Raul I. Palomares

Defect accumulation in swift heavy ion-irradiated CeO2 and ThO2

Neutron total scattering was used to investigate defect accumulation mechanisms in CeO2 and ThO2 irradiated with 2.2 GeV Au ions. Pair distribution function (PDF) analysis was applied to characterize the local structural evolution and irradiation-induced defects as a function of irradiation fluence. CeO2 exhibits a greater amount of disorder than ThO2 under the same irradiation conditions. The local structures of the two materials evolve differently as a function of ion fluence, even if similar defects are produced. The PDF analysis indicates that oxygen dimer and/or peroxide defects with 〈O–O〉 distances of ∼1.45 A are formed in CeO2, while irradiation-induced defects in ThO2 result in a change in the mean O–Th–O bond angle and a distortion of local ThO8 polyhedra. Understanding how bound oxygen defects, such as peroxide, affect bulk oxygen transport in CeO2 will aid in better predicting and improving properties of fluorite structure materials for fast ion conductor applications.

In situ defect annealing of swift heavy ion irradiated CeO2 and ThO2 using synchrotron X‐ray diffraction and a hydrothermal diamond anvil cell

Hydrothermal diamond anvil cells (HDACs) provide facile means for coupling synchrotron X-ray techniques with pressure up to 10 GPa and temperature up to 1300 K. This manuscript reports on an application of the HDAC as an ambient-pressure sample environment for performing in situ defect annealing and thermal expansion studies of swift heavy ion irradiated CeO2 and ThO2 using synchrotron X-ray diffraction. The advantages of the in situ HDAC technique over conventional annealing methods include rapid temperature ramping and quench times, high-resolution measurement capability, simultaneous annealing of multiple samples, and prolonged temperature and apparatus stability at high temperatures. Isochronal annealing between 300 and 1100 K revealed two-stage and one-stage defect recovery processes for irradiated CeO2 and ThO2, respectively, indicating that the morphology of the defects produced by swift heavy ion irradiation of these two materials differs significantly. These results suggest that electronic configuration plays a major role in both the radiation-induced defect production and high-temperature defect recovery mechanisms of CeO2 and ThO2.

Pressure-induced phase transitions of β-type pyrochlore CsTaWO6

The β-type pyrochlore CsTaWO6 was studied by synchrotron X-ray diffraction (XRD) and Raman scattering methods up to pressures of 43 GPa using a diamond anvil cell (DAC). With increasing pressure, the cubic pyrochlore in space group of Fd-m transforms to an orthorhombic structure (space group: Pnma) at 5.9 GPa and then to a monoclinic structure (space group: P21/c) at ∼18 GPa. The structural evolution in CsTaWO6 is a continuous process and experimental results suggest that the initial cubic phase has a tetragonal distortion at ambient conditions. Both XRD and Raman measurements indicate that the pressure-induced phase transitions in CsTaWO6 are reversible. These results may provide a structural explanation of previous experimental resistivity measurement results for the isostructural superconductor K(Cs)Os2O6 at high pressure conditions.