Formation of Co2+-doped nanocrystals in La2O3–MgO–Al2O3–SiO2 glass-ceramics

Transparent glass-ceramics were synthesized by heat-treatment of glass with a composition of 5La2O3–13.2MgO–28.8Al2O3–46SiO2–4.5TiO2–2.5ZrO2–0.15CoO (LMAS) (wt.%). The activation energy of crystallization and the Avrami parameter for the LMAS glass were determined from the DTA curves at different heating rates. The most two intense bands of Raman spectrum of initial glass at ~ 810 cm−1 and ~ 900 cm−1 were connected with the presence of [SiO4] and [TiO4] tetrahedral, respectively. After heat-treated at 700 °C/10 h+820 °C/8 h, the intensity of the band for [TiO4] tetrahedral weakened, while an intensive band at ~ 800 cm−1 for the Ti–O bond appeared. Other bands were characteristics of high-silicate network and x(MgTi2O5)·y(Al2TiO5) polycrystals. The changes reflected phase separation after heat-treatment of the initial glass. The strong absorption band of glass-ceramics centered at 580 nm can be assigned to 4A2(4F)→4T1(4P) and the broad absorption band at 1100–1700 nm to 4A2(4F)→4T1(4F) transitions of tetrahedral coordinated Co2+ ion. Two broad emission bands, one was around 660 nm, the other was from 800 nm to 1050 nm, of glass-ceramics correspond to the 4T1(4P)→4A2(4F) and 4T1(4P)→4T2(4F) transitions of tetrahedral coordinated Co2+ ions. The absorption and emission features clearly demonstrated that Co2+ ions were incorporated into nanocrystals and located in tetrahedral sites.