The role of Ca2+ ions in the formation of high optical quality Cr4+,Ca:YAG ceramics

The distribution of Ca2+ ions in high optical quality Cr4+,Ca:YAG ceramics after vacuum sintering followed by air annealing was successfully investigated by HRTEM, STEM, EDX, XPS and optical absorption spectroscopy. The HRTEM microscopy reveals the formation of clear grain boundaries without any impurity phase. A highly-doped thin Ca-rich layer was detected at the grain boundary with Ca2+ concentration up to 4.9% RTM, while the concentration of Ca(2+ )ions in the grain volume is less than 0.25%. The layer suppressed grain growth allowing the production of high optical quality ceramics with the average grain size of 1.95 +/- 0.27 mu m, which is five times smaller than in calcium-free ceramics. The air annealing of Cr4+,Ca:YAG ceramics results in a 10-fold decrease in Ca2+ ion concentration at the grain boundaries, practically removing the Ca-rich layer, moreover, the procedure generates Cr4+ ions within the grains. Most of the calcium originated from the Ca-rich layer diffuses outside the ceramics or dissolves into Al2O3 impurities without interfering with the generation of Cr4+.