Rare Earth and Actinide Oxides: Thermodynamic and Electron Miscroscopy Studies

Partial molar thermodynamic quantities for oxygen in sub-stoichiometric
cerium oxides (CeO_2-x), plutonium oxides (PuO_2-x) and mixed uranium -plutonium oxides ((U, Pu)O_2-x) were determined by thermogravimetric analysis in atmospheres of controlled oxygen pressures (CO₂/CO mixtures) in the temperature range 900-1450 °C. Detailed analysis of the data obtained showed that the non-stoichiometric phase ranges for the three oxide-systems, which were previously described as a single, grossly nonstoichiometric phase, can be divided into several subregions each consisting of an apparent non-stoichiometric single phase. The finer details of the thermodynamic data, however, suggest that some of these subregions can be further split into ordered intermediate phases with compositions following the series M_nO_2n-2. In order to verify some of the thermodynamic findings, supplementary high-temperature X-ray diffraction studies were made on CeO₂ at temperatures up to 855 °C. At the higher temperatures between 790 and 855 °C, a new phase of low symmetry was obtained. Indexing the powder pattern for this phase showed it to be isostructural with Pr₆O₁₁ and with a monoclinic unit cell with a = 6,781± 0,006Å, b = 11,893 ± 0,009 Å, c = 15,829 ± 0,015 Å and β = 125,04 ± 0,04^o. The Ce₆O₁₁ phase observed in the X-ray studies corresponds to one of the intermediate phases inferred from the thermodynamic data. Supplementary high resolution electron microscopy studies were also conducted on reduced single crystals of CeO₂. On some particles reduced by beam heating in the microscope, a lamellae structure was observed and a model involving crystallographic shearing is proposed to explain this observation. In other beam-heated particles diffraction patterns were observed corresponding to the monoclinic superstructure found in the highi temperature X-ray studies. Finally patterns on particles reduced by a heat treatment in vacuum under well defined conditions showed that twinning can also take place in this oxide system.