Photo-EPR studies of photorefractive BaTiO_3 heavily doped with Cr~(3+): Evidence of photo-induced dissociation of Cr~(3+) dimers

摘要

Barium titanate crystals were grown by top seeded solution growth technique, nominally pure and also 0.05% and 1% Cr~(3+) impurity. We have conducted electron paramagnetic resonance (EPR) and photo-EPR studies at room temperature to investigate the role of Cr~(3+) impurity in photoinduced electron transfer. Nominally pure crystals contained Fe~(3+) asimpurity, and its EPR is consistent with work reported by previous investigators. The Cr~(3+) doped crystals also contained Fe~(3+) impurities. It was observed that the site symmetry and the strength of the axial field parameter for Fe~(3+) complex were significantly different in Cr~(3+) doped crystals compared to nominally pure BaTiO_3. The EPR spectra of Cr~(3+) were distinguished using the hyperfine structure of odd isotope ~(53)Cr (I=3/2). By Photo-EPR technique we observe that in the presence of Cr~(3+), Fe~(3+) is not significantly photosensitive. In contrast Cr~(3+) exhibited higher photosensitivity in the presence of Fe~(3+). This was monitored by locking the magnetic field to 1/2reversible 1/2 transition of Cr~(3+), and recording intensity as a function of time, under insitu laser illumination. In lightly doped crystals the intensity of Cr~(3+) 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 Cr~(3+) was followed by slow and steady build up of Cr~(3+) signal. The growth of Cr~(3+) signal was attributed to photoinduced decoupling of Cr~(3+) dimers. Thus, by doping BaTiO_3 with Cr~(3+)more efficient grating formation can be achieved and time dependent phenomena are observed.