Spectroscopic and structural properties of Cr in silicate glasses: Cr does not probe the average glass structure

Cr³⁺-containing alkali, alkaline earth and mixed alkali-alkaline earth silicate glasses have been investigated using Cr K-edge extended x-ray absorption fine structure (EXAFS) and optical absorption spectroscopy. The local environment of Cr³⁺ appears similar in all glasses regarding EXAFS analysis, in particular for Cr-O distance (1.99 Å). By contrast, optical absorption spectra show variations of crystal field values and disorder effects with the nature of the modifier cation, revealing that some differences exist in the local surrounding of Cr³⁺ in glasses. In addition, in mixed alkali-alkaline earth glasses optical absorption parameters remain close to the values found in binary silicate glasses with the same alkali, which reveals a preference for alkalis in the surrounding of Cr³⁺. As a whole, these data show that Cr³⁺ is not probing the average glass structure and demonstrate its heterogeneous distribution at a local scale.