Crystal field spectroscopy of Cr 3+ in glasses: Compositional dependence and thermal site expansion

Abstract

Optical absorption (= crystal field) spectra of Cr 3+ in silicate glasses have been investigated as a function of chemical composition and temperature. In a large range of glass compositions, most spectra show a variation of Crystal Field Stabilization Energy (CFSE) as a function of glass chemistry. Cr 3+ CFSE ranges between 215 and 234 kJ/mol-Cr 3+ and it increases with the theoretical glass basicity. Alkali-bearing glasses make a noticeable exception, as CFSE only varies with the nature of the alkali and not with its concentration. In glasses representing simplified magmatic compositions, CFSE varies by 7 kJ/mol-Cr 3+ and the lowest CFSE values are observed in silica-rich compositions. This CFSE variation may contribute to the compositional dependence of mineral/liquid Cr-partition coefficients. Near UV–visible absorption spectra have been recorded up to 800 K, using a homemade diode array spectrometer fitted with a furnace. The modification of the optical spectra at high-temperature is characterized by a red shift of the Cr 3+ absorption bands. CFSE shows a nearly linear negative dependence on temperature, with an overall variation of about 7 kJ/mol-Cr 3+ over 500 K, a value similar to that characterizing the CFSE chemical dependence. This systematic red shift is related to the local thermal expansion of the Cr site and may be treated in glasses using a polyhedral approach. The Cr–O linear thermal expansion coefficient is 15–20 × 10 − 6 K − 1 in alkali-bearing silicate and borosilicate glasses and is similar to those determined in silicate minerals. The local thermal expansion coefficient exhibits a weak compositional dependence and is larger than the bulk thermal expansion coefficient of the glasses. The compositional and thermal dependence of the Cr 3+ crystal field spectra shows that this ion is a sensitive probe of the evolution of glass structure as a function of composition or temperature. These data are consistent with the location of Cr 3+ ions in cationic domains in glasses.