V. A. Cherepanov

Crystal structure, electrical and magnetic properties of La1−xSrxCoO3−y

The crystal structure and properties of La1 − xSrxCoO3 − y with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2–400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO3 is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co4+ (cobalt ions with one positive effective charge, CoCo.) and oxygen vacancies (Vo..). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers — and the resultant electrical conductivity — decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations — probably between oxygen vacancies and strontium ions — and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.

Oxygen nonstoichiometry of La1−xSrxCoO3−δ (0 < x ≤ 0.6)

Abstract The oxygen nonstoichiometry (δ) of La 1− x Sr x CoO 3− δ (0 x ≤ 0.6) has been studied as a function of temperature (573–1673 K) and oxygen partial pressure (1–10 −3 atm) using TGA and coulometric titration techniques. The absolute values of oxygen nonstoichiometry were determined by the direct reduction in a flux of hydrogen (TGA) or by decreasing P O 2 in a coulometric titration cell. The boundaries of phase stability of La 0.7 Sr 0.3 CoO 3−δ and (La 0.7 Sr 0.3 ) 2 CoO 4± v were evaluated. A possible mechanism of disordering processes has been discussed.The oxygen nonstoichiometry ({delta}) of La{sub 1{minus}x}Sr{sub x}CoO{sub 3{minus}{delta}} (0 < x {le} 0.6) has been studied as a function of temperature (573-1,673 K) and oxygen partial pressure (1{minus}10{sup {minus}3} atm) using TGA and coulometric titration techniques. The absolute values of oxygen nonstoichiometry were determined by the direct reduction in a flux of hydrogen (TGA) or by decreasing P{sub O{sub 2}} in a coulometric titration cell. The boundaries of phase stability of La{sub 0.7}Sr{sub 0.3}CoO{sub 3{minus}{delta}} and (La{sub 0.7}Sr{sub 0.3}){sub 2}CoO{sub 4{plus minus}v} were evaluated. A possible mechanism of disordering processes has been discussed.

Phase equilibria in the Ln-Mn-O system (Ln = Pr, Nd) and general aspects of the stability of the perovskite phase LnMeO3

Abstract Phase equilibria in the Pr-Mn-O and Nd-Mn-O systems have been studied between 1100 and 1500 K and in atmospheres with an oxygen partial pressure ranging between 10 −15 and 1 atm. LnMnO 3 and LnMn 2 O 5 have been found to form in both systems. PrMn 2 O 5 was synthesized for the first time and has the orthorhombic DyMn 2 O 5 -type structure with cell parameters: a = 7.51, b = 8.63 and c = 5.69 A. The thermodynamic stability of complex oxides was studied using TG and emf measurements in a galvanic cell with solid electrolytes. The equilibrium oxygen pressure, the standard Gibbs potential of the decomposition reactions 3LnMn 2 O 5 = 3LnMnO 3 + Mn 3 O 4 + O 2 and LnMnO 3 = 1 2 Ln 2 O 3 + MnO + 1 4 O 2 , and the standard Gibbs potential of the formation of the ternary oxides have been calculated. The variation of LnMeO 3 stability for the rare earth manganates, and for the 3 d -transition metal (Cr, Mn, Co, Ni, Cu) compounds for a given rare earth element is discussed based on geometrical crystallographic criteria and the outer 3 d n electron configurations.

Phase equilibria in the La-Me-Co-O (Me=Ca, Sr, Ba) systems

The phase equilibria of the La-Me-Co-O systems (Me = Ca, Sr and Ba) were studied in air at 1100 °C. Two types of solid solution of general composition La1−xMexCoO3−δ and (La1−y Mey)2CoO4 were found to exist in the systems. The limiting composition of La1−xMexCoO3−δ lies at x=0.8 for Me = Sr, Ba and between 0.3–0.5 for Me = Ca. It is shown that the rhombohedral distortion of the perovskite type La1−xMexCoO3−y decreases while x increases. La1−xMexCoO3−δ (Me = Sr, Ba) shows an ideal cubic structure at x=0.5. The stability range of (La1−yMey)2CoO4 was found to be 0.25≤y≤0.35 for Me = Ca, 0.3≤y≤0.55 for Me = Sr and 0.3≤y≤0.375 for Me = Ba. All phases have tetragonal K2NiF4-type crystal structure. Based on the XRD and neutron diffraction patterns of quenched samples, the phase diagrams (Gibbs triangles) are constructed for all systems. The phase equilibrium at low oxygen pressure is shown for the example of the La-Sr-Co-O system. The decomposition mechanism of La1−xSrxCoO3−δ at 1100 °C for the samples with 0.5log(Po2)>−2.25 can be written as follows: La1−x′ Srx′CoO3−δ′=n La1−x″Srx″CoO3−δ″+m SrCoO2.5+q/2 O2 where x′>x″. The decomposition mechanism of La1−xSrxCoO3−δ for the samples with x < 0.5 within the oxygen pressure range −2.25>log(Po2)>−3.55 changes and can be written as follows: La1−xSrxCoO3−δ′=r La1−x′Srx′CoO3−δ″+w (La1−y′Sry′)2CoO4+v CoO+f/2 O2. The results are shown in “logPo2-composition” diagrams.

Phase equilibria and crystal structures of phases in the La-Fe-Ni-O system at 1370 K in air

The phase equilibria in the La-Fe-Ni-O system have been studied at 1370 K in air, and the La-Fe-Ni-O phase diagram at constant temperature and pressure has been constructed. Based on x-ray diffraction results for samples prepared by standard solid-state reactions and via citrate and nitrate routes, the following solid solutions have been shown to exist at 1370 K in air: LaFe1 − xNixO3 − δ (0 < x ≤ 0.4, sp. gr. Pbnm; 0.6 ≤ x ≤ 0.8, sp. gr. R-3 c), La4(Ni1 − yFey)3O10 − δ (0 < y ≤ 0.3), La3(Ni1 − zFez)2O7 − δ (0 < z ≤ 0.05), La2Ni1 − vFevO4 + δ (0 < v ≤ 0.05), NikFe3 − kO4 (0.81 ≤ k ≤ 1.05), Ni1 − mFemO (0 < m ≤ 0.05), and Fe2 − pNipO3 (0 < p ≤ 0.04). The lattice constants and structural parameters of single-phase samples have been refined by the Rietveld profile analysis method.

Phase equilibria in the La-Ba-Co-O system

Phase equilibria in the La-Ba-Co-O system were studied at 1,100 C in air. The existence of oxide phases LaCoO{sub 3}, BaCoO{sub 3{minus}y}, Ba{sub 2}CoO{sub 4}, and La{sub 2}BaO{sub 4} in quasibinary systems in air at 1,100 C was found, in agreement with previous data. Two types of solid solutions were found in the quasiternary system: La{sub 1{minus}x}Ba{sub x}CoO{sub 3{minus}{delta}} and (La{sub 1{minus}z}Ba{sub z}){sub x}CoO{sub 4}. The homogeneity range of La{sub 1{minus}x}Ba{sub x}CoO{sub 3{minus}{delta}} was found to be 0 {le} x {le} 0.8. As the content of alkali-earth metal (x) increased, a rhombohedral distortion of La{sub 1{minus}x}Ba{sub x}CoO{sub 3{minus}{delta}} decreased; La{sub 0.55}Ba{sub 0.45}CoO{sub 3{minus}{delta}} had an ideal cubic structure. The composition of single phase samples of (La{sub 1{minus}z}Ba{sub z}){sub 2}CoO{sub 4} composition was obtained for z = 0.300, 0.325, 0.350, and 0.375. These samples had the tetragonal K{sub 2}NiF{sub 4}-type structure.

Crystal structure and physicochemical properties of layered perovskite-like phases LnBaCo2O5 + δ

Complex oxides LnBaCo2O5 + δ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y) were obtained by solid-state synthesis at 1373 K in air. The crystal structure of layered perovskites LnBaCo2O5 + δ was studied by X-ray and neutron diffraction analyses. It was shown that cobaltites LnBaCo2O5 + δ crystallized in tetragonal (space group P4/mmm, 0.6 > δ > 0.45) or orthorhombic (space group Pmmm, 0.6 > δ > 0.45) structures depending on the radius of the lanthanide ion and the oxygen content. The dependences of the unit cell parameters of LnBaCo2O5 + δ on the radius of the lanthanide ion were obtained. The average thermal expansion coefficients at temperatures from 298 to 1373 K in air were determined. The dependence of the chemical component of thermal expansion on the oxygen content was evaluated for LnBaCo2O5 + δ (Ln = Nd, Sm, Gd, Y).

Phase equilibria and crystal structures of solid solutions in the system LaCoO3−δ-SrCoO2.5±δ-SrFeO3−δ-LaFeO3−δ

AbstractThe composition region and structure of La1−xSrxCo1−yFeyO3−δ solid solutions are determined by x-ray powder diffraction using the Rietveld profile analysis method. The solid solutions based on lanthanum cobaltite, LaCoO3−δ, have a rhombohedrally distorted perovskite-like structure (sp. gr. R

Magnetoresistance and Hall effect in La0.8Sr0.2MnO3

\bar 3