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Photo-EPR studies of photorefractive BaTiO3 heavily doped with Cr3+: evidence of photo-induced dissociation of Cr3+ dimers

Barium titanate crystals were grown by top seeded solution growth technique, nominally pure and also 0.05% and 1% Cr3+ impurity. We have conducted electron paramagnetic resonance (EPR) and photo-EPR studies at room temperature to investigate the role of Cr3+ impurity in photoinduced electron transfer. Nominally pure crystals contained Fe3+ as impurity, and its EPR is consistent with work reported by previous investigators. The Cr3+ doped crystals also contained Fe3+ impurities. It was observed that the site symmetry and the strength of the axial field parameter for Fe3+ complex were significantly different in Cr3+ doped crystals compared to nominally pure BaTiO3. The EPR spectra of Cr3+ were distinguished using the hyperfine structure of odd isotope 53Cr (I=3/2). By Photo-EPR technique we observe that in the presence of Cr3+, Fe3+ is not significantly photosensitive. In contrast Cr3+ exhibited higher photosensitivity in the presence of Fe3+. This was monitored by locking the magnetic field to 1/2↔1/2 transition of Cr3+, and recording intensity as a function of time, under insitu laser illumination. In lightly doped crystals the intensity of Cr3+ signal is sharply reduced immediately after switching the laser OFF showing non-exponential decay. In heavily doped crystals photo-EPR signal clearly shows that the fast decay of Cr3+ was followed by slow and steady build up of Cr3+ signal. The growth of Cr3+ signal was attributed to photoinduced decoupling of Cr3+ dimers. Thus, by doping BaTiO3 with Cr3+ more efficient grating formation can be achieved and time dependent phenomena are observed.