NIR absorption and light-induced charge transfer in photorefractive Ba0.77Ca0.23TiO3 crystals doped with iron

Barium-calcium-titanate crystals of the congruently melting composition Ba077Ca0.23TiO3 doped with iron to enhance photorefractive properties, is investigated by Fourier IR spectroscopy under Ar+-Laser irradiation at low temperatures. The light-induced absorption in the NIR and visible range (VIS) is investigated as a function of light intensity, temperature and time. The dependence on temperature and on light intensity of the decay of the light-induced absorption after switching-off the laser light reveals a characteristic behaviour. The light induced absorption of Ti3+ polarons around 0.7 eV can be quantitatively fit in its temperature, intensity and time dependence by a simple charge transfer model Fe2++Ti4+ <-> Fe3++Ti3+. The time- and temperature-dependence reveal tunnelling and thermally activated hopping contributions to the polaron decay. At room temperature both, the (NIR) Ti(3+-)polarons and the VIS-Centers are observed to be thermally ionized and no NIR light-induced absorption can be observed any more under steady state conditions.