Spectral Properties and Thermal Quenching of Mn4+ Luminescence in Silicate Garnet Hosts CaY2MgMAlSi2O12 (M = Al, Ga, Sc)

Abstract

Multi-component silicate garnet ceramics CaY₂MgMAlSi₂O₁₂ comprising different cations M = Al, Ga or Sc in octahedral sites doped with Mn⁴⁺ ions have been synthesized and studied as novel red-emitting phosphors aiming at warm white pc-LED applications. All synthesized phosphors exhibit Mn⁴⁺ luminescence in rather deep red region, the shortest-wavelength spectrum of Mn⁴⁺ luminescence (peak wavelength at 668 nm) being obtained for the host with the largest cation M³⁺ = Sc³⁺ in the octahedral site. The effect of increasing the energy of the emitting Mn⁴⁺²E level with the size of the host cation in octahedral sites is supposed to be the result of decrease of the covalence of the “Mn⁴⁺-ligand” bonding with increase of the interionic Mn⁴⁺–O^2– distance. All studied phosphors demonstrate rather poor thermal stability of Mn⁴⁺ 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––Mn⁴⁺ charge-transfer state is observed with the increase of the M³⁺ cation radius, i.e. with the increase of the O^2––Mn⁴⁺ interionic distance. The thermal quenching temperature of Mn⁴⁺ luminescence in the studied phosphors correlates with the energy of the O^2––Mn⁴⁺ charge transfer state which is supposed to serve as a quenching state for Mn⁴⁺ luminescence.