Application of spectroscopic probes in study of ferroelectrics

Photoluminescence (PL), thermally stimulated luminescence (TSL), and photosensitivity of SrTiO3 and Cd2Nb2O7 crystals with Mn4+ and Cr3+ spectroscopic probes, respectively, were studied. Manganese doping of SrTiO3 results in structured luminescence of Mn4+ consisting of narrow zero-phonon (ZPL) R-line (E-2 --> (4)A(2)) and vibrational sidebands. It is suggested that unusually large temperature shift of ZPL is caused mainly by the contribution of the lowest TO soft optical branch into interaction of emitting center with a lattice. The PL spectrum of Cd2Nb2O7 :Cr3+ consists of four narrow emission lines accompanied by vibrational sidebands. Emission lines are attributed to the zero-phonon transitions E-2 --> (4)A(2) of two kinds of octahedral coordinated Cr3+ centers. The change in activation energies of PL thermal quenching at T-c = 196 K is tentatively explained by a change of the adiabatic potential of the Cr3+ centers at the ferroelectric phase transition of Cd2Nb2O7 at T-c . The low-temperature photosensitivity of both materials is attributed to the photoinduced change of charge state of manganese and chromium centers.