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

Abstract

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°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 complex clusters generates shallow defect states very close to valence band maxima leading to PL emission in the blue region.