corundum, synthetic corundum.7, gems

Large crystals of synthetic ruby (not of gem quality) were grown by the temperature gradient technique from seed plates using Al₂O₃ powders mixed with 0.5–3% Cr₂O₃. Crystallization started at 2,050 °C; cooling rates were 0.5–2.5 °C/hour. The largest crystal measured 75 mm in diameter, 45 mm long, and weighed 1,076 g. This crystal was dark red in the center and even darker red (almost black) at its edges and bottom. Wave-like growth bands were observed in the top portion, while clouds of minute inclusions were concentrated near the seed plate. The concentration of the inclusions decreased from edge to center and from bottom to top, resulting in a “trumpet”-shaped distribution along the growth direction similar to that found in synthetic Ti-doped sapphire crystals grown by the same technique. Electron-microprobe analysis showed that the inclusions were mainly uncrystallized Al₂O₃ powder. Gas bubbles or voids also were observed. Microprobe and spectroscopic (visible range) data showed that the Cr³⁺ concentration increased in two directions (i.e., from center to edge and from bottom to top), corresponding to the growth progression. The authors propose that the quality of synthetic ruby crystals produced by this method could be improved by optimizing the cooling rate, purifying the raw materials, and controlling the temperature during growth.