Cation-exchange reaction to prepare Na2WO2F4:Mn4+ with intense zero phonon line at 619 nm avoiding disproportionation reaction

Abstract

We report that in the case of synthesizing the Na2WO2F4:Mn4+ red luminescent phosphor, the cation-exchange reaction is superior to a conventional co-precipitation reaction for a brighter luminescence, because it avoided the disproportionation reaction of K2MnF6. As evidenced by the UV–Vis transmittance spectra, during the co-precipitation reaction, the WO42− contained in Na2WO4 induced a catalytic effect on the disproportionation reaction of MnF62−, which led to valence change (from Mn4+ to Mn7+, Mn3+, etc.) and fluorescence quenching of Mn4+. Fluorination of Na2WO4 to get Na2WO2F4 for cation-exchange reaction with K2MnF6 under presence of HF solution was found to be effective in suppressing this disproportionation reaction. At optimized conditions, the phosphor prepared via the cation-exchange reaction shows a luminescence intensity 19-fold higher than that prepared via the co-precipitation method. The Na2WO2F4:Mn4+ obtained via cation-exchange reaction exhibits red luminescence peaking at 619 nm that is ascribed to the zero phonon line (ZPL) emission, and the ZPL to Stokes ν6 phonon sideband intensity ratio reached as high as 117.7%–138.2%.