Yu. V. Golikov

Phase diagrams of the CoMnO system in air

Abstract A retrospective critical analysis of phase diagrams of the CoMnO system in air has been made. A high-temperature phase equilibrium of the CoMnO system in air and phase diagrams of this system under different cooling conditions (quenching in water, quenching in air, cooling at rate of 25°K h −1 ) have been constructed. A comparative analysis of these diagrams shows that whatever the cooling rate, cooling does not preserve the high-temperature state of the system and is accompanied (depending on cooling conditions, temperature and Co/Mn ratio) by one or more of the following phenomena: (1) oxidation of the Co N Mn 1−N O solution to spinel-type solid solutions (2) merging of a cubic and tetragonal spinel phase and formation of a homogeneous tetragonally distorted spinel (3) tetragonal distortion of the spinel lattice (4) decomposition of the cubic spinel into a cubic and tetragonal spinel. (5) decomposition of the cubic spinel into a cubic, tetragonal and slightly distorted tetragonal spinel. Therefore, the form of the phase diagram of the CoMnO system in air is entirely determined by the method of cooling.

Phase equilibrium diagram of the system—Ni-Mn-O

Abstract Using the available published experimental data on phase equilibria in the system Ni-Mn-O we have constructed “oxygen pressure-composition” and “temperature-composition” (in air) projections, with largely hypothetical boundaries, of the phase equilibrium diagram of this system and the projection of this diagram on the composition triangle. Having compared these data within the framework of one diagram we discovered a number of phase equilibria not revealed experimentally.

X-Ray studies of phase diagrams of the system Mn-Cr-O in air

Abstract X-Ray analysis was made of solid solutions and heterogeneous compositions of the general formula Mn 3− x Cr x O m (0⩽ x ⩽ 3, Δx = 0.05). Subsolidus portions of the high temperature phase equilibria diagram and the diagram of water-quenched phases of the system Mn-Cr-O in air were established in the temperature range 700–1400°C. Comparative analysis of these diagrams has revealed the mechanism of the processes occurring upon rapid cooling of the system (in water and in air in a ceramic crucible) from high temperatures to room temperature. Rapid cooling is shown to preserve the phase composition and crystal structure of nonspinel phases, but for equilibrium solid solutions with spinel or hausmannite structure, this is not always the case. Depending on the quench temperature and the solution composition the following phenomena are observed: (1) tetragonal distortion of a high temperature phase with spinel structure; (2) merging of a tetragonally distorted spinel and spinel and formation of a homogeneous phase with hausmannite structure; (3) decomposition of the homogeneous phase with spinel structure into two spinel-type phases with spinel and hausmannite structures. Possible causes of these phenomena are discussed.

Homogeneity regions of yttrium and ytterbium manganites in air

Homogeneous solid solutions and heterogeneous systems of the general formula R2 − xMnxO3 ± δ (0.90 ≤ x ≤] 1.10 for R = Y and 0.88 ≤ x ≤ 1.14 for R = Yb; Δx = 0.02) were produced by ceramic synthesis from oxides in air within the temperature range 900–1400°C. The solubility boundaries of simple oxides R2O3 (R = Y, Yb), Mn3O4, and binary oxide RMn2O5 in yttrium and ytterbium manganites RMnO3 ± δ were determined X-ray powder diffraction of these solutions and systems. The results were presented as fragments of phase diagrams of the systems Y-Mn-O and Yb-Mn-O in air. The solubility of Y2O3 and Mn3O4 in YMnO3 ± δ was found to increase with increasing temperature, and the solubility of Yb2O3 and Mn3O4 in YbMnO3 ± δ to be insensitive to varying temperature. It was suggested that the solubility of Y2O3 and Mn3O4 in YMnO3 ± δ and of Yb2O3 and Mn3O4 in YbMnO3 ± δ is caused by crystal structure defects of yttrium and ytterbium manganites and their related oxygen nonstoichiometry. In dissolving RMn2O5 in RMnO3 ± δ (R = Y, Yb) in air within a narrow (∼20°C) temperature range adjacent to the RMn2O5 = RMnO3 + 1/3Mn3O4 + 1/3O2 equilibrium temperature, the solubility of RMn2O5 in RMnO3 ± δ ecreases abruptly until almost zero. Conclusion is made that structural studies are necessary necessary to determine the oxygen nonstoichiometry δ of R2 − xMnxO3 ± δ solid solutions as a function of x and synthesis temperature; together with the results of this work, these studies will allow one to construct unique crystal-chemical models of these solid solutions.

Phase relations in alkaline earth-manganese-oxygen systems: Equilibrium and metastable states

This paper presents a systematic review of the literature concerned with the physicochemical analysis of the thermodynamic systems Ca-Mn-O, Sr-Mn-O, and Ba-Mn-O at variable temperature and oxygen pressure in the gas phase. Available data are systematized in the form of projections of the phase diagrams of these systems onto the alkaline earth-managanese-oxygen composition triangles. Compatibility triangles are identified in the projections, and the phase relations involved are discussed. The conclusion is drawn that the physicochemical data reported to date are insufficient for the thermodynamic analysis of heterogeneous equilibria in the A-Mn-O systems, which is hindered primarily by the lack of information about the equilibrium oxygen pressure as a function of temperature for the thermal dissociation (or oxidation) of AxMnyOz (A = Ca, Sr, Ba) oxides.

Phase equilibria in the CaCuO system under variable temperatures and oxygen pressures

Abstract Synthesis of samples in the CaCuO system in air by the ceramic method was carried out. Existence of Ca 2 CuO 3 and CaCu 2 O 3 compounds was confirmed. Phase equilibria during thermal dissociation of Ca 2 CuO 3 and its mixtures with CaO and CuO (953–1173K) were studied by the circulation method, in addition to the EMF method and X-ray phase analysis. The results are shown as in isothermal section (1073K) of the CaCuO phase diagram ( P O 2 - x diagram) and its projection on the P O 2 - T plane. Thermodynamic analysis was also performed.

Homogeneity range of neodymium manganite in air

The solubility boundaries for Nd2O3 and manganese oxides in NdMnO3 ± δ have been determined by X-ray powder diffraction analysis of homogeneous phases and heterogeneous compositions of the general formula Nd2 − xMnxO3 ± δ (0.90 ≤ x ≤ 1.20; Δx = 0.02) prepared by ceramic technology from constituent oxides in air in the temperature range 900–1400°C. The results are presented in the form of a fragment of the Nd-Mn-O phase diagram in air. It is suggested that the Nd2O3 solubility in NdMnO3 ± δ is due to crystal defects and the solubility of manganese oxides is in addition due to the disproportionation reaction 2Mn3+ = Mn2+ + Mn4+ and the subsequent partial substitution of divalent for tervalent manganese ions in the cuboctahedral positions of the perovskite-like crystal lattice. To verify this suggestion, it is necessary to systematically study the oxygen nonstoichiometry δ in Nd2 − xMnxO3 ± δ as a function of x and synthesis temperature and structurally study this oxide with these parameters being varied.

Stability region of Gd2 − xMnxO3 ± δ solid solutions in air

The solid-solution range of Gd2 − xMnxO3 ± δ has been determined using X-ray diffraction analysis of samples with 0.90 ≤ x ≤ 1.20 (Δx = 0.02) prepared from oxide mixtures by solid-state reactions in air between 900 and 1400°C. The results have been used to construct a partial phase diagram of the Gd-Mn-O system in air. It is shown that gadolinium manganite with the perfect metal stoichiometry, GdMnO3 ± δ, does not exist. The material of this composition in air consists of two phases: a Gd2 − xMnxO3 ± δ solid solution with an orthorhombic perovskite-like structure and the binary oxide Gd2O3. The solid solution extends to the composition Gd0.96Mn1.04O3 ± δ. Over the entire temperature range studied, gadolinium oxide does not dissolve in Gd0.96Mn1.04O3 ± δ. Mn3O4 exhibits significant solubility in Gd2 − xMnxO3 ± δ. In particular, Gd0.86Mn1.14 O3 ± δ is single-phase by X-ray diffraction in the temperature range 1185–1400°C. Below 1185°C, Gd2 − xMnxO3 ± δ is in equilibrium with another gadolinium manganite, GdMn2O5. With decreasing synthesis temperature, the GdMn2O5 solubility in Gd2 − xMnxO3 ± δ drops precipitously. At 900°C, the only single-phase sample was Gd0.96Mn1.04O3 ± δ.

Phase diagrams of the MnAlO system

Abstract A high temperature phase equilibrium diagram of the MnAlO system in air has been refined and phase diagrams of unstable states occurring upon cooling the system from an equilibrium state to room temperature in air by different methods (quenching in water, quenching in air, cooling at the rate of 25 K/h) have been constructed with the use of radiography methods. It is shown that cooling at any rate does not preserve the high temperature phase composition of the system and is accompanied by one (or a combination) of the following phenomena: (1) a tetragonal distortion of the spinel lattice; (2) the coalescence of a tetragonal and cubic spinel phase into one phase with the hausmannite structure; (3) decomposition of a homogeneous spinel with the formation of two spinel-type phases. Projections of the phase equilibrium diagram of the MnAlO system on the composition triangle and the ‘oxygen pressure-composition’ plane have been constructed. Correlating the data obtained within the framework of a single P - T - x diagram has revealed a number of phase equilibria in the system which have not been determined experimentally.

Composition ranges of Ln2 − xMnxO3 ± δ (Ln = Y, Ho, Er) solid solutions between 900 and 1400°C in air

The metal stoichiometry ranges of the Ln2 − xMnxO3 ± δ (Ln = Y, Ho, Er) manganites have been determined using X-ray diffraction analysis of ceramic samples prepared by reacting oxide mixtures in air at temperatures from 900 to 1400°C. The results are represented as partial phase diagrams of the Ln-Mn-O systems in air. Comparison of the phase diagrams demonstrates that the phase boundaries of the manganites are determined not only by the effective cation radius of the rare-earth metal. The solubilities of the binary oxides Y2O3, Ho2O3, and Mn3O4 in yttrium and holmium manganites increase with temperature, with significant quantitative distinctions. The Er2O3 and Mn3O4 solubilities in erbium manganite remain unchanged over the entire temperature range studied. The LnMn2O5 solubility in LnMnO3 ± δ is an intricate function of temperature. We analyze the possible causes of the Ln2O3, Mn3O4, and LnMn2O5 solubility in the LnMnO3 ± δ (Ln = Y, Ho, Er) manganites.