Effect of reduction on the optical properties of Sr0.61Ba0.39Nb2O6 single crystals grown by optical floating zone technique

The effect of thermal reduction during growth of strontium barium niobate Sr0.61Ba0.39Nb2O6 (SBN61) crystal on its optical and electrical properties has been investigated. Single crystals of SBN61 were grown by the Optical Floating Zone technique in air ambience. Growth challenges such as formation of core and crystal cracking were eradicated by optimizing the synthesis and growth process parameters. The evidence of reduction was observed in the grown crystals that resulted in a characteristic gray colour. Further, the crystal became colourless on annealing in oxygen ambiance at ∼1200 °C for 20 h. The as-grown crystal is therefore identified as reduced SBN crystals due to formation of oxygen vacancies and the accompanied colour centers and charge compensated defects e.g. Nb4+ polaron in the as-grown crystals. X-ray powder diffraction analysis confirmed the formation of the desired phase and no structural change in the reduced crystal. Also, the absorption spectra revealed a broad absorption band centred around 500 nm in the as-grown reduced crystals confirming the thermal reduction of the crystal. Reduced crystals depicted a slight decrease in the optical band gap energy due to defect induced band tailing. An increase in the pyroelectric coefficient (p3) was observed for reduced SBN61 crystal in comparison to the crystal annealed in oxygen. The impedance measurement revealed higher electrical conductivity with smaller activation energy for the reduced crystal in comparison to the annealed SBN61 crystal. It shows that the thermal reduction induced defects influence the electrical conduction processes, causing the materials to be more conducting, which is desirable for a relaxor ferroelectric materials to be used as a thermoelectric generator application.