Rare Earth and Actinide Oxides: Thermodynamic and Electron Miscroscopy Studies

Partial molar thermodynamic quantities for oxygen in sub-stoichiometric cerium oxides (CeO2-x), plutonium oxides (PuO2-x) and mixed uranium -plutonium oxides ((U, Pu)O2-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 MnO2n-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 ± 0,04o.

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.