Application of Spectroscopic Probes in Study of Ferroelectrics

Abstract

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 (²E → ⁴A₂) 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 ²E → ⁴A₂ 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.

Keywords:

• Strontium titanate
• cadmium pyroniobate
• Cr³⁺
• Mn⁴⁺
• luminescence
• phase transition