DISORDERING IN PYROCHLORE PHASES FOR RADIONUCLIDES DISPOSAL: A MULTISCALE STRUCTURE STUDY

Oxides with the general formula A2B2O7 and the pyrochlore structure have drawn much attention in the field of radionuclides immobilization. The Gd2Zr2O7 composition, in particular, has demonstrated high radiation resistance, making it an ideal host-matrix for encapsulating radioactive elements within the crystal structure. Despite the average structure, chemical composition can induce intrinsic structural distortions and control the degree of ordering of cations and anions in pyrochlore oxides, defining their response towards irradiation. To understand the origin of such distortions, an in-depth study of the structure at both the average and local scales is required. In the present thesis, a systematic investigation of three solid solutions related to the Gd2Zr2O7 composition is performed. Namely, the following solid solutions are examined: Gd2(Ti1-xZrx)2O7, (Nd1-xGdx)2Zr2O7 and Gd2(Zr1-xCex)2O7. All the samples are produced by solid-state synthesis and structural changes as a function of chemical composition are studied through High-resolution X-ray Diffraction (HR-XRD), laboratory Raman spectroscopy and the analysis of Pair Distribution Function (PDF). Cation antisite and anion Anti-Frenkel defects accumulate at the average scale as cations become similar in size. These defects are symptoms of structural distortions at the local scale and a weberite-type model is invoked for the description of the short-range order in the Gd2(Ti1-xZrx)2O7 system. Nevertheless, the deviation from the average structure at the local scale in disordered compositions appear to be more complex than a single structural model can describe.