Form Design of Bulky Ruby Crystals with Well-Developed (112̅3) FacesEpitaxial Growth of Crystal Films on Sapphire Substrates via MoO3 Flux

Ruby (Al2O3:Cr) is an aluminum oxide (Al2O3) crystal doped with chromium. Herein, we report the growth of a large bulky ruby crystal with well-developed n­(1123) faces via molybdenum trioxide flux evaporation. It is difficult to investigate the growth and dissolution rate for each crystal face during growth because a face with a slow growth rate develops, resulting in difficulty of modulation of the crystal form. In recent years, we developed a technique to grow ruby crystal films epitaxially on sapphire (Al2O3) substrates. In this study, for the first time, we quantified the relative differences in the growth rate of a ruby crystal and the dissolution rate of a sapphirecrystal for each crystal face. As a result, it was found that (1) the dissolution and growth rates differed significantly for each crystal face, and (2) no obvious positive correlation between the two parameters was observed. Based on the observed growth and dissolution rates, the bulky ruby crystal could be grown on a cylindrical sapphire seed crystal cut into a specific shape. Our study provides new insights into effectively modulating the crystal form, which is one of the key points of our continued studies on the epitaxial growth of ruby crystals.