Role of Na2CO3 Addition in Epitaxial Growth of Ruby Crystal Films on Sapphire Crystal Substrates via MoO3 Flux Evaporation

Ruby (Al2O3:Cr) has a wide variety of industrial applications because of its excellent properties. Herein, we report the role of sodium carbonate (Na2CO3) addition in the epitaxial growth of ruby crystal films on sapphire crystal substrates via molybdenum trioxide (MoO3) flux evaporation. It is empirically known that Na2CO3 easily reacts with MoO3 to act as an evaporation inhibitor of MoO3, but its mechanism has not been elucidated. First, the compound produced by sodium oxide (Na2O) and MoO3 was identified as Na6Mo10O33. It was found that Na6Mo10O33 acted as a chemical lid, suppressing MoO3 flux evaporation. Next, the effects of the inhibitor on the flux evaporation rate and on the growth rate of ruby crystal films were investigated. The crystal growth rate in Na2O-MoO3 flux was approximately half of that in MoO3 flux because the inhibitor slowed the flux evaporation rate. Finally, the effect of inhibitor on deviation from equilibrium in ruby crystal growth was investigated. The ruby crystal film epitaxially grew on the substrate as a seed crystal under stable conditions because crystal growth was very close to equilibrium. Control of the flux evaporation by Na2CO3 addition enables the growth of high-quality ruby crystal films with few defects.