Composition dependent spectral shift of Mn4+ luminescence in silicate garnet hosts CaY2M2Al2SiO12 (M = Al, Ga, Sc)

Multi-component silicate garnet ceramics CaY2M2Al2SiO12 having different cations M = Al, Ga or Sc at octahedral sites doped with Mn4+ ions have been synthesized and studied as possible red-emitting phosphors for warm white pc-LED applications. The short-wavelength shift of Mn4+ emission spectrum has been obtained with increasing the radius of the cation at octahedral sites as a result of decreasing the covalence of the “Mn4+-ligand” bonding, which leads to an increase of the energy of the emitting Mn4+ 2Eg level. However, simultaneously, stronger thermal quenching of Mn4+ luminescence is observed with the increase of the cation radius because of decreasing the energy of the 4T2g level as well as the O2- - Mn4+ CT state, leading to the decrease of activation barrier for thermal quenching due to non-radiative relaxation from the emitting Mn4+ 2Eg level to the Mn4+ ground state. All studied phosphors demonstrate rather poor thermal stability of Mn4+ luminescence with thermal quenching temperatures T1/2 < 240 K.