Fluorite-structured Oxides Research Articles

The effect of oxygen vacancy concentration on the elastic modulus of fluorite-structured oxides

The elastic modulus of fluorite-structured pure ceria, gadolinium doped ceria (GDC) and yttria stabilized zirconia (YSZ) with different levels of oxygen vacancy concentration were measured by the nanoindentation technique. The samples were heat treated at 800 °C under an oxygen partial pressure in the range of 0.21 to 10 − 25 atm for 15 h until equilibrium was reached at a sufficient depth. The defect concentrations were conserved to room temperature by fast cooling. The results reveal that the elastic modulus of both pure ceria and GDC decreases drastically in samples heat treated at very low partial pressures of oxygen. However, the elastic modulus change for YSZ, within the studied range, is insignificant. An analysis of the experimental results suggests that an increase in the point defect content weakens the attractive forces between atoms in fluorite-structured oxides.

The Effect of Lattice Vacancy Concentration on Mechanical Properties of Fluorite-structured Oxides

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Effect of Synthesis Conditions and Actinide Contents on Phase Composition of Actinide Bearing Ceramics

ABSTRACTPu-bearing zirconolite and pyrochlore based ceramics were prepared by melting under oxidizing and reducing conditions at 1550 °C. 239Pu content in the samples ranged between ∼10 and ∼50 wt.%. Phase composition of the ceramics and Pu partitioning were studied using X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive system (SEM/EDS). Major phases in the samples were found to be the target zirconolite and pyrochlore as well as a cubic fluorite structure oxide. Normally the Pu content in the Pu host phases was 10–12 wt.%. This corresponds to the Pu content recommended for matrices for immobilization of excess weapons plutonium. At higher Pu content (up to 50 wt.%) additional phases, such as a PuO2-based cubic fluorite-structured solid solution, perovskite, and rutile were found.

Synthesis and Examination of New Actinide Pyrochlores

ABSTRACTPyrochlore is a complex oxide with general formula VIIIA2VIB2O6X (X – additional anion in specific position, usually O2-). It has a fluorite-derived structure (space group Fd3m) where one-eighth of oxygens are missing. Two different structural sites for cations exist, resulting in the cell parameter of pyrochlore is twice that of the fluorite lattice. Some pyrochlore-based actinide waste forms have been produced in the systems: CaO-CeO2-TiO2, CaO-UO2(ThO2)-ZrO2, CaO-UO2(ThO2)-Gd2O3-TiO2-ZrO2, and CaO-ThO2-SnO2. The oxide precursors were cold pressed at 200 – 400 MPa and sintered at 1500 – 1550 °C for 6-10 hours in air or at 1300 – 1350 °C for 5-50 hours in pure oxygen (Ce-doped batches only). The use of pure oxygen was used to try to put Ce in the tetravalent state required to form a pyrochlore.The products were examined with XRD, SEM/EDS, and TEM. In the ceramics with bulk compositions: CaCeTi2O7, CaThZr2O7, (Ca0.5GdTh0.5)(TiZr)O7, (Ca0.5GdU0.5)(TiZr)O7, and CaThSn2O7, pyrochlore was the major phase, but fluorite-structured oxide was also present. In the samples with target compositions CaUZr2O7 and (Ca0.5GdU0.5)Zr2O7 pyrochlore was not identified, rather a fluorite-structured oxide appeared. Different occurrences of pyrochlore and defect fluorite in the ceramics produced are due to crystal chemical constraints.

Oxidative coupling of methane on improved bismuth oxide membrane reactors

AbstractFluorite‐structured yuria‐bismuth oxide membranes with improved chemical stability were prepared by doping samarium in the lattice. The samarium‐doped yuria‐bismuth oxide (BYS) membranes are in fluorite structure, with high oxygen permeability and good catalytic properties for oxidative coupling of methane (OCM). The oxygen partial pressure dependence of oxygen permeation flax shows a shift of the rate‐limiting step from the suface reaction to the hulk electron‐hole conduction as temperature increases from 750 to 950°C. OCM reactions were conducted in a membrane reactor made of the disk‐shaped BYS membrane. Merhane‐ and oxygen‐containing streams are fed into the opposite sides of the BYS membrane to allow OCM reactions to occur on the membrane surface exposed to methane. The membrane reactor gives C2, yields of 4–11% with C2, selectivity up to 74%. The performance of this membrane reactor is closely related to oxygen permeation properties of the BYS membrane. Experimental results show the effectiveness in improving the C2 yield by increasing the membrane surface area to reactor volume ratio. In this disk‐shaped membrane reactor the highest C2, yield achieved is 17%, with a C2 selectivity of about 80%.

Analysis of the Atomic-Scale Defect Chemistry at Interfaces in Fluorite Structured Oxides by Electron Energy Loss Spectroscopy

ABSTRACTGd3+ doped Ce oxides are a major candidate for use as the electrolyte in solid oxide fuel cells operating at ∼500 °C. Here, the effect of the atomic structure on the local electronic properties, i.e. oxygen coordination and cation valence, at grain boundaries in the fluorite structured Gd0.2Ce0.8O2-x ceramic electrolyte is investigated by a combination of atomic resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM). In particular, EELS analyses from grain boundaries reveals a complex interaction between segregation of the dopant (Gd3+), oxygen vacancies and the valence state of Ce. These results are similar to observations from fluorite-structured Yttria-Stabilized Zirconium (YSZ) bicrystal grain boundaries.

Sol-gel processed fluorite-structured PrOy-ZrO2 mixed oxides deposited on alumina

C. K. Narula, L. F. Allard and G. W. Graham, J. Mater. Chem., 1999, 9, 1155 DOI: 10.1039/A809200A

Electronic structure and chemical bonding in δ-Bi2O3

Abstract The cubic fluorite structure oxide, δ-Bi 2 O 3 , possesses a very high oxygen ionic conductivity which can be very useful in applications. Since the structural arrangement of the oxygen vacancies in Bi 2 O 3 cannot be unambiguously established experimentally, we have used the LMTO method in both the full-potential and ASA versions to study the δ-Bi 2 O 3 system with various oxygen vacancy configurations. Our results include band structures, densities of states, cohesive energies and partial pressures. We find that there are nonlinear changes in the cohesive energy with variations in the oxygen vacancy concentration, with a structure with two vacancies per unit cell being the most stable. Our total energy calculations lead to the conclusion that the most probable stable configuration is the 〈111〉 type of vacancy ordering for Bi 2 O 4 , corresponding to equal numbers of vacancies in all of the BiO planes, and formation of channels for oxygen diffusion in the crystals. The role of BiO and BiBi bonding in the stabilization of such structures is also discussed.

High-Energy Mechanical Synthesis of Nanophase Fluorite-Structured Mixed Oxide Catalysts with a High Redox Activity

ABSTRACTA series of nanostructured, highly defective, ternary solid solutions containing CeO2, ZrO2, and MnO2 or CuO were prepared by high-energy mechanical milling of individual components. Morphological and redox properties were studied by XRD, HRTEM and temperature-programmed reduction techniques. It was shown that the introduction of small amounts of copper and manganese strongly promotes the redox behavior of cerium at lower temperatures in comparison with CeO2 and CeO2-ZrO2. High temperature treatment of up to 1400K was also shown to further promote overall redox capacity without affecting low-temperature redox behavior. Moreover, evidence is provided to show that Cu and Mn are dissolved within the CeO2 lattice structure. Addition of dopants enhances catalytic redox properties in the oxidation of CO at low temperatures, which is associated with the high concentration of oxygen vacancies that form on the introduction of aliovalent elements into the ceria-zirconia lattice.

CeO2-based solid solutions with the fluorite structure as novel and effective catalysts for methane combustion

The partial substitution of Ce in CeO2 with isovalent elements like Hf and Zr greatly increases the overall activity of methane combustion measured as light-off and ignition temperature; the presence of a defective fluorite-structured oxide is recognized as a key factor in the activity enhancement.

X-ray-absorption studies of zirconia polymorphs. III. Static distortion and thermal distortion.

Static and dynamic Debye-Waller factors for the Zr(Y)-O shell and the Zr(Y)-Zr(Y) shell have been determined for four zirconia polymorphs from 10 K to room temperature. The apparent Einstein frequencies for cation-O vibrations and the Debye frequencies for cation-cation vibrations have also been obtained from the analysis. The higher phase stability and the stabilizing effect of Y dopants are manifest in the higher apparent vibrational frequencies. For tetragonal zirconia these frequencies correlate well with Raman data, and for cubic zirconia the Y-O frequency coincides with the unique Raman frequency for fluorite-structure oxides. Similar bonding in monoclinic and cubic zirconia yields similar vibrational frequencies, while a larger bond distance generally leads to a lower frequency. Comparison of the Debye-Waller factors obtained from extended x-ray-absorption fine structure and from neutron diffraction has also revealed the different extent of atom-atom correlation in atomic thermal motion for different bonding.

A computer simulation study of the defect structure of calcia-stabilized zirconia

Abstract The defect structure of calcia-stabilized zirconia, a topic of long-standing controversy, has been examined by atomistic computer simulation techniques. Defect association energies for an anion vacancy and a dopant calcium ion at both nearest-neighbour and next-nearest-neighbour (NNN) positions have been determined and suggest the preferential siting of oxygen vacancies in the NNN position with respect to the dopant cation, at low dopant concentrations. A structural explanation for this is proposed. Our calculations also suggest Schottky intrinsic disorder in cubic zirconia which may be atypical compared with other fluorite-structured oxides; we advance an explanation for this in terms of lattice relaxation. Excellent agreement between experimental and calculated values shows the reliability and potential of computer simulation techniques. Binding energies of various types of defect cluster were calculated to check their relative stabilities. Also the migration energy of the oxygen vacancy was ca...

Lattice Parameters, Ionic Conductivities, and Solubility Limits in Fluorite‐Structure MO2 Oxide [M = Hf4+, Zr4+, Ce4+, Th4+, U4+] Solid Solutions

Changes in the lattice parameters of fluorite type MO2 oxides (M = Hf4+, Zr4+, Ce4+, Th4+, U4+) due to the formation of solid solutions can be predicted by proposed empirical equations. The equations show the generalized relationship between dopant size and ionic conductivity in the binary systems of these oxides, illustrating that the smaller the difference between the dopant ionic radius and the critical dopant radius, the higher the conductivity. The solubility limit of the same periodic group elements in fluorite‐structure MO2 oxides decreaes linearly with the square of Vegard's slope for each solute as determined from the proposed equations.

Vibrational spectra of fluorite-structure oxides in systems based on CeO2 and ZrO2

Vibrational spectra of fluorite-structure oxides in systems based on CeO2 and ZrO2

Neutron diffuse scattering in Y2O3- and Sc2O3-doped CeO2

CeO2 is a fluorite structured oxide which exhibits fast oxygen-ion transport when doped with trivalent impurities. At low concentration the predominate defect formed is a single oxygen vacancy partially compensated by one M3+ ion. However, as concentration is increased, more complex clusters develop. To elucidate the differences in defect structure, we have measured the diffuse neutron scattering for CeO2: 6 m/o Y2O3, CeO2: 15 m/o Y2O3 and CeO2: 1 m/o Sc2O3 + 1 m/o Y2O3. It was found that the scattering in the Q range measured (0.5A−1 to 4.5A−1) is relatively insensitive to the number of associated point defects, but is dominated by the local lattice displacements. Best agreement is obtained with cluster models in which cations coordinating the vacancy move away from the vacancy along <111> by ∼0.2 Å and anions coordinating the vacancy move toward the vacancy along <100> by ∼0.3Å.

Fast anion transport in solids

The role of lattice simulation in the development and understanding of anionic conduction in the fluorite structure oxides is reviewed in this paper. The major influence of dopant-vacancy interactions in determining the activation energy for conduction is stressed, with reference to recent calculations on the effect of cation size on defect pair binding energies. Finally the relationship between the pair bonding energy and deviations from Vegard's law in these oxides is explored.

A new statistical thermodynamic theory for substoichiometric fluorite structure compounds and its application III. Application of the model theory based on “tetrahedral” defects to some substoichiometric fluorite-structure compounds: Phase diagrams and “residual structures” at high temperature

In previous papers, a theory was proposed to explain the thermodynamic functions of substoichiometric fluorite structure compounds, on the basis of a new defect, which was called the tetrahedral defect, constituted by an oxygen vacancy which is bound with two reduced cations in its coordination tetrahedron. In this paper, the relevant energy parameters are discussed, and their number reduced to a “bonding energy” δ of the tetrahedral defect, a “strain energy” Δ measuring in first approximation, an isotopic strain introduced by packing tetrahedral defects in the fluorite lattice, a long-range electrostatic interaction u between “free” charged point defects and a “dipolar interaction energy” ϵ between tetrahedral defects: the latter two parameters can be in principle evaluated. By varying suitably the energy parameters, and comparing the results with the phase diagrams of oxides of the lanthanide and actinide elements, the difference between PuO 2−x (and AmO 2−x) (for which no low-temperature subphase is known) and all other fluorite-structure oxide systems may be explained. Variations in the slope of experimental oxygen potential vs stoichiometry curves which are found at high temperature for all these system, are explained (and fairly reproduced) in terms of residual structures.

Configurational model for conductivity of stabilized fluorite structure oxides

The formalism developed here furnishes means by which ionic configurations, solid solution limits, and conductivity mechanisms in doped fluorite structures can be described. The present model differs markedly from previous models but reproduces qualitatively reality. The analysis reported is for the case of a MO2 oxide doped with a divalent oxide MeO.

Massively defective crystalline solutions in fluorite-structure oxides: the systems ThO2-Ln2O3 (Ln=La3+, Gd3+, Yb3+)

Subsolidus phase relations have been defined in three representative rare earth sesquioxide (La2O3, Gd2O3, Yb2O3)-thoria systems over a wide range of compositions by X-ray methods (from phase detection and precision lattice parameter measurements). Characterisation of the point defects in these fluorite structure materials shows good agreement between calculated anion vacancy models and density values measured. Currently accepted crystal chemical generalisations were found to be inadequate to rationalise phase equilibria and crystalline solubility limits in these systems.

Fluorescent Properties of Trivalent Rare Earths in Fluorite Structure Oxides*

The fluorescent properties of the trivalent rare earths were studied in the fluorite structure oxides, CeO2 and ThO2. In these compounds, the cations occupy sites of cubic symmetry, but the symmetry of the trivalent rare earth depends on the concentration and the manner of charge compensation. The emission spectra, fluorescent lifetimes, and excitation spectra were studied as functions of these variables. Multiple emission spectra of the trivalent ions were observed which varied with concentration and charge compensation. These changes were attributable to multiple sites for the rare earth under the various conditions. The multiple spectra could be well separated in the case of the 5D0–7F1 transition of Eu3+ in ThO2 which shows at least three distinct spectra. A twofold enhancement of fluorescent lifetime was found to occur when the rare earth was placed in a predominantly cubic environment. The excitation spectra did not change detectably from those expected from known energy levels, but energy transfer was found to occur from the CeO2 band edge (~38 000 cm−1) to Eu3+, Sm3+, and Er3+.