Luminescence of Yb3+ incorporated into SrTi1-xZrxO3 solid-solutions: inquiring into the charge transfer state nature in lanthanide-doped titanates

For more than forty years, quenching of the 3P0 → 3H4 emission in favor of the 1D2 → 3H4 one in Pr3+-doped titanates has been explained by a mechanism involving a metal-to-metal charge transfer (MMCT) state, profusely designated in the literature as IVCT Pr3+/Ti4+ → Pr4+/Ti3+ state. Here, we present experimental evidence making unnecessary this mechanism and supporting theoretical results that points towards an alternative process based on a ligand-to-metal charge transfer (LMCT) O2-/Ti4+ → O-/Ti3+ state as being responsible for that quenching (Z. Barandiarán et al. J. Phys. Chem. Lett. 2017, 8, 3095). Photoluminescent spectra of Yb3+ incorporated into SrTi1-xZrxO3 solid-solutions interpreted in the framework of phenomenological models leading to vacuum referred binding energy diagrams and compared with equivalent solid solutions but doped with Pr3+, indicate that the emission of both lanthanide ions is excited through the same host state: the LMCT O2-/Ti4+ → O-/Ti3+ state. Photoluminescence spectra and decay curves performed in Pr-doped SrTiO3 as a function of the temperature suggest a thermally activated mechanism as being responsible for the quenching of the emission associated with the 3P0 → 3H4 transition.