Fluorite-structured Solid Solutions Research Articles

Atmosphere- and Voltage-Dependent Electronic Conductivity of Oxide-Ion-Conducting Zr1-xYxO2-x/2 Ceramics.

Cubic, fluorite-structured solid solutions Zr1-xYxO2-x/2 (YSZ; x = 0.4-0.7) were prepared by sol-gel synthesis. Impedance measurements on pellets of 85% approximate density sintered at 1300 °C for 24 h showed strong evidence of oxide ion conduction with an inclined Warburg spike at low frequencies and capacitance values of ∼10-6 F cm-1 at 40 Hz. Arrhenius plots of total pellet conductivities were linear with activation energies of 1.4-1.56 eV. The conductivity decreased with x and was 2-4 orders of magnitude lower than that with optimized YSZ, x = 0.08. When the atmosphere was changed from N2 to O2 during impedance measurements, two reversible effects were seen: the Warburg spike contracted greatly, and the sample resistance decreased. These effects were more noticeable at higher x and are attributed to the introduction of p-type electronic conduction, in parallel with the preexisting oxide ion conduction. A similar reversible result was observed upon application of a direct-current (dc) bias during impedance measurements. When either pO2 is increased or a dc bias is applied, hole creation is believed to arise by the ionization of underbonded oxide ions situated near the Y3+ dopant ions. The ionized electrons are trapped at surface oxygen species, and the holes that are left on oxygen are responsible for p-type conduction. The electrolytic domain of x = 0.4-0.7 extends up to approximately 10-2 atm of O2 before p-type conduction is observed. The upper pO2 limit of the electrolytic domain of x = 0.08 is not known but is likely to be close to or slightly above 1 atm of O2.

Theoretical study of mixing energetics in homovalent fluorite-structured oxide solid solutions

Mixing energies (ΔHmix) for fluorite-structured (Zr1−xCex)O2 and (Th1−xCex)O2 solid solutions are computed from density functional theory (DFT), employing cluster-expansion (CE), special-quasirandom-structure (SQS), and continuum-elasticity approaches. These systems are of interest as models for actinide-dioxide mixtures, due to the availability of calorimetric data which allows a direct assessment of the accuracy of the different computational methods for calculating ΔHmix in such fluorite-structured solid solutions. The DFT-based SQS and CE results for solid solutions with random configurational disorder are in very good agreement, and are used along with the calorimetry data to test the accuracy of a linear-elasticity model which allows predictions of the ΔHmix under the assumption that the dominant contribution in these homovalent solid solutions arises from elastic strain energy. The linear-elasticity models describe the mixing energies to within an accuracy of approximately 2 and 0.1kJ/mol for the Zr and Th based systems, respectively. The excellent accuracy for the ThO2-based system is interpreted to result from the smaller size mismatch, and corresponding high accuracy of the linear elasticity approximation. We thus apply elasticity theory to estimate the magnitudes of ΔHmix for (Th1−xMx)O2 and (U1−xMx)O2 actinide-dioxide solid solutions, with M=U, Th, Ce, Np, Pu and Am, for which the degree of size mismatch is comparable to that in (Th1−xCex)O2; the results yield elastic contributions to ΔHmix with a maximum magnitude of 3kJ/mol.

First-principles computational study of defect clustering in solid solutions ofThO2with trivalent oxides

The energetics of mixing and defect ordering in solid solutions of fluorite-structured ThO$_{2}$ with oxides of trivalent cations (Sc, In, Y, Nd, La) are investigated by electronic density-functional-theory (DFT). Through DFT calculations of structures enumerated by lattice-algebra techniques, we identify the lowest-energy patterns for defect clustering for four separate dopant concentrations. The most stable structures are characterized by a repulsive interaction between nearest-neighbor vacancies on the oxygen sublattice. The enthalpies of formation with respect to constituent oxides are positive for all dopants considered, and show a tendency to decrease in magnitude as the size and electronegativity of the trivalent dopant decrease. Due to the small positive formation enthalpies and low oxygen-vacancy binding energy with La dopants, La$_{2}$O$_{3}$-ThO$_{2}$ solid solutions are predicted to have relatively high ionic conductivities relative to those for the other aliovalent dopants considered. Our results are compared with those for the more widely studied ZrO$_{2}$ and CeO$_{2}$ fluorite-structured solid solutions with trivalent cations.

Phonons in mixed superionic fluorites (BaF2)1-x(LaF3)x

In recent years, the fluorite-structured solid solutions with the general formula, (MF2)1-x(RF3)x (M = Ca, Sr, Ba, Pb and R is a rare-earth element or Y), have been the subject of numerous experimental studies focussed on their superionic properties. The overall cubic crystal symmetry (space group Fm3m) is conserved up to x ≶ xmax, where xmax ⊁ 0.4-0.5 depending on M and R. The zone centre phonons and phonon dispersion along three symmetry directions of the mixed superionic compound (BaF2)1-x(LaF3)x have been investigated by applying de Launey angular force model for x ≶ xmax. The calculated results are compared and explained with available experimental results.

New ceria-based catalysts for pollution abatement

In the present work, we describe the use of fluorite-structured solid solutions, based on mixtures of CeO 2/La 2O 3/Pr 2O 3, and similar systems, i.e. CeO 2/Ln 2O 3/Pr 2O 3 where Ln=Tb, Sm, and Gd as effective catalysts for methane combustion. Mixed lanthanide oxide doped ceria solid solutions were prepared using an evaporation method from nitrate precursors and were subsequently calcined to 1073 K. Sample characterisation was achieved using powder X-ray diffraction (PXRD) and temperature programmed reduction (TPR). PXRDs obtained subsequent to a 12 h calcination at 1073 K for 12 h all show peaks corresponding to a fluorite-based structure. There are no additional features suggestive of large-scale segregation or ordered superstructure formation. This was confirmed by Rietveld analysis which showed that the lattice possesses cubic nature with a= b= c=5.50 Å. Methane combustion tests show that the most effective catalysts are those lanthanide oxide doped ceria catalysts prepared with 5% Pr molar metal ratios. TPR experiments suggest PrO 2 is an essential component and both surface and bulk chemistries have important roles in catalytic activity.

Electrical and EPR studies of heterovalent solid solutions based on superionic β-PbF 2

Ionic thermocurrent (ITC), ac-conductivity, and electron paramagnetic resonance (EPR) experiments have been performed on solid solutions Pb 1− x RE x F 2+ x . These hetrovalent solid solutions were prepared as crystals and powders by different te chniques. The differences in the dependence on concentration of the electrical properties are explained by the nature of defect structures. Using the ITC method we were able to distinguish different types of dipole defects created by the RE 3+ cation and the charge compensating interstitial fluoride. The activation energies for all these complexes were determined and correlated with structural aspects. EPR-spectra (0.1 mol% GdF 3 as a probe) for both polycrystalline and single crystals show three different RE-sites and are compared to spectra obtained from other Gd-doped fluorite-structured solid solutions.

Structural and dynamical studies of δ-Bi 2O 3 oxide ion conductors : IV. An EXAFS investigation of (Bi 2O 3) 1− x( M2O 3) x for M = Y, Er, and Yb

The local structural environments of Bi 3+ and dopant cations in the fluorite-structured solid solutions ( M 2O 3) x (Bi 2O 3) 1− x ( M = Y, Er, Yb) have been studied using extended X-ray absorption fine structure techniques. The results show that the BiO shell is heavily disordered with an asymmetric radial distribution function. The Bi 3+ ion tends to be displaced from its centrosymmetric, cube center site. The first coordination shell of the dopant is comparatively ordered. Varying the dopant cation has a small effect on the local structural environment and increasing the dopant concentration causes a small increase in the degree of local order. Data obtained over a range of temperatures show that the large anisotropy in the BiO shell is attributable to static displacements from the perfect lattice sites. The degree of correlation between the thermal vibrations of the anion sublattice and those of the Bi atoms differs from that observed between those of the anion sublattice and the dopant atoms; the significance of this for ionic conductivity is discussed.

Enhanced ionic motion in some fluorite-structured solid solutions

Enhanced ionic motion in some fluorite-structured solid solutions