Characterizing the Structure and Radiation Resistance of Weberite-Type Complex Oxides

Weberite-type A3BO7 oxides, where A is a trivalent rare earth and B is a pentavalent element like Ta, have been a focus of research due to the discovery of the weberite-type local atomic arrangement in ceramics with a defect-fluorite structure. Earlier studies primarily examined their long-range structures, leaving gaps in understanding their short-range atomic behavior. This thesis investigates various weberite-type tantalates across all structural scales. There has been debate over the long-range structure of Y3TaO7, a medium-sized rare earth tantalate oxide, particularly regarding its spacegroup symmetry. This work identifies Y3TaO7 as an outlier, with neutron total scattering and DFT calculations indicating the C2221 spacegroup as the most fitting for its structure at both local and long-range levels. The thesis extends this structural analysis to a series of Ln3TaO7 weberite-type oxides. Neutron- and X-ray total scattering experiments reveal these materials\u27 complexity and establish a link between short- and long-range order. It is found that local distortions and the averaging of local atomic domains lead to distinct structural families in weberite-type oxides. A new experimental framework is developed to systematically explore local symmetries and derive accurate local atomic models, using a novel correlated differences approach. Additionally, the thesis explores the behavior of weberite-type oxides under swift heavy ion radiation, a previously unexamined area. It shows that while the long-range structures of weberite-type and pyrochlore oxides respond similarly to radiation, their short-range local atomic configurations differ post-irradiation, with weberite-type oxides maintaining configurations similar to their non-irradiated state. In summary, this comprehensive study of weberite-type oxides\u27 structures and radiation resistance emphasizes the importance of local atomic arrangements in understanding complex oxides. It demonstrates that these arrangements significantly affect amorphization resistance. The methodologies and insights gained are applicable to other materials, opening avenues for further research

Disorder in Ho2Ti2-x Zr x O7: pyrochlore to defect fluorite solid solution series.

Pyrochlore (A2B2O7) is an important, isometric structure-type because of its large variety of compositions and structural derivatives that are generally related to different disordering mechanisms at various spatial scales. The disordering is key to understanding variations in properties, such as magnetic behavior or ionic conduction. Neutron and X-ray total scattering methods were used to investigate the degree of structural disorder in the Ho2Ti2-x Zr x O7 (x = 0.0-2.0, Δx = 0.25) solid solution series as a function of the Zr-content, x. Ordered pyrochlores (Fd3̄m) disorder to defect fluorite (Fm3̄m) via cation and anion disordering. Total scattering experiments with sensitivity to the cation and anion sublattices provide unique insight into the underlying atomic processes. Using simultaneous Rietveld refinement (long-range structure) and small-box refinement PDF analysis (short-range structure), we show that the series undergoes a rapid transformation from pyrochlore to defect fluorite at x ≈ 1.2, while the short-range structure exhibits a linear increase in a local weberite-type phase, C2221, over the entire composition range. Enthalpies of formation from the oxides determined using high temperature oxide melt solution calorimetry support the structural data and provide insight into the effect of local ordering on the energetics of disorder. The measured enthalpies of mixing are negative and are fit by a regular solution parameter of W = -31.8 ± 3.7 kJ mol-1. However, the extensive short-range ordering determined from the structural analysis strongly suggests that the entropies of mixing must be far less positive than implied by the random mixing of a regular solution. We propose a local disordering scheme involving the pyrochlore 48f to 8a site oxygen Frenkel defect that creates 7-coordinated Zr sites contained within local weberite-type coherent nanodomains. Thus, the solid solution is best described as a mixture of two phases, with the weberite-type nanodomains triggering the long-range structural transformation to defect fluorite after accumulation above a critical concentration (50% Ti replaced by Zr)