Optical Transmission of Heat-Treated Strontium Titanate

The room-temperature optical transmission spectra of pure and doped SrTiO₃ crystals have been studied after heating them in oxygen and vacuum; spectral measurements were made in the quantum energy range from 0.1 to 3.4 ev.A blue color, with strong attendant absorption in the infrared, can be induced in the pure crystals by heating them in vacuum with absorption maxima occurring at 0.9 and 2.9 ev. This induced absorption can be removed by heating the crystals in oxygen. Prolonged heatings of the pure, stoichiometric crystals in oxygen have failed to induce any measurable change in the transmission spectra in the aforementioned energy range.Optical absorption can be induced in iron- and cobalt-doped SrTiO₃ crystal samples by heating them in oxygen; this absorption can be removed by heating the crystals in vacuum. The induced absorption occurs primarily at high quantum energies in the spectra of these doped crystals, with maxima occurring at 2.75 and 2.2 ev in the iron-doped samples, and at 3.0 and 2.6 ev in the cobalt-doped samples. It is suggested that the induced absorption observed in the doped crystals involves a controlled change in the valence states of these impurities.