Phase equilibria in the system Sm–Rh–O and thermodynamic and thermal studies on SmRhO3

Using isothermal equilibration, phase relations are established in the system Sm–Rh–O at 1273 K. SmRhO₃ with GdFeO₃-type perovskite structure is found to be the only ternary phase. Solid-state electrochemical cells, containing calcia-stabilized zirconia as an electrolyte, are used to measure the thermodynamic properties of SmRhO₃ formed from their binary component oxides Rh₂O₃ (ortho) and Sm₂O₃ (C-type and B-type) in two different temperature ranges. Results suggest that C-type Sm₂O₃ with cubic structure transforms to B-type Sm₂O₃ with monoclinic structure at 1110 K. The standard Gibbs energy of transformation is $ \Delta_{\text{tr}} G^{\text{o}} ( \pm 87)/{\text{J}}\,{\text{mol}}^{ - 1} = 3763 - 3.39\,(T/{\text{K}}) $ . Standard Gibbs energy of formation of SmRhO₃ from binary component oxides Rh₂O₃ and Sm₂O₃ with B-type rare earth oxide structure can be expressed as $ \Delta_{\text{f(ox)}} G^{\text{o}} ( \pm 75)/{\text{J}}\,{\text{mol}}^{ - 1} = - 64230 + 6.97(T/{\text{K}}) $ . The decomposition temperature of SmRhO₃ estimated from the extrapolation of electrochemical data is 1665 (±2) K in air and 1773 (±3) K in pure oxygen. Temperature-composition diagrams at constant oxygen pressures are constructed for the system Sm–Rh–O. Employing the thermodynamic data for SmRhO₃ from emf measurement and auxiliary data for other phases from the literature, oxygen potential-composition phase diagram and 3-D chemical potential diagram for the system Sm–Rh–O at 1273 K are developed.     

High temperature phase chemistry of system Eu-Pd-O

Abstract

An isothermal section of the phase diagram for the system Eu-Pd-O at 1223 K has been established by equilibration of samples representing 20 different compositions, and phase identification after quenching by optical and scanning electron microscopy, X-ray powder diffraction, and energy dispersive spectroscopy. Three ternary oxides, Eu₄PdO₇, Eu₂PdO₄, and Eu₂Pd₂O₅, were identified. Liquid alloys and the intermetallic compounds EuPd₂ and EuPd₃ were found to be in equilibrium with EuO. The compound EuPd₃ was also found to coexist separately with Eu₃O₄ and Eu₂O₃. The oxide phase in equilibrium with EuPd₅ and Pd rich solid solution was Eu₂O₃. Based on the phase relations, four solid state cells were designed to measure the Gibbs energies of formation of the three ternary oxides in the temperature range from 925 to 1350 K. Although three cells are sufficient to obtain the properties of the three compounds, the fourth cell was deployed to crosscheck the data. An advanced version of the solid state cell incorporating a buffer electrode with yttria stabilised zirconia solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode was used for high temperature thermodynamic measurements. Equations for the standard Gibbs energy of formation of the interoxide compounds from their component binary oxides Eu₂O₃ with C type structure and PdO have been established. Based on the thermodynamic information, isothermal chemical potential diagrams and isobaric phase diagrams for the system Eu-Pd-O have been developed.     

Phase relations in the system Cu-La-O and thermodynamic properties of CuLaO2 and CuLa2O4

Phase relations in the system Cu-La-O at 1200 K have been determined by equilibrating samples of different average composition at 1200 K, and phase analysis of quenched samples using optical microscopy, XRD, SEM and EDX. The equilibration experiments were conducted in evacuated ampoules, and under flowing inert gas and pure oxygen. There is only one stable binary oxide La₂O₃ along the binary La-O, and two oxides Cu₂O and CuO along the binary Cu-O. The Cu-La alloys were found to be in equilibrium with La₂O₃. Two ternary oxides CuLaO₂ and CuLa₂O₄₊ were found to be stable. The value of varies from close to zero at the dissociation partial pressure of oxygen to 0.12 at 0.1 MPa. The ternary oxide CuLaO₂, with copper in monovalent state, coexisted with Cu, Cu₂O, La₂O₃, and/or CuLa₂O₄₊ in different phase fields. The compound CuLa₂O₄₊, with copper in divalent state, equilibrated with Cu₂O, CuO, CuLaO₂, La₂O₃, and/or O₂ gas under different conditions at 1200 K. Thermodynamic properties of the ternary oxides were determined using three solid-state cells based on yttria-stabilized zirconia as the electrolyte in the temperature range from 875 K to 1250 K. The cells essentially measure the oxygen chemical potential in the three-phase fields, Cu + La₂O₃ + CuLaO₂, Cu₂O + CuLaO₂ + CuLa₂O₄ and La₂O₃ + CuLaO₂ + CuLa₂O₄. Although measurements on two cells were sufficient for deriving thermodynamic properties of the two ternary oxides, the third cell was used for independent verification of the derived data. The Gibbs energy of formation of the ternary oxides from their component binary oxides can be represented as a function of temperature by the equations:

Oxygen Coefficient of Uranium Oxides and Current Efficiency as Influenced by Electrolysis Parameters and Composition of Molten Salt Electrolytes in the Na2WO4-Na2W2O7-UO2WO4 System

The oxygen coefficient of uranium oxides, their structure, and current efficiency were studied as influenced by the electrolyte composition, deposition potential, and temperature of electrolysis. On passing from Na₂WO₄-UO₂WO₄ binary system to lower-melting ternary systems the dependences of the oxygen coefficient in the cathodic product on the electrolyte composition and electrolysis parameters remain essentially similar. Significant deviation of the experimental current efficiency with respect to uranium oxides from the theoretical value suggests significant chemical interaction between the cathodic product and electrolyte. The corrosion rate increases and the current efficiency decreases with increasing temperature and concentration of W₂O₇ ^2- ions. The structure of the resulting cathodic deposits is predominantly determined by their specific electrical conductivity, which is a function of the chemical composition of the electrolyte. The dendrite structure is typical for higher oxides.     

High–temperature phase chemistry of the system Gd–Pd–O

Anisothermal sectionof the phase diagram for the system Gd–Pd–O at 1223 K has been established by equilibrationof samples and phase identification after quenching by optical and scanning electron microscopy, X–ray powder diffraction, and energy dispersive spectroscopy. Three ternary oxides Gd₄PdO₇,Gd₂PdO₄ and Gd₂Pd₂O₅ were identified. Liquid alloys, the four inter–metallic compounds and Pd–rich solid solutionwere found to be inequilibrium with Gd₂O₃.

Based on the phase relations, four solid–state cells were designed to measure the Gibbs energies of formation of the three ternary oxides in the temperature range from 920 to 1320 K. Although three cells are sufficient to obtain the properties of the three compounds, the fourth cell was deployed to cross check the data. An advanced version of the solid–state cell incorporating a buffer electrode with yttria–stabilized zirconia solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode was used for high–temperature thermodynamic measurements. The standard Gibbs energy of formation of the inter–oxide compounds from their component binary oxides can be represented by the following equations:

Gd₄PdO₇(s) : Δ_f(ox)G⁰/J mol^–1 = –25,030 + 0.33T (±140), Gd₂PdO₄(s) : Δf(ox)_f(ox)G⁰/J mol^–1 = –25,350 + 0.84T (±135), Gd₂Pd₂O₅(s) : Δf(ox)_f(ox)G⁰/J mol^–1 = –48,700 + 0.38T (±270)

Based on the thermodynamic information, isothermal chemical potential diagrams and isobaric phase diagrams for the system Gd–Pd–O are developed.     

Activity of PtSnRh/C nanoparticles for the electrooxidation of C1 and C2 alcohols

A systematic investigation of alcohol adsorption and oxidation on binary and ternary electrocatalysts in acid medium was performed. Binary (PtRh) and ternary (PtRhSn) were prepared by the Pechini modified method on carbon Vulcan XC-72, and different nominal compositions were characterized by energy dispersive X-ray and X-ray diffraction (XRD) techniques. The XRD results showed that the Pt₈₀Rh₂₀/C and Pt₇₀Sn₁₀Rh₂₀/C electrocatalysts consisted of the Pt displaced phase, suggesting the formation of a solid solution between the metals Pt/Rh and Pt/Sn.Electrochemical investigations on these different electrode materials were carried out as a function of the electrocatalyst composition, in acid medium (0.5 mol dm^− 3 H₂SO₄), and in the absence and presence of different alcohols (methanol, ethanol and ethylene glycol). The electrochemical results obtained at room temperature have shown that the Pt₇₀Sn₁₀Rh₂₀/C catalyst display better catalytic activity for alcohol oxidation compared with the binary catalyst.In situ reflectance infrared spectroscopy measurements have shown that the oxidation of alcohols mentioned produced CO₂ at low potentials indicating that the materials synthesized could be used as efficient anodes in the fuel cell applications.     

Thermodynamic properties of LaFeO3−δ and LaFe12O19

The standard Gibbs energies of formation of lanthanum orthoferrite (LaFeO_3−δ) and hexaferrite (LaFe₁₂O₁₉) were determined using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the electrolyte and pure oxygen gas at ambient pressure as the reference electrode. From emf of the solid-state cell, the Gibbs energy of formation of nonstoichiometric orthoferrite (LaFeO_3−δ) is obtained. To derive values for the stoichiometric phase, variation of the oxygen nonstoichiometric parameter δ with oxygen partial pressure was measured using thermogravimetry under controlled gas mixtures. The results obtained for LaFeO₃ and LaFe₁₂O₁₉ can be summarized by the following equations, which represent the formation of ternary oxides from their component binary oxides: ½La₂O₃ + ½Fe₂O₃ → LaFeO₃; ΔG° (LaFeO₃) (±450) (J mol⁻¹) = −62920 − 2.12T (K), and ½La₂O₃ + 9/2Fe₂O₃ + Fe₃O₄ → LaFe₁₂O₁₉; ΔG° (LaFe₁₂O₁₉) (±200) (J mol⁻¹) = −103900 + 21.25T (K). These data are discussed critically in comparison with thermodynamic values reported in the literature from a variety of measurements. The values obtained in this study are consistent with calorimetric entropy and enthalpy of formation of the perovskite phase and with some of the Gibbs energy measurements reported in the literature. For the lanthanum hexaferrite (LaFe₁₂O₁₉) there are no prior thermodynamic measurements for comparison.     

Oxygen content determination of Bi2Sr2Ca2Cu4O11+x superconductor by thermogravimetric analysis

Thermogravimetric analysis of the individual, binary, ternary, and bismuth-based superconductor mixtures have been carried out to elucidate the excess oxygen content of the Bi₂Sr₂Ca₂Cu₄O_11+x . Our systematic approach eliminates the need to assume initial phase present, original oxygen content, and degree of reduction as in other TG studies. The excess oxygen content of the bismuth superconductor increases fromx=0.38 tox=0.64 after three heating cycles in oxygen atmosphere. Most of the excess oxygen is associated with the highly oxidized copper (Cu³⁺) in the superconducting phase/phases.     

Oxygen content determination of Bi2Sr2Ca2Cu4O11+x superconductor by thermogravimetric analysis

Thermogravimetric analysis of the individual, binary, ternary, and bismuth-based superconductor mixtures have been carried out to elucidate the excess oxygen content of the Bi₂Sr₂Ca₂Cu₄O_11+x . Our systematic approach eliminates the need to assume initial phase present, original oxygen content, and degree of reduction as in other TG studies. The excess oxygen content of the bismuth superconductor increases fromx=0.38 tox=0.64 after three heating cycles in oxygen atmosphere. Most of the excess oxygen is associated with the highly oxidized copper (Cu³⁺) in the superconducting phase/phases.     

Microstructure and phase transformation of zirconia-based ternary oxides for thermal barrier coating applications

Five dopant oxides, Sc₂O₃, Yb₂O₃, CeO₂, Ta₂O₅, and Nb₂O₅, were incorporated into 7YSZ to create ternary zirconia-based oxides with varying oxygen vacancies and substitutional defects. These ternary oxides were consolidated using a high-temperature sintering process. The resulting bulk oxides were subjected to microstructural study using scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The results show that the microstructures of the ternary zirconia-based oxides are determined by the amount of oxygen vacancies in the system, the dopant cation radius, and atomic mass. Increasing the number of oxygen vacancies in the lattice by the addition of trivalent dopant as well as the use of larger cations promotes the stabilization of the high-temperature cubic phase. The tetravalent cation, on the other hand, has the effect of retaining tetragonal phase to room temperature without the influence of oxygen vacancy. The addition of pentavalent oxide leads to the formation of monoclinic phase upon cooling.     

Alloy-oxide equilibria in the system Pt-Rh-O

The composition of Pt-Rh alloys that co-exist with Rh₂O₃ in air have been identified by experiment at 1273 K. The isothermal sections of the phase diagram for the ternary system Pt-Rh-O at 973 K and 1273 K have been computed based on experimentally determined phase relations and recent thermodynamic measurements on Pt_1−X Rh _X alloys and Rh₂O₃. The composition dependence of the oxygen partial pressure for the oxidation of Pt_1−X Rh _X alloys at different temperatures, and temperature for the oxidation of the alloys in air are computed. The diagrams provide quantitative information for optimization of the composition of Pt_1−X Rh _X alloys for high temperature application in oxidizing atmospheres.     

Oxygen potentials,Gibbs' energies and phase relations in the Cu-Cr-O system

Thermodynamic properties of ternary compounds, cuprous and cupric chromites (CuCro₂, CuCr₂O₄), and oxygen potentials corresponding to three three-phase regions in the Cu-Cr-O system have been measured in the temperature range 900 to 1350 K using a solid state galvanic cell incorporating calcia-stabilized zirconia. Cuprous chromite was found to be nearly stoichiometric. The compositions of non-stoichiometric cupric chromite saturated with CuO and Cr₂O₃ have been determined using electron microprobe and energy dispersive X-ray analysis. The results of this study resolve discrepancies in Gibbs' energies of cuprous and cupric chromites reported in the literature. A ternary phase diagram for the Cu-Cr-O system at 1150 K and phase relations in air for the Cu₂O-CuO-Cr₂O₃ system as a function of temperature have been derived based on the new thermodynamic data. The phase diagram given in the literature is found to be inaccurate.     

Determination of the oxygen potential of U0.718Ce0.282O2 ± x using a solid electrolyte galvanic cell

To assess the nature of variations in the thermodynamic properties of uranium plutonium oxide fuel, which is highly radioactive and toxic, we have measured characteristics of uranium cerium oxides similar in properties to the nuclear fuel. The emf method, a solid electrolyte galvanic cell, and solid-state coulometric titration have been used to find out general trends in the variation of the oxygen potential with the oxygen/metal ratio for U_0.718Ce_0.282O_{2 ± x} .     

Synthesis and characterization of Cu2O·TeO2 and CuI·Cu2O·TeO2 glasses

Cu₂O·TeO₂ and CuI·Cu₂O·TeO₂ glasses were synthesized and characterized by complex impedance measurement, Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy techniques. Samples of the binary and the ternary systems are found to have both Cu⁺ and Cu²⁺ with their relative concentration being composition dependent. Bonds like -O-Cu²⁺-O-, leading to the formation of bridging oxygen are found to form in the binary system. Structural units like (Te₃O₈ ^4–)_n are also found to form when Cu₂O content is high in the binary system. Phase separation is observed in the ternary system. The glass structure and hence the ionic conduction behavior are found to depend upon chemical composition. When CuI content exceeds 60 mol%, the crystalline phase of -Cul gets stabilized at room temperature, thus causing the enhancement in conductivity.     

Gibbs’ energy of formation of YBa2Cu3O7-x (tetragonal)

The high temperature ceramic oxide superconductor YBa₂Cu₃O_7-x (1–2–3 compound) is generally synthesized in an oxygen-rich environment. Hence any method for determining its thermodynamic stability should operate at a high oxygen partial pressure. A solid-state cell incorporating CaF₂ as the electrolyte and functioning under pure oxygen at a pressure of 1·01 × 10⁵ Pa has been employed for the determination of the Gibbs’ energy of formation of the 1–2–3 compound. The configuration of the galvanic cell can be represented by: Pt, O₂, YBa₂Cu₃O_7−x , Y₂BaCuO₅, CuO, BaF₂/CaF₂/BaF₂, BaZrO₃, ZrO₂, O₂, Pt. Using the values of the standard Gibbs’ energy of formation of the compounds BaZrO₃ and Y₂BaCuO₅ from the literature, the Gibbs’ energy of formation of the 1–2–3 compound from the constituent binary oxides has been computed at different temperatures. The value ofx at each temperature is determined by the oxygen partial pressure. At 1023 K for O content of 6·5 the Gibbs’ energy of formation of the 1–2–3 compound is −261·7 kJ mol⁻¹.     

Subsolidus phase equilibria in the Al2O3-CeO2-PbO and Al2O3-CeO2-RuO2 systems

The subsolidus phase equilibria in air for the Al₂O₃-CeO₂-PbO and Al₂O₃-CeO₂-RuO₂ systems were studied with the aim of obtaining information on possible interactions between a CeO₂-based solid electrolyte in solid-oxide fuel cells (SOFCs) and other oxides. No ternary compound was found in either of the systems. The tie line in the Al₂O₃-PbO-CeO₂ system is between Al₂Pb₂O₅ and the CeO₂.     

Thermodynamic properties of ternary phases in the Cu-Tl-Se system

The Cu-Tl-Se system has been studied at temperatures from 300 to 420 K using emf measurements with Cu₄RbCl₃I₂ as a Cu+ ion conducting solid electrolyte. The emf data have been used to map out the subsolidus phase diagram of the Cu-Tl-Se system in the composition region Tl₂Se-CuTlSe-CuSe-Se. We have calculated the partial molar thermodynamic functions of the copper in the alloys and the standard thermodynamic functions of formation and standard entropies of the ternary compounds CuTlSe₂, CuTlSe, and Cu₂TlSe₂. The results confirm that the thermodynamic properties of copper-containing ternary systems can be studied using the approach in question even when they contain an element (thallium in this study) located to the left of copper in the electrochemical series.     

Characterization of the interface between (U,M)O2±x and yttria-zirconia

Solid-state electrochemical cells have been prepared by co-sintering pre-reacted electrode and electrolyte materials together. The electrodes investigated were the non-stoichiometric oxides of the general formula (U, M)O_2±x (M = Sc, Y) and the electrolyte used was yttria-stabilized zirconia. The specimens were characterized by X-ray diffraction, Rutherford back-scattering spectrometry, scanning electron microscopy with energy-dispersive analysis by X-rays, and optical microscopy. For M = Sc, an intermediate phase is formed at the interface and is responsible for the strong bonding of the electrode layer to the electrolyte. The thickness of the intermediate layer was about 2 to 3Μm. Considerable loss of uranium, which in some cases led to destabilization of the fluorite phase, was observed from the surface of the uraniascandia electrode layers. The intermediate phase is thought to be formed as a result of reaction between the electrolyte and volatile uranium-containing species produced by decomposition of the urania-scandia electrode material. For M = Y, no evidence for the formation of such a phase was found and the adhesion of the electrode to electrolyte was poor.     

Relative thermodynamic stabilities of constituent oxides of highT c superconductors

A thermodynamic scale of the relative stabilities of constituent oxides can help the choice of stoichiometric multi-component compositions and can provide guidance for judicious selection of heat-treatment conditions for high-T _c superconductors. A thermodynamic analysis was undertaken to study Y₂O₃-BaO-CuO, BaO-K₂O-Bi₂O₃ and La₂O₃-SrO-NiO systems. The relative stability of the oxides was expressed in terms of two ratios,S _v andS _p computed using free-energy of formation and vapour pressure data. CuO, Bi₂O₃ and NiO were taken as reference oxides for YBa₂Cu₃O_7\t-\gd, Ba_0·6K_0·4BiO₃ and La_1·9Sr_0·1NiO₄ respectively. Thermodynamically, the reference oxide was found to be the least stable amongst the constituent oxides in each of these systems.