCO film changed from the a-axis to the c-axis with increasing laser powers from 77 to 158 W (the deposition temperatures from 951 to 1087 K). The a-axis-oriented YBCO film showed in-plane epitaxial growth of YBCO [0 0 1]//LAO [0 0 1], and the c-axis-oriented YBCO film showed that of YBCO [0 1 0]//LAO [0 0 1]. A c-axis-oriented YBCO film with a high critical temperature of 90 K was prepared at a deposition rate of 90 μm h⁻¹, about 2-1000 times higher than that of metalorganic chemical vapor deposition.     

The effect of La-substitution on magnetic properties of nanosized Sr1−xLaxTi0.05Zn0.2(Fe)11.75−δ(Fe)δO19 powders

A series of La-substituted M-type Sr hexaferrite powders Sr_1−xLa_xTi_0.05Zn_0.2Fe³⁺_11.75O₁₉, wherein x ranges from 0.1 to 0.5 with a step of 0.1, have been prepared by the conventional ceramic method and were then milled in a high energy mill to prepare nanosized powders. XRD investigation of the calcined and the milled powders shows that single phase hexaferrite structure has been formed after calcining and has not changed after milling. The lattice parameters and the mean crystallite sizes of the samples have been determined from the XRD data and Scherrer's formula. The results show that the lattice parameters (“а” and “c”) decrease with increase in La-substitution and the mean crystallite size of the milled powders is about 17 nm. Coercivities and magnetizations of the samples in a magnetic field of 16 kOe have been determined from the room temperature hysteresis loops. It was found that both parameters increase with La substitutions up to 0.3 and then decrease for higher substitutions. These variations were attributed to the enhancement of hyperfine field and spin-canting magnetic structure when La content increases. In addition, the magnetizations were smaller for the nanosized samples in comparison with those of bulk ones, which were discussed according to the core-shell model. Also the results show that annealing of the nanosized samples up to 500 °C can enhance coercivity and magnetization of the samples, which is discussed based on crystallite size growth.     

Flame spray deposition of nanocrystalline dense Ce0.8Gd0.2O2 − δ thin films: Deposition mechanism and microstructural characterization

Nanocrystalline dense sub-micron thin films of Ce_0.8Gd_0.2O_2 − δ have been successfully deposited by flame spray deposition. The deposition mechanism has been identified as droplet deposition. The deposition temperature of the substrate was as low as 200 °C and the deposition rate ∼ 30 nm/min. Under these conditions the droplets have a very limited residence time in the hot zone of the flame and are deposited as liquid, forming smooth films free of particles. They exhibited a dense and crack-free microstructure. Grain growth, lattice constant, crystallographic density and surface roughness have been investigated as a function of annealing temperature. The good quality of the films, in association with the high deposition rates at low deposition temperatures shows the preeminence of the flame spray method for depositing thin films for micro-solid oxide fuel cells.     

Structural and optical properties of Ga2(1−x)In2xO3 films prepared on α-Al2O3 (0 0 0 1) by MOCVD

Ga_2(1−x)In_2xO₃ thin films with different indium content x [In/(Ga + In) atomic ratio] were prepared on α-Al₂O₃ (0 0 0 1) substrates by the metal organic chemical vapor deposition (MOCVD). The structural and optical properties of the Ga_2(1−x)In_2xO₃ films were investigated in detail. Microstructure analysis revealed that the film deposited with composition x = 0.2 was polycrystalline structure and the sample prepared with x up to 0.8 exhibited single crystalline structure of In₂O₃. The optical band gap of the films varied with increasing Ga content from 3.72 to 4.58 eV. The average transmittance for the films in the visible range was over 90%.     

Investigation on cation distribution and magnetic properties in self-doped manganites La0.6−xSr0.4MnO3−δ

Self-doped manganites with nominal composition La_0.6−xSr_0.4MnO_3−δ (0≤x≤0.175) have been prepared by the sol–gel method. The X-ray diffraction (XRD) patterns and magnetic measurements indicate that the samples have two phases with the ABO₃ perovskite structure being the dominant phase and Mn₃O₄ being the minor phase when the doping level x≥0.05. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions in the perovskite phases were estimated, in which there are Mn²⁺ ions and no vacancies at A sites. The ion contents have been corrected by Rietveld fitting of the powder samples' X-ray diffraction data. The change tendency of the Curie temperature T_C vs. the Mn⁴⁺ ion content ratio at the B sites of ABO₃ perovskite phase is in accord with the experimental result of the samples La_1−xSr_xMnO₃.