Harnessing reaction media as charge compensator to improve red emission from CaWO4:Eu3+ a new strategy

Abstract

This is probably the first report wherein we have correlated the optical properties of red emitting doped scheelite (CaWO4:Eu3+) synthesized by solid state (SS) reaction and sodium tungstate employed ethylene glycol (EG) method. The presence of sodium in EG sample was confirmed using neutron activation analysis whereas defect formation was probed using positron annihilation lifetime spectroscopy (PALS). The positron lifetimes suggested formation of calcium vacancies in CaWO4:Eu3+synthesized by SS method whereas Na from Na2WO4 in EG method enters the lattice and charge compensates for trivalent Eu3+ doping at divalent Ca2+ sites and plays pivotal role in removal of the calcium vacancies. Due to the removal of Ca vacancies, Eu doped sample prepared by EG method demonstrated higher internal quantum efficiency (IQE) and longer excited state lifetime with lower non-radiative channels and improved branching ratio compared to SS. The comparison with externally added sodium ion in SS samples with EG showed slightly higher IQE in former but at the cost of branching ratio and asymmetry factor. This work paved a new strategic pathway for research community working in the area of phosphor converted light emitting diodes to employ a alkali rich precursor to compensate for aliovalent doping and in lowering the non-radiative channels to improve the luminescence output of the phosphor.