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

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 coprecipitation 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.6 phonon sideband intensity ratio reached as high as 117.7%-138.2%.

Automated summary

We found that the cation-exchange reaction is better than the conventional co-precipitation reaction for synthesizing Na2WO2F4:Mn4+ red luminescent phosphor, as it avoids the disproportionation reaction of K2MnF6. The WO42- present in Na2WO4 during the coprecipitation reaction catalyzes the disproportionation reaction of MnF62-, causing a change in valence and fluorescence quenching of Mn4+. Fluorination of Na2WO4 to obtain Na2WO2F4 and then utilizing cation-exchange reaction effectively suppresses this disproportionation reaction. Under optimized conditions, the luminescence intensity of the phosphor prepared via cation-exchange reaction is 19 times higher than that prepared via the co-precipitation method. The obtained Na2WO2F4:Mn4+ exhibits red luminescence at 619 nm, with a high ZPL to Stokes.6 phonon sideband intensity ratio ranging from 117.7% to 138.2%.