Flux growth of ruby crystals

The growth of ruby (Al2O3:Cr) crystals from PbF2-B2O3 flux is reported. The crystal growth was conducted by heating a mixture of solute and flux at 1100 degrees C for 10 h, followed by cooling to 500 degrees C at a rate of 5, 10, 20 or 40 degrees C h(-1). Red and transparent ruby crystals were obtained. A combination of slow cooling with evaporation of the flux produced crystals of ruby. The simple PbF2 flux gave ruby crystals of up to 12 mm x 9 mm x 1 mm in size. The crystal sizes decreased with increasing rate of cooling. The evaporation loss and crystal size decreased with increasing amount of B2O3 added to PbF2. The PbF2-B2O3 flux produced a lot of small ruby crystals. Inclusions were rarely found in the ruby crystals. The form of the ruby crystals was a hexagonal thin plate bounded by c{0001}, r{10 (1) over bar 1} and d{01 (1) over bar 2} faces. On the basis of microscopic observation, the central parts of c faces were slightly swollen. The growth hillocks were observed on the c faces of crystals. The step heights of the hillocks were in the region of 0.2-15 mu m. The chromium was incorporated preferentially in the center parts of crystals. The refractive index of transparent ruby crystals was 1.73+/-0.01.