Fluorite-related Oxides Research Articles

Dominant mechanisms of thermo-mechanical properties of weberite-type RE3TaO7 (RE=La, Pr, Nd, Eu, Gd, Dy) tantalates toward multifunctional thermal/environmental barrier coating applications

Weberite-type RE3TaO7 (RE=La, Pr, Nd, Eu, Gd, Dy) tantalates have been studied as multifunctional thermal/environmental barrier coating materials with working temperatures above 1500 K. This study proposes the dominant mechanisms of their thermo-mechanical properties. The relative ionicity of the RE-O bonds increased with increasing RE+3 ionic radius, which weakened the bond strength and led to decreases in Young's modulus and hardness. A model was proposed to derive the high-temperature thermal conductivity based on the relationship between the phonon thermal conductivity and temperature, and the thermal radiative conductivity was estimated. RE3TaO7 exhibited a lower thermal conductivity than other fluorite-related oxides because its weak bond strength could slow down the phonon speed and enhance the an-harmonic vibrations of the lattices. At low temperatures, the RE atomic weight was a better descriptor of the thermal conductivity than the RE+3 ionic radius, and which could aid in further regulating the thermal conductivity. The thermal expansion coefficients of the different axes were affected by both RE-O and Ta-O bonds when weak RE-O bonds dominated the linear thermal expansion. The low oxygen ion conductivity derived from the strong covalent Ta-O bonds, which increased the activation energy for oxygen hopping in lattices of RE3TaO7. This study elucidates the dominant mechanisms of properties from the characteristics of crystals and chemical bonds, which are significant for high-temperature applications of tantalates.

Fundamental aspects about the first steps of irradiation-induced phase transformations in fluorite-related oxides

Fundamental aspects of ion-irradiation-induced phase transformations in fluorite-related oxides, from cubic to monoclinic structures, are investigated through xenon implantation at medium energy in yttrium sesquioxide (Y2O3). For that purpose, an original approach considering this material fabricated as a thin film is proposed to control the pristine structure before irradiation and to allow fine measurements of internal stresses. It is evidenced that this oxide can be strongly disordered up to an unexpected amorphization, depending on the initial damage state. We emphasize that point defects created during the irradiation process generate strong internal stresses which turn out to be a major parameter in the mechanism involved during such phase transition. When the oxide structure is initially weakly disordered, the evolutions of the biaxial and triaxial residual stresses, together with the lattice parameter, presents two different steps. Those steps are interpreted at the atomic scale by the nucleation of oxygen vacancy aggregates which collapse in extended dislocation loops that are considered as monoclinic nuclei. The geometry of these extended defects has been considered to explain why the monoclinic phase appears with clearly defined crystallographic orientation relationship (4 0 2¯)B//(2 2 2)C relative to the initial cubic phase. The link between the energy deposited by irradiation and the critical size of these defects, analyzed from a thermodynamical approach by the evaluation of the Gibbs free energy, also elucidates the presence of sharp thresholds observed between the different phases (amorphous/cubic and monoclinic/amorphous) along the depth of the film.

Disorder–order phase transformation in a fluorite-related oxide thin film: In-situ X-ray diffraction and modelling of the residual stress effects

This work is focused on the transformation of the disordered fluorite cubic-F phase to the ordered cubic-C bixbyite phase, induced by isothermal annealing as a function of the residual stresses resulting from different concentrations of microstructural defects in the yttrium oxide, Y2O3.This transformation was studied using in-situ X-ray diffraction and was modelled using Kolmogorov–Johnson–Mehl–Avrami (KJMA) analysis. The degree of the disorder of the oxygen network was associated with the residual stress, which was a key parameter for the stability and the kinetics of the transition of the different phases that were present in the thin oxide film. When the degree of disorder/residual stress level is high, this transition, which occurs at a rather low temperature (300°C), is interpreted as a transformation of phases that occurs by a complete recrystallization via the nucleation and growth of a new cubic-C structure. Using the KJMA model, we determined the activation energy of the transformation process, which indicates that this transition occurs via a one-dimensional diffusion process. Thus, we present the analysis and modelling of the stress state. When the disorder/residual stress level was low, a transition to the quasi-perfect ordered cubic-C structure of the yttrium oxide appeared at a rather high temperature (800°C), which is interpreted as a classic recovery mechanism of the cubic-C structure.

δ-Phase to defect fluorite (order–disorder) transition in the R2O3–MO2 (R=Sc, Tm, Lu; M=Zr, Hf) systems

Abstract We have studied the δ-phase to defect fluorite F * (order–disorder) transition in the R 4 M 3 O 12 (R = Sc, Tm, Lu; M = Zr, Hf) compounds. The temperature of the δ– F * phase transition in Tm 4 Zr 3 O 12 is ∼1600 °C. The rate of this transition in R 4 Zr 3 O 12 (R = Sc, Tm, Lu) decreases markedly with decreasing difference in ionic radius between the R 3+ and Zr 4+ , leading to stabilization of the δ-phases R 4 Zr 3 O 12 with R = Sc and Lu at high temperatures (∼1600 °C). During slow cooling (5 °C/h), the high-temperature defect fluorites F *-R 2 Hf 2 O 7 (R = Tm, Lu) decompose reversibly to form the δ-phases R 4 Hf 3 O 12 . Some of the materials studied exhibit microdomains formation effects, typical of the fluorite-related oxide compounds in the R 2 O 3 –MO 2 (M = Ti, Zr, Hf) systems of the heavy rare earths. The high-temperature defect fluorites F *-R 4 M 3 O 12 (R = Tm, Lu; M = Zr, Hf) as a rule contain antiphase microdomains of δ-R 4 Zr 3 O 12 . After slow cooling (5 °C/h), such microdomains are large enough for the δ-phase to be detected by X-ray diffraction. The conductivity data for R 4 M 3 O 12 (R = Sc, Tm, Lu; M = Zr, Hf) and Ln 2 Hf 2 O 7 (Ln = Dy, Lu) prepared by different procedures show that the rhombohedral phases δ-R 4 M 3 O 12 (R = Sc, Tm, Lu; M = Zr, Hf) are poorer conductors than the defect fluorites, with 740 °C conductivity from 10 −6 to 10 −5 S/cm. The conductivity drops with decreasing rare-earth ionic radius and, judging from the E a values obtained (1.04–1.37 eV), is dominated by oxygen ion transport. The highest conductivity, ∼6 × 10 −4 S/cm at 740 °C, is offered by the rapidly cooled F *-Dy 2 Hf 2 O 7 . In the fluorite homologous series, oxygen ion conductivity decreases in the order defect pyrochlore > defect fluorite > δ-phase.

Mechanical properties of fluorite-related oxides subjected to swift ion irradiation: Pyrochlore and zirconia

The modifications of the mechanical properties of related-fluorite oxides (cubic zirconia [c-ZrO 2] and pyrochlores [Gd 2(Ti 1− x Zr x ) 2O 7 with x = 0.5 and x = 1]) induced by swift heavy ion irradiation are investigated. Polycrystalline pellets of both materials were irradiated at room temperature with 940 MeV Pb or 870 MeV Xe ions at the GANIL accelerator in Caen at fluences ranging from 2 × 10 11 to 10 13 cm −2. Residual macroscopic stresses induced by irradiation were determined using X-ray diffraction and the sin 2ψ method. The microhardness and the fracture toughness of irradiated samples were studied by Vickers micro-indentation. Amorphization occurs in Gd 2TiZrO 7 and not in Gd 2Zr 2O 7 and c-ZrO 2. The mechanical behavior of materials is found to be closely related to the residual stresses induced in the surface layer by irradiation. Compressive stresses are generated in c-ZrO 2 and Gd 2TiZrO 7 (leading to an increase of fracture toughness), whereas tensile stresses (inducing a large decrease of fracture toughness) are observed in Gd 2Zr 2O 7 due to the lattice contraction related to a pyrochlore fluorite→transition.

Phase formation and dielectric properties of Ln 2(Ln′ 0.5Nb 0.5) 2O 7 (Ln = rare earth element)

Weberites and pyrochlores ( A 2 B 2O 7), both fluorite-related superstructures, are attractive dielectric ceramics due to their ability to accommodate diverse cations, thus allowing their properties to be tailored. This study focuses on the fundamental understanding of the structure–dielectric property relationships in fluorite-related oxides. Specifically, Ln 3NbO 7 and Ln 2(Ln′ 0.5Nb 0.5) 2O 7 (where the ionic radius of Ln′ is smaller than that of Ln) compounds are investigated. It has been previously shown that weberite-type Ln 3NbO 7 exhibits a composition dependent dielectric relaxation above room temperature. It is here shown that a dielectric relaxation also occurs in La 2(Ln′ 0.5Nb 0.5) 2O 7 (Ln′ = Yb 3+, Er 3+, and Dy 3+) compounds near or below −158 °C. The temperature, at which the maximum permittivity occurs, is different for different compositions (−132 °C for La 2(Yb 0.5Nb 0.5) 2O 7, −197 °C for La 2(Er 0.5Nb 0.5) 2O 7, and −187 °C for La 2(Dy 0.5Nb 0.5) 2O 7 at 1 MHz) and is correlated with the distortion of the NbO 6 octahedra. The room temperature dielectric permittivity of all three compounds was measured to be between 40 and 50 at 1 MHz.

Magnetic properties of orthorhombic fluorite-related oxides Ln3SbO 7 ( Ln=rare earths)

Ternary rare earth antimonates Ln 3SbO 7 ( Ln=rare earths) were prepared and their structures were determined by X-ray diffraction measurements. They crystallize in an orthorhombic superstructure of cubic fluorite (space group Cmcm for Ln=La, Pr, Nd; C222 1 for Ln=Nd–Lu), in which Ln 3+ ions occupy two different crystallographic sites (the 8-coordinated and 7-coordinated sites). Their magnetic properties were characterized by magnetic susceptibility and specific heat measurements from 1.8 to 400 K. The Ln 3SbO 7 ( Ln=Nd, Gd–Ho) compounds show an antiferromagnetic transition at 2.2–3.2 K. Sm 3SbO 7 and Eu 3SbO 7 show van Vleck paramagnetism. Measurements of the specific heat down to 0.4 K for Gd 3SbO 7 and the analysis of the magnetic specific heat indicate that the antiferromagnetic ordering of the 8-coordinated Gd ions occur at 2.6 K, and the 7-coordinated Gd ions order at a furthermore low temperature.

Crystal structures and magnetic properties of fluorite-related oxides Ln3NbO 7 ( Ln=lanthanides)

Crystal structures and magnetic properties of the ternary oxides Ln 3NbO 7 ( Ln=La, Pr, Nd, Sm–Lu) are reported. Their powder X-ray diffraction measurements and Rietveld analyzes show that they have the fluorite-related structures with space group Pnma ( Ln=La, Pr, Nd), C222 1 ( Ln=Sm–Tb), or Fm-3 m ( Ln=Dy–Lu). Magnetic susceptibility measurements were carried out from 1.8 to 400 K. The Ln 3NbO 7 compounds for Ln=Pr, Gd, Dy–Yb show Curie–Weiss paramagnetic behavior, and Sm 3NbO 7 and Eu 3NbO 7 show van Vleck paramagnetism. On the other hand, two magnetic anomalies were observed for both Nd 3NbO 7 (0.6 and 2.7 K) and Tb 3NbO 7 (2.0 and 3.2 K). From the results of specific heat measurements, it was found that these anomalies are due to the antiferromagnetic ordering of Ln ions in two different crystallographic sites (the 8-coordinated and 7-coordinated sites).

Design of Micro-Structure at Atom Level in Dy Doped CeO<sub>2</sub> Solid Electrolytes for Fuel Cell Applications

Rare earth doped ceria compounds are fluorite related oxides which show oxide ionic conductivity higher than yttria stabilized zirconia in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in application of these materials for ‘low (below 500°C)’ temperature operation of solid oxide fuel cells (SOFCs). In this study, the nano-sized powders of DyxCe1-xO2-x/2 (x=0.15 and 0.2) were prepared using ammonium carbonate co-precipitation method. To design the nano-structure in aforementioned materials, the round shape particles were prepared in nano-scale. The combined process of Spark Plasma Sintering (SPS) and Conventional Sintering (CS) was examined for fabrication of nano-structured doped CeO2 solid electrolytes. The nano-structural features in the (SPS+CS) specimen and CS specimen were observed using transmission electron microscopy (TEM). This micro-analysis suggested that the micro-domain with distorted pyrochlore structure exists in the grain of these materials. The conducting properties in the specimens were strongly influenced by the micro-domain size. It is found that the present combined process minimized the micro-domain size and maximized the conductivity in the specimens. Also nano-structured Dy doped CeO2 sintered bodies in the present study had wide ionic domain and high transport number of oxygen. This suggests that fabricated sintered bodies are suitable for the solid electrolyte in low temperature operated SOFCs. It is concluded that a control of micro-domain size is a key for development of high quality doped CeO2 electrolytes for fuel cell application. It is expected that advanced solid electrolytes for clean energy production will be produced by a design of nano-structure in rare earth doped CeO2 solid electrolyte.

Simulation of defects and defect processes in fluorite and fluorite related oxides: Implications for radiation tolerance

Atomic scale computer simulation is used to study four defect related processes across a broad range of A 2 3 + B 2 4 + O 7 composition that exhibit pyrochlore or the parent fluorite structures. The first set of results concern the energy for a local disorder process, through which the compositional ranges of stability of the structures are discussed. Second, the propensity of a given composition to exhibit either trivalent excess or tetravalent excess non-stoichiometry is considered. Third, the volume expansion upon transformation from pyrochlore to fluorite or pyrochlore to amorphous is presented. Lastly, the activation energy for oxygen ion migration is determined and the migration mechanisms considered. All these processes have a bearing on the propensity of a composition to resist radiation induced amorphisation. By considering the relative energies and volume changes as a function of composition these simulations identify regions of compositional significance.

Correlation effects for cation diffusion via vacancy-pairs in fluorite-related oxides

Recent research has suggested that diffusion via vacancy-pairs could be an important contribution to cation diffusion in fluorite-related oxides, such as yttria-stabilized zirconia. In this paper, a combination of analytical development and Monte Carlo computer simulation is used to analyze various diffusion correlation effects for cation and oxygen ion diffusion via tightly bound vacancy-pairs in the fluorite structure. It is shown that the application of sum-rule relations provides exact expressions for the collective correlation functions. It is also shown that a formalism inspired by Manning's diffusion kinetics formalism gives accurate expressions for tracer correlation factors when tested against Monte Carlo simulation results. It is also shown that the tracer correlation factors follow the impurity-form, thereby simplifying an interpretation of the diffusion isotope effect.

Crystal growth, observation, and characterization of the low-temperature structure of the fluorite-related ruthenates: Sm(3)RuO(7) and Eu(3)RuO(7).

The compounds Sm(3)RuO(7) and Eu(3)RuO(7) were grown as single crystals from molten hydroxide fluxes. They crystallize in the orthorhombic space group Cmcm and are part of a well-known family of fluorite-related oxides of stoichiometry Ln(3)MO(7). This structure contains rare earth cations in two different coordination environments, 8-fold pseudocubic and 7-fold pentagonal bipyramidal, and contains Ru(V) cations that are octahedrally coordinated. The RuO(6) octahedra are trans vertex-sharing to yield chains oriented along the c-axis. Upon cooling, single crystals of Sm(3)RuO(7) and Eu(3)RuO(7) undergo a structural transition at 190 and 280 K, respectively, from space group Cmcm to P2(1)nb. The structure transition results in a loss of lattice centering, a doubling of the b-axis, a distortion of the vertex-shared Ru-O chains, and a reduction in the coordination of one of the rare earth cations from 8-fold to 7-fold. Accompanying this structural transition are anomalies in the magnetic susceptibility at about 190 and 280 K for Sm(3)RuO(7) and Eu(3)RuO(7), respectively. The structures of these low-temperature phases of Ln(3)RuO(7) have been determined for the first time and are described.

Sol–gel deposition and characterization of In 6WO 12 thin films

Indium tungsten oxide (In 6WO 12) powders and thin films were prepared from a 2,4-pentanedione solution containing In(NO 3)· xH 2O and WCl 6. X-ray diffraction results indicated that phase-pure In 6WO 12 powder formed after drying and heating the solution at 900 °C. Glancing X-ray diffraction results indicated that thin-film samples prepared on fused quartz substrates at 600 and 700 °C contained a disordered fluorite-related phase whereas those prepared at 800 °C contained the ordered compound—In 6WO 12. Films prepared at 900 °C reacted with the substrate to form In 2Si 2O 7. Despite the chemical and structural similarity of In 6WO 12 to several indium-containing fluorite-related oxides, including In 2O 3, In 3Sn 4O 12, and Ga 3− x In 5+ x Sn 2O 16, the resistivity of the films prepared in this study ranged from 70 to 720 Ω cm, which is several orders of magnitude higher than reported for the related oxides (approx. 10 −3 Ω cm). The band gaps of the films prepared at 600 and 700 °C were estimated from optical transmittance measurements to be 4.1 and 3.8 eV, respectively. Films prepared at 700 °C exhibited substantial coloration when exposed to n-butyl lithium whereas those prepared at 800 °C did not.

Research on the electrochemistry of oxygen ion conductors in the former Soviet Union

This review is focused on the analysis of experimental results on oxygen ion-conducting ceramic materials based on HfO2, CeO2, and ThO2, published in the former Soviet Union. In particular, the physicochemical and transport properties of fluorite-related oxides and the characteristics of electronic conduction in these solid electrolytes are briefly reviewed. Emphasis is given to electrocatalytic and electrochemical properties of cerium-containing oxides, which are promising materials for electrodes of electrochemical cells operating in reducing atmospheres, and mixed-conducting membranes. A comparative analysis of specific features of the solid-electrolyte ceramics based on hafnia, zirconia, ceria, and thoria is performed in order to reveal basic tendencies of oxygen ionic transport in fluorite-type oxides, and to identify the potential applicability of these materials in various high-temperature electrochemical devices.

The Structural Principles and Their Consequences for the Anion-Deficient Fluorite-Related Oxides of the Higher Rare Earths

The Structural Principles and Their Consequences for the Anion-Deficient Fluorite-Related Oxides of the Higher Rare Earths

Percolation model for the anomalous conductivity of fluorite-related oxides

The Monte Carlo simulation technique has been used to study the anomalous conductivity of aliovalently doped fluorite-related oxides. In order to investigate the effect of the interaction between the dopants and the oxygen vacancies on the transport behavior, three simple model interactions have been tested: Nearest-neighbor attraction, nearest-neighbor repulsion, and a barrier model, which is based on the assumption of a reduced oxygen mobility in the neighborhood of the dopants. We find that percolation theory is essential to correlate the local interactions and the long-range mobility of the ions. Our numerical results show that satisfactory agreement with experiment can be obtained only for the barrier model.

The Structures of Higher Rare Earth Oxides: Role of the Coordination Defect

A milestone in our understanding of the chemistry of the defect solid state has been achieved through new insights provided by recently published neutron diffraction studies of five rare earth oxides with compositions in the range M2O3-MO2. For more than four decades the exact way by which gross concentrations (i.e., up to 25%) of vacant sites can be accommodated in the fluorite MO2 lattice has remained largely unresolved. There is now compelling evidence that the vacancies do not exist in isolation but as octahedral 'coordination defects' of composition {MIII/2MIV/1.5□O6}. It is now clear that the intricate planar patterns formed by anion vacancies in the fluorite lattice arise from the unique and structure-determining topology of these coordination defects. The decisive role played by the coordination defect in generating extended defects in these fluorite-related oxides is highlighted in this paper.

Structures and Stabilities of Trivalent and Tetravalent Rare Earth Ions in Sevenfold Andeightfold Coordination in Fluorite-Related Complex Oxides

ABSTRACTThis paper reports the preparation and characterization of a series of oxides containing3+ or 4+ lanthanide (M = Ce, Pr, or Tb) ions, with different ionic sizes and varying M4+/M3+ reduction potentials, in nearly cubic coordination. The objective of the study was to demonstrate how oxidation-reduction characteristics and ionic-size trends explain the properties of these oxides and to compare the oxidation-reduction stability of M3+ and M4+ lanthanide ions in high (CN 7 or 8) coordination in fluorite-related oxides versus low (CN 6) coordination in perovskite oxies. Efficient preparative methods are reported, as well as powder diffraction and thermogravimetric measurements for oxides CaMTi2O7-x and CaMZr2O7-x. These oxides were characterized by X-ray powder diffraction and by thermogravimetric analysis. CaCeTi2O7 is a pyrochlore, a = 10.142(4) Å with Ce4+ much more easily reducible than in the perovskite BaCeO3. By contrast, a preparation with the stoichiometry “CaPrTi2O7-x” is a two-phase mixture of perovskite CaTiO3 and a presumably Pr3+-rich pyrochlore Pr2Ti2O7(?). CaTbTi2O7-x appears to be a Tb3+ pyrochlore, a = 10.149(2) Å. CaCeZr2O7 is a pyrochlore, a = 10.524(1) sÅ. A preparation of “CaPrZr2O7-x” also appeared to yield a two-phase mixture, perovskite CaZrO3 and pyrochlore Pr2Zr2O7. In this paper, the structures, f-element ion sites, and M(IV)- MM1I stability trends in the CaMTi2O7-x and CaMZr2O7-x oxides are compared with the structural and stability trends in the perovskites BaMO3 which have M4+ ions in sixfold (tilted octahedra) coordination.

Oxide-ion conduction in Ba 2In 2O 5 and Ba 3In 2MO 8 (M=Ce, Hf, or Zr)

Oxide-ion conductivity in oxygen-deficient perovskites has been explored in two structures having ordered oxygen vacancies at room temperature. Ba 2In 2O 5 was shown to be isostructural with Brownmillerite, Ba 3In 2MO 8 (M=Ce, Hf, or Zr) with Ca 3Fe 2T iO 8. Ba 2In 2O 5 exhibits a low extrinsic O 2--ion conductivity below 650°C, an intrinsic conductivity across an ordering energy gap △ H g that decreases with increasing temperature in the interval 650°C< T< T t, and a first-order transition to a fast oxide-ion conductor above T t≈930°C. The compounds Ba 3In 2MO 8 exhibit an extrinsic conductivity below 450°C that is markedly superior to that of the fluorite-related oxides, e.g. Gd-CeO 2 and Y-ZrO 2; in particular, Ba 3In 2ZrO 8 has twice the dc conductivity at 400°C as Ca-ZrO 2 at 800°C. Disordering of the M 4+ cations over the cation array and their preference for octahedral versus tetrahedral coordination introduces a disordering of the oxygen vacancies at lower temperatures. An extrinsic activation energy E a≈0.6 eV was found for Ba 3In 2ZrO 8.

Ionic conductivity and tracer diffusion in a lattice containing random traps

A model where the lattice contained a number of randomly distributed traps equal to the number of diffusing particles was investigated rigorously. Monte Carlo methods were used to determine the relevant correlation factors in the tracer diffusion and ionic conductivity. It was found that the temperature dependence of the correlation factors contributed a relatively small component to the activation energy of the overall transport process. The relevance of the findings to atomic transport in fluorite-related oxides containing divalent dopant ions is commented on.