Spectral properties and thermal quenching of Mn$^{4+}$ luminescence in silicate garnet hosts CaY$_{2}$MgMAlSi$_{2}$O$_{12}$ ($M$ = Al, Ga, Sc)

Multi-component silicate garnet ceramics CaY$_{2}$MgMAlSi$_{2}$O$_{12}$ comprising different cations $M$ = Al, Ga or Sc in octahedral sites doped with Mn$^{4+}$ ions have been synthesized and studied as novel red-emitting phosphors aiming at warm white $pc$-LED applications. All synthesized phosphors exhibit Mn$^{4+}$ luminescence in rather deep red region, the shortest-wavelength spectrum of Mn$^{4+}$ luminescence (peak wavelength at 668 nm) being obtained for the host with the largest cation $M^{3+}$ = Sc$^{3+}$ in the octahedral site. The effect of increasing the energy of the emitting Mn$^{4+}$ ${}^{2}E$ level with the size of the host cation in octahedral sites is supposed to be the result of decrease of the covalence of the "Mn$^{4+}$-ligand" bonding with increase of the interionic Mn$^{4+}$–O$^{2-}$ distance. All studied phosphors demonstrate rather poor thermal stability of Mn$^{4+}$ photoluminescence with a thermal quenching temperature $T_{1/2}$ below 200 K, the lowest value being observed for the host with $M$ = Sc. As expected, the decrease of the energy of the O$^{2-}$–Mn$^{4+}$ charge-transfer state is observed with the increase of the $M^{3+}$ cation radius, i. e. with the increase of the O$^{2-}$–Mn$^{4+}$ interionic distance. The thermal quenching temperature of Mn$^{4+}$ luminescence in the studied phosphors correlates with the energy of the O$^{2-}$–Mn$^{4+}$ charge transfer state which is supposed to serve as a quenching state for Mn$^{4+}$ luminescence.