Growth and characteristics of bicolor Fe-doped MgAl 2 O 4 crystals

Bicolor Fe-doped MgAl2O4 crystals were successfully grown using the floating zone technique. These bicolor crystals were grown with composition ratios of MgO to Al2O3 (x) ranging from 0.3 to 1.0 and Fe2O3 concentrations of 0.1 mol%-2.0 mol%. Their growth process involved changing the O2 concentrations in the growth atmospheres: 0 vol% O2-100 vol% Ar, 50 vol% O2-50 vol% Ar, and 100 vol% O2-0 vol% Ar. The color of the resultant crystals changes with the growth conditions employed. Crystals manifested in six colors: pink, green, purple, violet, blue, yellow, and brown, based on the combination of Fe2O3 concentrations, O2 concentrations, and x. Additionally, bicolor crystals could be grown by suddenly changing the O2 concentration in the growth atmosphere, while tri-color crystals could be achieved through gradual alterations in the O2 concentration in the growth atmosphere. The color change of the bicolor crystals is considered to be caused by the change in the optical absorption ratio at lambda = 455 (blue light) and lambda = 550 nm (green light) due to the change in the concentration ratio of Fe2+ and Fe3+ in the crystal.