“…The equivalently doped A 2 XF 6 :Mn 4+ systems, for example, trigonal Na 2 GeF 6 :Mn 4+ , generally show a lifetime >5 ms at room temperature; 6 Mn 4+ -doped fluorides with lifetime shorter than 4 ms are difficult to achieve in equivalently doped systems but have been achieved in several non-equivalent doping cases. 7,8 As the selection rule for an electric dipole transition becomes relaxed, an intense ZPL will commonly appear as an accompanying feature. As the ZPL is of higher energy and blue-shifted when compared with Stokes phonon sideband emissions, the human visual sensitivity can be improved by 10%, when ZPL dominates the luminescence spectrum, 9 as in Na 2 WO 2 F 4 :Mn 4+ , which helps improve the luminous efficiency.…”