  Luminescence quenching via non-radiative recombination channels limits the
efficiency of optical materials such as phosphors and scintillators and
therefore has implications for conversion efficiency and device lifetimes. In
materials such as Ce-doped yttrium aluminum garnet (YAG:Ce), quenching shows a
strong dependence on both temperature and activator concentration, limiting the
ability to fabricate high-intensity white-light emitting diodes with high
operating temperatures. Here, we reveal by means of first-principles
calculations an efficient recombination mechanism in YAG:Ce that involves
oxygen vacancies and gives rise to thermally activated concentration quenching.
We demonstrate that the key requirements for this mechanism to be active are
localized states with strong electron-phonon coupling. These conditions are
commonly found for intrinsic defects such as anion vacancies in wide band-gap
materials. The present findings are therefore relevant to a broad class of
optical materials and shine light on thermal quenching mechanisms in general.
