Optical properties of Sr₂LuNbO₆:Mn⁴⁺ deep-red phosphor: crystal-field splitting and multimode vibrational coupling

In this study, the optical properties of Sr₂LuNbO₆:Mn⁴⁺ deep-red phosphor synthesized with a molten salt solid-phase method were investigated in detail by using photoluminescence (PL), PL excitation (PLE), time-resolved PL, and Raman spectroscopic techniques. At room temperature, the PL spectra of the phosphor sample show a stepped shape. At cryogenic temperatures, however, the measured PL spectra are featured by two sets of multi-mode vibronic luminescence lines. Based on the variable-temperature PL spectra and Raman spectra, the dual zero-phonon lines and the two sets of multimode vibronic lines with a constant energy separation of ∼2.8 meV are identified. This study leads to the unprecedented determination of the splitting energy of the ²E_g excited-state and the confirmation of multiple vibrational modes involved within the vibronic luminescence.