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Georges Calas FSGT is University Institute of France Chair of Mineralogy at Sorbonne Université in Paris. His research 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, with applications in environmental and materials sciences, including cultural heritage, functional glasses and nuclear waste materials.

Georges Calas

Georges Calas*1; Laurence Galoisy1; Myrtille Hunault2; Gérald Lelong1

1Sorbonne Université, Institut de Minéralogie de Physique des Matériaux et Cosmochimie, Paris France
2SOLEIL Synchrotron, l’Orme des merisiers Saint-Aubin, Gif-Sur-Yvette, France

The presence of transition elements in alkali borate glasses results in a unique and original chemical composition dependence of their color. We will discuss the case of alkali borate glasses containing Cr, Co, Ni, Cu in light of optical absorption spectrometric data. As a function of the chemical composition of the glass, the coordination numbers, crystal-field splitting, site distribution or site geometry vary, depending on the existence of either only one or several sites occupied by the transition element. These spectroscopic properties will be discussed at the light of recent findings on the medium-range organization in borate glasses, in particular on the role played by [3]B in the geometry of borate superunits. Alkali concentration in borate glasses influences drastically the speciation of transition elements, with major changes observed when [3]B is present, i.e. at alkali concentration lower than 20 mol%. The local rigidity induced by planar [3]B triangles also explains the formation of ordered transition element clusters, already demonstrated by EXAFS and XANES in low-alkali borate glasses containing Ni, Co or Zn. Finally, the significance of [5]Ni and [5]Co will be discussed at the light of similar local structures observed in crystalline borates. The similarities between the chemical dependence of the coordination state of the various transition elements, as well as the evidence for a similar structural variation in uranium-bearing borate glasses (cf. talk by Myrtille Hunault), shed light on the structural relations between the local environment of cations and the nature of borate superunits.