Formation, Absorption and Emission Spectra of Cr4+ Ions in Li2O-SiO2 System Transparent Glass-Ceramics

Abstract

The formation mechanism, optical absorption and emission spectra of Cr4+ ions-containing Li2O-SiO2 system transparent glass-ceramics were investigated. In this material, the main crystalline phase was Li2O•2SiO2, and the percent crystallinity and crystal size were 67-72 and 20-33 nm, respectively. The remarkable change in color and absorption spectra was observed upon crystallization. The characteristic emission of tetrahedrally coordinated Cr4+ ions was identified in the near infrared region, 1000-1600 nm. It was found that tetrahedrally coordinated Cr4+ ions exist in this transparent glass-ceramics from absorption and emission measurement. The Cr4+ ions exist in residual high SiO2 glassy phase, and their ligand field parameters are estimated to be: 10Dq=10,610 cm-1, B=690 cm-1 and Dq/B=1.54. This Dq/B value is just below the crossing point of 3T2 and 1E levels (Dq/B=1.6). The Cr4+ ions in Li2O-SiO2 system transparent glass-ceramics occupy the little stronger ligand field sites than those in aluminate glass reported previously (Dq/B=1.2). It is considered that the Cr4+ ions can be formed by the reduction of Cr6+ ions (chromate ion [CrO4]2-) associated with decreasing the basicity of residual glassy phase during crystallization. In this process the behavior of Li+ ions plays a significant role. The [CrO4] formed is equivalent to [SiO4] and substitutes [SiO4] sites in the residual high SiO2 glassy phase.