Spectroscopic properties of Cu2+ in alkaline earth metaphosphate, fluoride-phosphate and fluoride-phosphate-sulfate glasses

Divalent copper ions (Cu2+) are not only important colorants in consumer glasses. They can also be used as a probe for assessing the local structural arrangement within a solid host. Here, we report on the incorporation of Cu2+ into alkaline earth metaphosphate (MP), fluoride-phosphate (FP) and fluoride-phosphate-sulfate (FPS) glasses. As a d9 transition metal ion, Cu2+ exhibits a broad optical absorption band at wavelengths of 740–880 nm when incorporated into oxide glasses. The specific properties of this band are strongly affected by the local basicity of the host, which reflects in a straightforward dependence of the band position on the mean-field optical basicity in alkaline earth MP. For FP and FPS, however, distinct deviations from this simplistic correlation are found due to selective cation localization on the various oxoanions and/or halogen species. In addition, high basicity favours the formation of monovalent Cu+. Electron paramagnetic resonance indicates a transition from tetragonally distorted octahedral coordination of Cu2+ in MP to rhombohedral distortion in FP and FPS glasses.