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

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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+–O2– distance. All studied phosphors demonstrate rather poor thermal stability of Mn4+ photoluminescence with a thermal quenching temperature T1/2 below 200 K, the lowest value being observed for the host with M = Sc. As expected, the decrease of the energy of the O2––Mn4+ charge-transfer state is observed with the increase of the M3+ cation radius, i.e. with the increase of the O2––Mn4+ interionic distance. The thermal quenching temperature of Mn4+ luminescence in the studied phosphors correlates with the energy of the O2––Mn4+ charge transfer state which is supposed to serve as a quenching state for Mn4+ luminescence.