Improved phase stability of the metastable K2GeF6:Mn4+ phosphors with high thermal stability and water-proof property by cation substitution

•The metastable phase of K2GeF6 was stabilized by substituting Ge4+ with Si4+ ion.•The spectrum of the obtained K2(Ge,Si)F6:Mn4+ shows obvious strong ZPL.•K2(Ge,Si)F6:Mn4+ phosphors show high thermal stability.•K2(Ge,Si)F6:Mn4+ exhibits an excellent water-proof property for a long time. K2GeF6:Mn4+ (KGFM) phosphors with very poor water-proof property in HF solution transform automatically from P63mc hexagonal phase to P3¯m1 trigonal phase. By substituting Ge4+ ion with Si4+ ion, the metastable P63mc hexagonal K2(Ge1-xSix)F6:Mn4+ (KGSFM) with an obvious zero-phonon line peaking at 623.8 nm is stabilized in HF solution. When x = 0.3, the photoluminescence intensity of KGSFM keeps 78% after 168 h in water as well as it increases by 14% at 150 °C (423 K) more than that of KGFM. By using KGSFM the fabricated WLEDs exhibit the high color-rendering index (Ra = 92) and low correlated color temperature (CCT = 3810 K). These results demonstrate that KGSFM solid solution with efficient red emission, meritorious water-proof and high thermal stability is a promising phosphor for the commercial application in the WLEDs and wide-gamut displays.