article posted 01 June 2016
Georges Calas FSGT, is Chair of Mineralogy at the University Institute of France and a professor at the Pierre & Marie Curie University in Paris.
He was the 2014–2015 Chair in “Sustainable Development – Environment, Energy, and Society” at the Collège de France in Paris. His research, based on solid-state spectroscopic
and diffraction methods and numerical modeling, centers on structural properties of materials linked to the presence of minor components and impurities or radiation-induced
defects. This helps rationalize how the molecular-scale organization of minerals, glasses, and melts controls their properties, encompassing a broad range of applications in
environmental mineralogy and materials sciences, including cultural heritage, functional glasses and glass nuclear waste forms. A member of Academia Europaea and a Foreign
Fellow of the Royal Society of Canada, he is a fellow of the Society of Glass Technology, the Mineralogical Society of America and the Geochemical Society.
Nanoscale structural hetererogeneity in glasses: the point of view from cations.
Georges Calas*, Laurent Cormier, Laurence Galoisy and Gérald Lelong
University Pierre & Marie Curie, Paris
A chemically-selective approach of glass structure allows to use cations to probe the molecular-scale structure of multi-component glasses. In addition to determining
the coordination sites of cations, the possibility to get information on the medium-range order in glasses, including the relationships between cation sites or the linkage
with the glass framework, sheds light on the actual structure of these glasses. Pauling rules, site distortion and geometric constraints or the competition for charge
compensation represent some driving forces that explain the non-homogeneous distribution of cations in glasses.
We present a comparative vision of the structural behavior of 3d and 4d transition elements in oxide glasses. The local environment of these transition elements is
investigated by comparing the data obtained from various structural methods, including UV-visible and X-ray absorption spectroscopy (XANES and EXAFS), EPR and scattering
methods, as well as numerical modeling. Various examples will be presented on glasses including binary silicate and borate glasses as well as more complex boro-silicate
glasses. In the former, a major progress has been accomplished by recognizing the importance of heterogeneous distribution of cations, including cation clustering. In the
latter, some cations indicate a clear preference for either a silicate or a borate surrounding. This may shed light on glass stability and helps understand optical and
mechanical properties of glasses.