Microstructure and Optical Absorption Spectra of Transparent Glass-Ceramics Containing ZnAl₂O₄: Cr³⁺

Abstract

The microstructure and optical properties of transparent glass-ceramics containing ZnAl₂O₄: Cr³⁺ have been examined by using X-ray diffraction, transmission electron microscopy and optical absorption measurement. The glass-ceramics were prepared by the heat treatment of glass with 20ZnO·20Al₂O₃·50SiO₂·6TiO₂·4ZrO₂·0.1Cr₂O₃ composition (mol%). ZnAl₂O₄ and ZrTiO₄ precipitated from the glass when the heat treatment temperature was above 850°C. The heat treatment below 1100°C successfully yielded the transparent glass-ceramics. The crystallite size of ZnAl₂O₄ increased from 5nm to 20nm as the heat treatment temperature was increased from 850°C to 1100°C. At 1100°C, the ZnAl₂O₄ crystallites precipitated as a cubic form. The optical absorption spectra at room temperature indicate that the absorption due to the ⁴A₂→⁴T₂ transition, which corresponds to the 10Dq, takes place at the wavelength of 650nm for the glass and at 557 to 530nm for the glass-ceramics. This fact suggests that the ligand field strength for the Cr³⁺ ion sites increases as the Cr³⁺ ions are incorporated into ZnAl₂O₄. The wavelength of the absorption band due to the ⁴A₂→⁴T₂ transition increases monotonically with an increase in crystallite size. This phenomenon is explaned by taking into account the surface state of fine. ZnAl₂O₄: Cr³⁺ particles and the variation of the concentration of Cr³⁺ ions in ZnAl₂O₄: Cr³⁺ with heat treatment temperature.