Multiphoton light emission in barium stannate perovskites driven by oxygen vacancies, Eu and La: accessing the role of defects and local structures.

Defect engineering in perovskites has been found to be the most efficient approach to manipulate their performance in ultraviolet-to-visible photon conversion. Under UV irradiation, BaSnO exhibited multicolor photoluminescence (MCPL) in the bluish white region. Its origin has not been well studied in the literature and has been probed in this work using synchrotron radiation, positron annihilation and density functional theory. To achieve desirable performance of doped BaSnO in optoelectronics, it is imperative to have correct information on the dopant local site, doping induced defect evolution and efficacy of host to dopant energy transfer (HDET). Extended X-ray absorption fine structure (EXAFS) showed that Eu ions stabilize at both Ba and Sn sites consistent with the highly negative formation energy of around -6.26 eV. Eu doping leads to an intense D→F orange emission and a feeble D→F red emission and an internal quantum yield (IQY) of ∼21% mediated by ET from the defect level of Eu and Eu sites to the valence band maximum (VBM). X-ray absorption near edge structure (XANES) ruled out any role of Sn in the PL of BaSnO or Eu in the PL of BaSnO3:Eu. Interestingly, when co-doped, Eu stabilizes at Sn sites whereas La stabilizes at Ba sites with a formation energy value of -6.44 eV. Based on the asymmetry ratio in emission spectra, it was found that La ions lead to lowering of symmetry around Eu due to increased vacancies and structural distortions, and also suppress the luminescence IQY. We have performed experimental positron annihilation lifetime spectroscopy (PALS) to probe the defects in BaSnO in pristine samples and on doping/co-doping. The positron lifetimes for saturation trapping of positrons in various kinds of defects envisaged in BaSnO and in the defect free system were calculated using the MIKA Doppler program. Such deep insight into the effect of local structures, dopant sites, defect evolution, ET, etc. on the optical properties of BaSnO is expected to provide very deep insight for material scientists into the fabrication of perovskite-based optoelectronic and light-emitting devices.