Order-disorder correlation on local structure and photo-electrical properties of La3+ ion modified BZT ceramics

Rare earth lanthanum (La) doped barium zirconate titanate, Ba1-xLa2x/3Zr0.3Ti0.7O3 (BLZT) ceramics, with x = 0.00, 0.02, 0.04, 0.06, 0.08 and 0.10 were prepared using solid state reaction route. Structural characterizations of the materials were done by using X-ray diffraction and Raman spectroscopy. The Rietveld refinement technique employed to investigate the details of the crystal structure revealed a single phase cubic perovskite structure for all the compositions, belonging to the space group Pm-3m. Raman spectroscopy was used to probe the order-disorder correlation in local symmetry and it was verified that the presence of disorder in cubic structure is increased due to La3+ ion substitution at A-site. In addition, the signature of relaxor behavior and diffuse types of phase transition can be detected by monitoring the relative intensity of Raman features. Room temperature photo-electronic properties were investigated by using ultraviolet-visible (UV-Vis) and photoluminescence (PL) spectroscopy. Heterovalent doping (La3+) is accompanied by creation of ionic defects to maintain the charge neutrality; as a result the intermediate energy levels are formed within the band gap. These intermediate energy levels play a significant role in electronic band transitions in higher La concentration, x >= 0.08; enhancing the self-trapping mechanism leads to slightly decreasing in band gap values and shifting the PL emission spectra towards violet-blue regions. The temperature dependence of the dielectric constant was investigated and relaxor type of phase transition was observed in the material. The degree of relaxor behavior was enhanced with increase in La3+ ion concentration.