Understanding the Interactions between Vibrational Modes and Excited State Relaxation in Y<sub>3–<i>x</i></sub>Ce<sub><i>x</i></sub>Al<sub>5</sub>O<sub>12</sub>: Design Principles for Phosphors Based on 5<i>d</i>–4<i>f</i> Transitions

The oxide garnet Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> (YAG), when a few percent of the activator ions Ce<sup>3+</sup> substitutes for Y<sup>3+</sup>, is a luminescent material widely used in phosphor-converted white lighting. However, fundamental questions surrounding the defect chemistry and luminescent performance of this material remain, especially in regard to the nature and role of vibrational dynamics. Here, we provide a complete phonon assignment of YAG and establish the general spectral trends upon variation of the Ce<sup>3+</sup> dopant concentration and temperature, which are shown to correlate with the macroscopic luminescence properties of Y<sub>3–<i>x</i></sub>Ce<sub><i>x</i></sub>Al<sub>5</sub>O<sub>12</sub>. Increasing the Ce<sup>3+</sup> concentration and/or temperature leads to a red-shift of the emitted light, as a result of increased crystal-field splitting due to a larger tetragonal distortion of the CeO<sub>8</sub> moieties. Decreasing the Ce<sup>3+</sup> concentration or cosubstitution of smaller and/or lighter atoms on the Y sites creates the potential to suppress thermal quenching of luminescence because the frequencies of phonon modes important for nonradiative relaxation mechanisms are upward-shifted and hence less readily activated. It follows that design principles for finding new Ce<sup>3+</sup>-doped oxide phosphors emitting at longer wavelengths require tetragonally distorted environments around the CeO<sub>8</sub> moieties and a sufficiently rigid host structure and/or low activator-ion concentration to avoid thermal quenching of luminescence.