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Down-/Up-Conversion Emission Enhancement by Li Addition: Improved Crystallization or Local Structure Distortion?

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journal contribution
posted on 27.06.2017 by Daniel Avram, Bogdan Cojocaru, Ion Tiseanu, Mihaela Florea, Carmen Tiseanu
Local symmetry distortion by Li addition is acknowledged as an effective strategy for enhancing the luminescence of lanthanide (Ln) doped into a wide range of lattice hosts. Despite extensive literature, direct evidence that supports Li-induced modification of the local crystal-field at the Ln sites is still missing. Herein, we show that the emission enhancement by Li addition in Ln,Li–Y2O3 is due to improved crystallization and not to local structure distortion. Our approach is based on the premise that any distortion/lowering of the local symmetry would reflect into the alteration of the emission shapes and shortening of the emission decays. To this aim, we have extensively investigated the evolution with Li addition and calcination temperature of down (optical and X-ray induced) and up-conversion (UPC) emission of Ln-Y2O3 measured across the visible to near-infrared range. First, a center to center (corresponding to Ln in the C2 and S6/C3i sites of the cubic Y2O3 lattice) as well as global comparison of the emission properties of Li free and Li codoped Y2O3 are presented by use of Eu, Sm, Tb and Dy as local probes in the visible range. Next, the effect of Li on the up-conversion emission of Er- Y2O3 is analyzed in terms of UPC pathways, emission shape and intensity, decays and excitation spectra. It is concluded that Li addition does not change either the local structure around C2 or S6 Ln centers or the relative contribution of these. Moreover, it is found that the effects of Li doping on the emission properties of Ln–Y2O3 are like extending the calcination temperature of Li-free Ln–Y2O3 from 800 °C to ∼1000–1100 °C. Additionally, a relatively intense 1500 to 980 nm UPC emission is evidenced for the first time for Er–Y2O3, while a relatively intense emission around 1500 nm was measured under X-ray excitation. Taken together, our findings highlight the need for revisiting the traditional optimization strategy based on Li modification but also the promise of Er–Y2O3 nanoparticles for optical/X-ray applications in the near-infrared range.