Photorefractive properties of tetragonal KTa_052Nb_048O_3:Fe crystals and explanation by the three-valence charge-transport model

Photoconductivities, dynamics of light-induced absorption changes, and dark relaxations of holographic gratings are measured in as-grown and oxidized tetragonal potassium tantalate niobate crystals (KTa0.52Nb0.48O3) doped with iron. A sublinear intensity dependence of photoconductivity, buildup, steady-state value, and relaxation of the induced absorption changes, nonexponential dark relaxations of holographic gratings, and further holographic measurements are well explained, qualitatively and quantitatively, in terms of the three-valence charge-transport model, i.e., considering one impurity center occurring in three different valence states. Most probably the responsible photorefractive center is Fe3+,4+,5+.