Nature of defects in blue light emitting CaZrO3: spectroscopic and theoretical study

The optical behaviour of a perovskite ceramic CaZrO3 is investigated. The orthorhombic CaZrO3 was obtained by gel combustion synthesis which yielded phase pure product at temperatures as low as 600 degrees C. Transmission electron microscopy shows the formation of highly monodisperse nanospheres of calcium zirconate. Despite the absence of any activator, undoped CaZrO3 showed distinct excitation and emission spectra attributed to the presence of local defects in the perovskite phase. Photoluminescence decay and EPR spectroscopy shows the presence of oxygen vacancies which is responsible for intense violet blue emission in the CaZrO3 nanospheres. The presence of oxygen vacancies was further confirmed by comparing the intensity of emission and the EPR spectrum of the sample annealed in completely reducing and completely oxidizing atmospheres with that of the as prepared sample. To explain the PL emission in the blue region, a distortion model is proposed. Our DFT based hybrid functional calculations show distortion in the Ca network causes less disorder in the unit-cell compared to the Zr network. DFT calculations also show distortion in the Ca network comprising [CaO7 center dot V-O(center dot center dot)] and [CaO7 center dot V-O(center dot)] complex clusters generates shallow defect states very close to valence band maxima leading to PL emission in the blue region.