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.