Raman scattering and luminescence of CoMF$_6$ $\cdot$ 6H$_2$O (M = Si, Ge, Ti) activated by Mn$^{4+}$ ions

Mn$^{4+}$-doped red-emitting fluorides are a promising class of materials for improving the color rendering and luminous efficiency of white light-emitting diodes (WLEDs). Phosphors based on hydrated cobalt hexafluoridometallates CoMF$_6$ $\cdot$ 6H$_2$O (M = Si, Ge) activated with Mn$^{4+}$ ions demonstrate red luminescence in the range of 600–650 nm upon excitation into the $^4$A$_2$–$^4$T$_2$ (450–480 nm) and $^4$A$_2$–$^4$T$_1$ (350–370 nm) bands. The luminescence spectra are formed by electron-phonon components at the $^2$E–$^4$A$_2$ transition involving vibrations of the MnF$_6^{2-}$ octahedron. The quantum yield of CoSiF$_6$ $\cdot$ 6H$_2$O : Mn$^{4+}$ is maximized at 357 nm excitation and is equal to 5% at 5.5% manganese concentration. When the crystal lattice of CoTiF$_6$ $\cdot$ 6H$_2$O is formed, the depth of the potential minimum for the Mn$^{4+}$ ion is smaller than in the case of CoSiF$_6$ $\cdot$ 6H$_2$O and CoGeF$_6$ $\cdot$ 6H$_2$O, which leads to random fluctuations of the crystal field in the ensemble of positions occupied by this ion.