The photoconductivity of LiNbO3 crystals codoped with Cr2O3 and different concentrations of MgO from 0 up to 6 mol % is reported. The spectral excitation of photocurrent presents an intense band corresponding to the Cr3+ induced blue absorption edge, but illuminating at the (4)A(2) -> T-4(1) ionic transition band a weak photocurrent is also excited. The photoconductivity is interpreted in terms of Cr3+ ionization and the formation of Nb-Nb(4+) small polarons as charge carriers. Photoconductivity below about 140 K is suppressed. The analysis of the photocurrent temperature dependence results in a 0.14 +/- 0.02 eV binding energy of the polaron at low temperature. Photoconductivity induced by excitation in the intraionic Cr3+ can be explained by a tunnelling process between the excited Cr3+ and the polaron state at the next-nearest-neighbour Nb-Nb(5+).
- defect structures