Defect Fluorite Phase Research Articles

Tem Study of Radiation Damage and Annealing of Neutron Irradiated Zirconolite

AbstractNeutron irradiation was used to simulate alpha-decay damage in zirconolite, resulting in a transformation from the crystalline to the amorphous state at doses of 4–25 × 1019 n/cm2 (E ≥ 1 MeV). With increasing dose, the radiation damage microstructures resemble damage caused by: 1) alpha-decay of 232Th and 238U in natural zirconolites, 2) alpha-decay of 238Pu or 244Cm in synthetic samples, and 3) collision cascades in samples irradiated with heavy ions. Heavily damaged zirconolite recovers to a defect fluorite phase on annealing at temperatures up to 1000 °C. The main stage of structural recovery was found to occur at temperatures of 600–800 °C. The microstructures after heating depend on the initial level of damage: zirconolite grains with low to moderate levels of damage anneal to imperfect single crystals, whereas heavily damaged grains recrystallize to a polycrystalline microstructure. Complications encountered in this work include the production of fission tracks (due to trace amounts of U) and a non-uniform distribution of damage at higher dose levels (possibly due to electron beam heating).

Electrical properties of defect-fluorite phase in the system R 2O 3-Ta 2O 5 (R  Gd, Y, Er and Yb)

The electrical properties of the new fluorite-type phase R 0.8Ta 0.2O 1.7⦜ ⊥ 0.3 in the system R 2O 3-Ta 2O 5 (R  Gd, Y, Er and Yb) were studied. This fluorite phase had a p-type conductivity above 10 −3 atm of oxygen partial pressure in the temperature range 650–1250 °C. In the oxygen partial pressure range between 10 −4 and 10 −21 atm, it showed an ionic conductivity which could occur by the diffusion of oxygen ions through oxygen vacancies.

Oxygen ion conductivity of yttria-niobia mixed oxide with fluoride related structure

The oxygen ion conductivity of Y 2O 3Nb 2O 5 with a fluorite-like structure was studied. Substitutional solid solutions of Nb 2 O 5 in Y 2O 3 lattice formed the defect fluorite phase and remarkably enhanced the oxygen ion conductivity. Doping with tetravalent cations, especially Ti 4+ or Ce 4+, in yttria-niobia oxide is effective in enhancing the oxygen ion conductivity. Although the n-type semiconducting property appeared below P O2 = 10 −18 atm at 1243 K, the yttria-niobia mixed oxide doped with Ce 4+, Ti 4+, and Zr 4+ stably exhibited oxygen-ion conduction in the wide range of oxygen partial pressures studied.

The Defect Fluorite Phase in the ZrO2-PrO1.5 System and Its Relationship to the Structure of Pyrochlore

A detailed transmission electron microscope and X-ray powder diffraction study has been made of the so-called 'defect fluorite' phase field in the ZrO2-PrO1.5 system and of its close relationship to the pyrochlore solid solution field in the same system. Even for the lowest possible PrO1.5 content within the 'defect fluorite' phase field, it is clear that the sharp Bragg reflections characteristic of the underlying fluorite average structure are accompanied by some of the 'satellite reflections' characteristic of the pyrochlore solid solution field. As the PrO1.5 content increases, these satellite reflections increase systematically in intensity as well as sharpening very considerably. It is shown that this 'defect fluorite' phase field cannot be adequately described either in terms of random point defects within an average fluorite-type matrix or in terms of a diphasic texture of pyrochlore domains embedded coherently into a fluorite matrix, but must be regarded as enuinely intermediate between these two end-member structures and of commensurately modulated fluorite type. A group theoretical approach is used to propose a model for the structural deviation from the underlying fluorite average structure.

Neutron Powder Diffraction Studies of Fluorite and Pyrochlore NdxZr1−xO2−x/2 Solid Solutions with 0.25<x<0.55

AbstractNeutron powder diffraction studies on stoichiometric and non‐stoichiometric pyrochlore phases of the ZrO2‐ Nd2O3 system show remarkable differences in the occupation factor of the 8b site of space group Fd3m to which the pyrochlore phase belongs. Stoichiometric Nd2Zr2O7 can be described as a perfect pyrochlore phase in which the 8b site is either vacant or at least has a low occupation factor. The occupation factors of the 8b sites of non‐stoichiometric NdxZr1−xO2−x/2 pyrochlores reveal that these contain a large degree of disorder in the anion sublattice. Best refinement results on non‐stoichiometric materials are achieved when the overall structure of the non‐stoichiometric pyrochlore phase is described as a hybrid phase consisting of a major volume fraction of the stoichiometric pyrochlore phase and a minor fraction of the defect fluorite phase.

X-ray diffraction study of Zr(Ca,Y)O2−x. II. Local ionic arrangements

Above the 1300 K transition and in the defect fluorite phase, there is diffuse X-ray scattering which is similar for both Ca- and Y-stabilized zirconias. This is largely due to ionic displacements from the average structure, principally oxygen ions displaced locally in (100) directions. It has been proposed that some of this scattering is due to fine precipitates of ZrCa4O9 or ZrY4O9. The quantitative analysis of this diffuse scattering reported here does indeed indicate some similarities to this phase; in particular the stabilizing ions form rods in the ({\bar 3}{\bar 3}2) directions similar to those in the precipitate. However, in this compound the stabilizing ion is a second neighbor to an oxygen-ion vacancy, whereas the present results indicate that these tend to be first neighbors. This is the first direct evidence for this association in these materials. This probably occurs to reduce local distortions and to provide local charge balance. It is concluded that the diffuse intensity is from locally ordered regions, not precipitates.