MOLECULAR SCALE CONTROL OF THE STABILITY OF ZR-BEARING GLASSES: ENVIRONMENTAL AND MATERIALS SCIENCE IMPLICATIONS

More disordered than nanomaterials, amorphous solids such as glasses and gels have benefited from a better structural knowledge through molecular scale approaches. Glass stability is required for environmental applications, including the development of nuclear waste matrices. On the other hand, nanophase crystallization is favored in glasses to form glass ceramics. It is interesting that the same glass components, such as Zr, are used to develop such different properties.

Zr decreases the solubility of simulated nuclear glasses. In these glasses, Zr occurs in octahedral coordination, a reticulating role confirmed by combining molecular dynamics MD-simulations with Zr K-edge EXAFS. At the glass-water interface, Zr L-edge XANES shows that glass alteration under SiO2-saturated conditions does not correspond to a modification of Zr-coordination: Zr retains its reticulating role in the alteration gel with a small nanoporosity, providing a protective role. By contrast, during alteration under open conditions, Zr coordination changes from 6 to 7 at the glass-water interface, due to the lack of local charge compensation. This is a precursor of the formation of a mixed hydrous zirconia-silica gel at the glass surface, with a poor protective role.

In glass ceramics, such as Mg- or Li- aluminosilicate glasses in which high cation field strength increases the disorder of the glassy network, Zr is used for enhancing the nucleation of nanocrystals. Zr coordination is intermediate between 6 and 7, indicating the absence of an appropriate charge compensation for a reticulating position, and explaining the structural instability of Zr during thermal treatment. In situ high-T XANES measurements indicate that molecular scale transformations occur about 30°C below the onset of crystalline nucleation. Tetragonal ZrO2 nuclei are formed, consistent with thermodynamics predictions that system energy is minimized moving from amorphous to tetragonal with increasing particle size. Surface stress effects cause a core-shell structure of these nuclei, illustrating the peculiar character of the nanophases formed under these conditions.