Solid solution of Cr3+ doped ZnGa2O4 and Zn2SnO4 to create cation inversion and its role on persistent deep red emission

With the advent of multitude of applications arising from deep red emitting rare earth free phosphors, lot of research is being carried out on Cr3+ doped phosphors. Herein we have made an effort of engineering photoluminescence (PL) intensity, PL quantum yield (PLQY) and persistent luminescence (PerL) by gradually replacing Ga3+ site by Sn4+ in ZnGa2O4:Cr3+ normal and moving to the extreme Zn2SnO4:Cr3+ inverse spinel. It was observed that lower level of Sn4+ up to (40%) leads to enhancement in PL intensity and producing more deep red emission than ZnGa2O4:Cr3+. Higher Sn4+ inclusion in ZnGa2O4:Cr3+ leads to reduction in red emission and Zn2SnO4:Cr3+ and generates much fainted red emission with tinge of pinkish white. Interestingly both PLQY and persistent luminescence (PerL) degraded monotonically with Sn4+ and the same has been probed using positron annihilation lifetime and electron paramagnetic resonance spectroscopy. Incorporation of Zn2+ and Sn4+ in lattice sites of Ga3+ reduced the average positron lifetime drastically suggesting removal of the zinc vacancies that were inherently present in ZnGa2O4 leading to reduction in PerL. EPR on the other hand suggested that as we move from normal to inverse spinel, more and more Cr ions come out of the lattice positions occupying highly disordered sites with large number of antisite defect state resulting in quenching of PLQY.