System Cu-Rh-O: Phase diagram and thermodynamic properties of ternary oxides CuRhO2 and CuRh2O4

An isothermal section of the phase diagram for the system Cu-Rh-O at 1273 K has been established by equilibration of samples representing eighteen different compositions, and phase identification after quenching by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive analysis of X-rays (EDX). In addition to the binary oxides Cu₂O, CuO, and Rh₂O₃, two ternary oxides CuRhO₂ and CuRh₂O₄ were identified. Both the ternary oxides were in equilibrium with metallic Rh. There was no evidence of the oxide Cu₂Rh₂O₅ reported in the literature. Solid alloys were found to be in equilibrium with Cu₂O. Based on the phase relations, two solid-state cells were designed to measure the Gibbs energies of formation of the two ternary oxides. Yttria-stabilized zirconia was used as the solid electrolyte, and an equimolar mixture of Rh+Rh₂O₃ as the reference electrode. The reference electrode was selected to generate a small electromotive force (emf), and thus minimize polarization of the three-phase electrode. When the driving force for oxygen transport through the solid electrolyte is small, electrochemical flux of oxygen from the high oxygen potential electrode to the low potential electrode is negligible. The measurements were conducted in the temperature range from 900 to 1300 K. The thermodynamic data can be represented by the following equations: {fx741-1} where Δ_f(ox) G ^o is the standard Gibbs energy of formation of the interoxide compounds from their component binary oxides. Based on the thermodynamic information, chemical potential diagrams for the system Cu-Rh-O were developed.     

Effect of rare-earth oxides on fracture properties of ceria ceramics

The influences of the sintering additive content of rare-earth oxide (Y₂O₃, Gd₂O₃, Sm₂O₃) on microstructure and mechanical properties of ceria ceramics were investigated by scanning electron microscopy and small specimen technique. A small punch testing method was employed to determine the elastic modulus and biaxial fracture stress of the ceria-based ceramics, and the fracture toughness was estimated by Vickers indentation method. Grain growth in the rare-earth oxides doped ceria ceramics was significantly suppressed, compared to the pure ceria ceramics. However, the elastic modulus, fracture stress and fracture toughness were decreased significantly with increasing additive content of the rare-earth oxides, possibly due to the oxygen vacancies induced by the rare earth oxides doping. The experimental results suggest that the change in the mechanical properties should be taken into account in the use of ceria-based ceramics for solid oxide fuel cells, in addition to the improvement of oxygen ion conductivity.     

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.     

Engineering of Electronic States on Co3O4 Ultrathin Nanosheets by Cation Substitution and Anion Vacancies for Oxygen Evolution Reaction

Due to the earth abundance and tunable electronic properties, etc., transition metal oxides (TMOs) show attractive attention in oxygen evolution reaction. O‐vacancies (V_o) play important roles in tailoring the local surface and electronic environment to lower the activation barriers. Herein, an effective strategy is shown to enhance the oxygen evolution reduction (OER) performance on Co₃O₄ ultrathin nanosheets via combined cation substitution and anion vacancies. The oxygen‐deficient Fe‐Co‐O nanosheets (3–4 nm thickness) display an overpotential of 260 mV@10 mA cm^?2 and a Tafel slope of 53 mV dec^?1, outperforming those of the benchmark RuO₂ in 1.0 m KOH. Further calculations demonstrate that the combined introduction of Fe cation and V_o with appropriate location and content finely tune the intermediate absorption, consequently lowering the rate‐limiting activation energy from 0.82 to as low as 0.15 eV. The feasibility is also proved by oxygen‐deficient Ni‐Co‐O nanosheets. This work not only establishes a clear atomic‐level correlation between cation substitution, anion vacancies, and OER performance, but also provides valuable insights for the rational design of highly efficient catalysts for OER.     

The influence of foreign ions on the crystal lattice of barium titanate

From investigations of phase diagrams of ternary oxides the lattice sites of foreign ions and compensating vacancies are established. Large trivalent ions occupy barium sites and are completely compensated by titanium vacancies. Small pentavalent ions occupy titanium sites and are mainly compensated by titanium vacancies. During these investigations a new compound was found, Ba La₄ Ti₄ O₁₅ isomorphous with Ba₅ Nb₄ O₁₅.     

Electrical property and defect structure of lanthanum-doped polycrystalline barium titanate

Electrical conductivity of two types of lanthanum-doped barium titanote ceramics with different dopant levels was measured at temperatures between 900 and 1250° C andP _{O 2}from 10^–5 to 1 atm. The activation energies of the conduction for the two are interpreted in terms of the formation energy of ionized oxygen vacancies even in such a highP _{O 2}region. This fact is in contrast with a well -known controlled -valency model proposed for rare- earth -doped semi-conducting perovskites. In a lightly lanthanum-doped specimen, semiconduction achieved at elevated temperatures is retained on cooling the specimen to room temperature, whereas in a heavily doped specimen, the resultant high-temperature semiconduction changed to insulation on cooling. The former behaviour on cooling is successfully explained by a metastabilization of oxygen vacancies accompanied by electrons formed at elevated temperatures.     

New annealing method and CaGe garnet film coercivity origin

A new annealing method which drastically reduces the coercivity of GaGe-garnet films grown by LPE has been developed. The as-grown films of (YSmLuCa)₃(FeGe)₅O₁₂ and (YEuTmCa)₃(FeGe)₅ O₁₂ are sealed with Y₂O₃ and metal Ca in an evacuated tube having three chambers. When heated, the tube is filled with Ca and O₂ gases which are supplied from the metal Ca and Y₂O₃, respectively. Ca and oxygen are believed to be incorporated into the film occupying the dodecahedral cation vacancies and oxygen vacancies, respectively. By this annealing, coercivities higher than 5.0 Oe are drastically reduced to less than 0.5 Oe. At the same time, the 4πMs and lattice constant af are decreased, while the Curie temperature T_c is increased. A point defect structure model containing c-site vacancies, oxygen vacancies and a-site Ge²⁺ ions is proposed. The origin of the coercivity is believed to be the oxygen vacancies.     

Effects of doping with La and Mn on the crystallite growth and phase transition of BaTiO3 powders

The effects of La and Mn dopants on the crystallite growth and the phase transformation of BaTiO₃ powders were studied. The barium titanate powders were obtained by calcining barium titanyl oxalate tetrahydrate in the temperature range 800 to 1200 °C. Crystallite growth of BaTiO₃ powders was promoted by the use of Mn dopant due to the increase of oxygen vacancies. The dissolution of La dopant into BaTiO₃ structure may decrease the oxygen vacancies so that the growth of BaTiO₃ crystallites is inhibited at high temperature ( 900 °C). When the crystallite size is small, the barium titanate can exist as a cubic phase due to the manifestation of the surface energy. Undoped cubic BaTiO₃ powders can be stable at a size < 30 nm. Doping with La and Mn would bring the crystallite size for the cubic-to-tetragonal phase transformation to 100 nm, resulting from the presence of cation or oxygen vacancies.     

Relationships between the chemical composition and properties of the high-temperature superconductor Bi2+x Sr2-y Ca1+y Cu2O8+d

Samples of the high-temperature superconducting solid-solution Bi_2+x Sr_2-y Ca_1+y Cu₂O_8+d (2212 phase, T _c ≤ 95 K) were prepared with different cation concentrations in order to study the influence of the cation concentration on the crystal structure, oxygen content and on several other physical properties. The measurements show that the c-axis parameter, the critical temperature, T _c, the resistivity, R, and the oxygen ion conductivity, σ_O2^- of the phase decrease with increasing calcium content. With increasing bismuth content, T _c and R decrease, whereas the c-axis parameter increases. σ_O2^- is not strongly dependent on the bismuth content of the phase. The oxygen content of the phase increases with increasing bismuth and calcium content. This revised version was published online in November 2006 with corrections to the Cover Date.     

Size effect of trivalent oxides on low temperature phase stability of 2Y-TZP

2 mol% Y₂O₃stabilized-TZPs (2Y-TZPs) doped with oversized trivalent cations (Sc³⁺< Yb³⁺< Y³⁺< Sm³⁺< Nd³⁺< La³⁺) whose ionic radius is larger than Zr⁴⁺was sintered for 1 h at 1500°C over the range containing trivalent oxides from 0 to 2 mol% with 0.5 mol% interval to evaluate the effect of trivalent cation alloying on low temperature phase stability of 2Y-TZP by investigating the variation of Raman spectra and lattice parameters. For a given concentration of dopant, tetragonality (c/aaxial ratio) increases with raising the dopant size. However, monoclinic (m)-ZrO₂content for the specimens annealed for 500 h at 220°C in air firstly decreases with increasing dopant size and then increases as dopant size is greater than Y³⁺ion. Raman modes of Zr-O_II(260 cm^–1) and Zr-O_I(640 cm^–1) shift to higher wavenumbers only when Sm₂O₃, Nd₂O₃, and La₂O₃are added. Although full width at half maximum (FWHM) of 640 cm^–1is constant, FWHM of 260 cm^–1mode decreases with increasing dopant size, indicating that an ordered structure (pyrochlore phase) may be formed. Therefore, dopant size is dependent on phase stability of 2Y-TZP in this system.     

Photoluminescence properties of sesquioxide doped ceria synthesized by modified sol-gel route

Highly crystalline and porous sesquioxide (Sm₂O₃, La₂O₃) doped ceria with different molar ratio is successfully synthesized by a simple modified sol-gel route. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) are used to investigate their phase, microstructure and composition. XRD analysis confirmed the formation of highly crystalline cubic fluorite phase in all samples. The Raman spectroscopy revealed a single triple degenerated F_2g mode as the attestation of the oxygen vacancy in the doped and undoped samples. Strong photoluminescence lines due to interconfigurational transition and vacancy mediated transition were observed in doped CeO₂. The oxygen vacancy induced luminescence of CeO₂ was strongly enhanced due to La doping. The effect of rare earth dopant on the photoluminescence properties has been studied in details.     

Quantitative nanoscale tracking of oxygen vacancy diffusion inside single ceria grains by in situ transmission electron microscopy

Oxygen vacancy formation and migration in ceria is critical to its electrochemical and catalytic properties in systems for chemical and energy transformation, but its quantification is rather challenging especially at atomic-scale because of disordered distribution. Here we report a rational approach to track oxygen vacancy diffusion in single grains of pure and Sm-doped ceria at −20 °C to 160 °C using in situ (scanning) transmission electron microscopy ((S)TEM). To create a gradient in oxygen vacancy concentration, a small region (∼30 nm in diameter) inside a ceria grain is reduced to the C-type CeO_1.68 phase by the ionization or radiolysis effect of a high-energy electron beam. The evolution in oxygen vacancy concentration is then mapped through lattice expansion measurement using scanning nano-beam diffraction or 4D STEM at a spatial resolution better than 2 nm; this allows direct determination of local oxygen vacancy diffusion coefficients in a very small domain inside pure and Sm-doped ceria at different temperatures. Further, the activation energies for oxygen transport are determined to be 0.59, 0.66, 1.12, and 1.27 eV for pure CeO₂, Ce_0.94Sm_0.06O_1.97, Ce_0.89Sm_0.11O_1.945, and Ce_0.8Sm_0.2O_1.9, respectively, implying that activation energy increases due to impurity scattering. The results are qualitatively supported by density functional theory (DFT) calculations. In addition, our in situ TEM investigation reveals that dislocations impede oxygen vacancy diffusion by absorbing oxygen vacancies from the surrounding areas and pinning them locally. With more oxygen vacancies absorbed, dislocations show extended strain fields with local tensile zone sandwiched between the compressed ones. Therefore, dislocation density should be reduced in order to minimize the resistance to oxygen vacancy diffusion at low temperatures.     

Current progress in ternary LREBa2Cu3Oy materials and their application

The J_c and H_irr values at 77 K of ternary light-rare-earth compounds, LREBa₂Cu₃O_y “LRE-123”, are usually high enough to serve in various applications. Several sources of vortex pinning can be in these composites tailored to fit the needs of the particular application. The list comprises LRE/Ba solid solution, oxygen vacancies, large particles of secondary phases, twin planes, nanoscale lamellas, etc. By means of the latter defects one can achieve a very high irreversibility field. Refinement of secondary phase particles and the optimal choice of their amount enhance the electromagnetic performance in a broad temperature range, up vicinity of T_c, allowing levitation at liquid oxygen, 90.2 K. An optimum content of MoO₃ doubles the self-field super-current at 77 K, H||c-axis. Altogether, the pinning tailoring in ternary LRE-123 materials provides a flexible and reliable way to fit the electromagnetic performance with the needs of sophisticated high-temperature and high-magnetic-field applications.     

Oxygen ion conductivity in oxygen-deficient perovskite-related oxides

Oxygen-deficient oxides with perovskite and perovskite-related structures have been prepared and their electrical conduction properties studied. The following materials were prepared in the form of dense single phase sintered specimens: CaTi_0.7Al_0.3O_2.85□_0.15, Sr₃NbO_5.5 □_0.5, Sr₂CaTaO_5.5□_0.5, Sr₂CaNbO_5.5□_0.5, LaAl_0.85Mg_0.15O_2.925□_0.075, LaAl_0.8Mg_0.2O_2.9□_0.1, SrZr_0.8Yb_0.2O_2.9□_0.1, SrZr_0.8Mg_0.2O_2.8□_0.2, and LaNb_2.6Zr_0.4O_8.8□_0.2, where □ represents an oxygen vacancy. At high oxygen partial pressures, p-type electronic conductivity is predominant, but oxygen ion conductivity predominates at low oxygen pressures. Ionic conductivity effects are related to oxygen vacancy concentrations and the size of the dopant ions.     

In situ high-temperature X-ray and neutron diffraction of Cu–Mn oxide phases

Copper–manganese oxides were analyzed by in situ high-temperature powder neutron and X-ray diffraction to investigate their crystal structure. Cu–Mn spinel was found to form a continuous solid solution with cubic symmetry between Mn₃O₄ and Cu₂MnO₄. A high-temperature phase with approximate composition Cu₅Mn₄O₉ was shown to have hexagonal symmetry. The cation distribution and lattice parameters of Cu–Mn spinel were resolved through Rietveld refinement of in situ neutron diffraction data. The results demonstrated that the Cu ion has a lower octahedral site preference than manganese ions, and quenching is not a reliable method to determine the equilibrium structure in the system.     

Thermodynamic analysis and crystal-chemical modeling of the defect structure of calcium iron garnets

This paper presents detailed analysis of the phase equilibria and thermodynamic stability of CaO, Fe₂O₃, Gd₂O₃, Nd₂O₃, Pr₂O₃, ZrO₂, NiO, and ThO₂, and detect formation during high-temperature synthesis of gadolinium (yttrium) calcium and zirconium calcium iron garnets doped with praseodymium, neodymium, nickel, cerium, thorium, and other oxides. The origin of point defects in each garnet is discussed. Crystal-chemical modeling indicates that all calcium-containing garnets are nonstoichiometric, but differ in the origin of non-stoichiometry, δ. The values of δ are shown to correlate with the activation energy for oxygen-ion conduction determined by emf measurements on solid electrolytes. The mechanisms of formation of different garnets from binary oxides are compared.     

The study of optical properties of In2O3 and of mixed oxides In2O3−MoO3 system deposited by coevaporation

A discussion of the optical properties of two systems of dielectric films i.e. In₂O₃ and of mixed oxides In₂O₃−MoO₃ system is presented. Film thickness, substrate temperature, annealing and composition (in molar%) have a profound effect on the structure and optical properties of these films. The decrease in optical band gap with the increase in film thickness of In₂O₃ is interpreted in terms of incorporation of oxygen vacancies in the In₂O₃ lattice. The decrease in optical band gap with the increase in substrate temperature and annealing of In₂O₃ thin films is ascribed to the release of trapped electrons by thermal energy or by the outward diffusion of the oxygen-ion vacancies, which are quite mobile even at low temperature. For the mixed oxides In₂O₃−MoO₃ system the results are found to be compatible with the reduction in the value of optical band gap of these materials as the molar fraction of MoO₃ increases in the In₂O₃ thin films and is attributed to the incorporation of Mo(VI) ions in an In₂O₃lattice that causes the indium orbital to become a little less tightly bound. The decrease in optical band gap of mixed oxides In₂O₃−MoO₃ system, with increasing film thickness is interpreted in terms of incorporation of oxygen vacancies in both In₂O₃ and MoO₃ lattice which are also believed to be the source of conduction electrons in In₂O₃–MoO₃ complex. The decrease in optical band gap with increasing substrate temperature and annealing of mixed oxides In₂O₃−MoO₃ system is due to the increasing concentration of oxygen vacancies, formation of indium and molybdenum species of lower oxidation state and indium interstitials. The blue colouration of mixed oxides In₂O₃–MoO₃ samples is due to the inter-electron transfer from oxygen 2p to molybdenum 4d level due to which Mo species of lower oxidation states are formed.     

Solid-state reaction pathways of Sillenite-phase formation studied by high-temperature X-ray diffractometry and differential thermal analysis

This paper reports on the use of high-temperature X-ray diffractometry, in conjunction with differential thermal analysis, to study the solid-state reactions and phase transformations involved in and leading to the synthesis of various (bismuth oxide-based) Sillenite-type compounds. These, which have γ-body centered cubic (bcc) structures related to γ-Bi₂O₃, are generally represented by the formula Bi₁₂MO₂₀; here, the second cation M=Ge, Si, Ti, Zn, Al, as well as the two-cation combination {Si_0.5Ge_0.5}. In each case, using the two techniques, measurements were performed on starting powder-mixtures of bismuth and cation M oxides, formulated with desired proportions to favor the Sillenite (γ-)phase formation. Henceforth, the proceeding of solid-phase reactions and occurring of phase transitions, with rising temperature, were monitored and elucidated by the evolution of the X-ray diffractometry patterns and the appearance of thermal events in the differential thermal analysis curve, as both sets of results were also correlated.     

Mechanochemical Synthesis and Thermal Behavior of Metastable Mixed Oxides in the CaO–Sb2O3–Bi2O3 System

The phase composition of crystalline mechanochemical synthesis products in the CaO–Sb₂O₃–Bi₂O₃ system was determined. Of the known phases in this system, only three could be prepared mechanochemically: Ca₂Sb₂O₅, CaSb₂O₄, and CaBiO_2.5 (fcc). A new metastable phase, "-Bi₂O₃, with an orthorhombic structure close to that of the high-temperature, fluorite phase -Bi₂O₃, was obtained by mechanical processing at 30°C. A number of new metastable fluorite solid solutions of binary and ternary oxides were obtained as single-phase powders by mechanochemical synthesis. The mechanochemical yield of primary crystalline products was shown to be several times higher than that of secondary products. A broad composition range was revealed in which perovskite and fluorite phases are in mechanochemical equilibrium. The composition dependence of the lattice parameter of the metastable fluorite phase Bi_{2 – x} Sb _x O₃ was found to be the opposite of the one predicted by Vegard's law. Metastable mixed oxides undergo phase transformations during heating (starting at 280°C in the case of the ternary perovskite phase). Bi_{2 – x} Ca _x O_{3 – 0.5x} fluorite solid solutions experience a transformation at 400°C, accompanied by oxygen loss. During heating in air, Sb₂O₃-containing fluorite phases partially stabilize owing to oxidation but, nevertheless, undergo structural transformations above 480°C. The transformation of Sb_{2 – x} Ca _x O_{3 – 0.5x} metastable fluorite solid solutions near 500°C in air is accompanied by the formation of needle-like Sb₂O₃ crystals. A mechanism is proposed for the extremely rapid growth of such crystals: extrusion of the Sb₂O₃ resulting from fluorite decomposition in agglomerates through triple junctions of aggregates and through cracks in the surface layer of agglomerates.     

Mass spectrometric study of vapor composition over the Ce-O system

Vapor compositions over cerium oxides were determined by high-temperature mass spectrometry and x-ray diffraction. Two congruently subliming compositions were found in the Ce-O system, CeO_1.99 and Ce₂O_2.96, and their thermodynamic properties were studied. The results, combined with earlier data, were used to construct a partial phase diagram of the Ce-O system.