K2NaAlF6:Mn4+ red phosphor: room-temperature synthesis and electronic/vibronic structures†
In this work, we propose a co-precipitation method for the synthesis of K2NaAlF6:Mn4+ red phosphor at room-temperature using methanol as the precipitating agent with a synthetic yield of ∼77%. The prepared sample (Mn ion doping concentration of 2.31 at%) exhibits a broad absorption band peaked at 466 nm and sharp emission lines centered at 631 nm with a high photoluminescence quantum yield up to 85%. The electronic/vibronic structures of the Mn4+ ions in the K2NaAlF6 crystal were investigated in detail for the first time by means of low-temperature (3 K) and high-resolution spectroscopy. This revealed that the Mn4+ ions mainly occupy three distinct crystallographic sites, which differs from the hypothesis that the Mn4+ ions will substitute the Al3+ ions in the K2NaAlF6 crystal, likely due to the charge mismatch between the Mn4+ and Al3+ ions. In particular, the unusually high intensity of the zero-phonon line at 622.40 nm implies that the corresponding Mn4+ ions resided in a non-centrosymmetric local site, which was corroborated by the observed energy level splitting of the 2E state. By employing the 2.31 at% sample as a red phosphor, we fabricated a warm white light-emitting diode (WLED) with a low correlated colour temperature (3911 K), a high colour rendering index (Ra = 94) and a luminous efficacy of 89 lm W−1. These findings provide deep insights into the optical properties of Mn4+ ions in hexafluoroaluminate hosts, and they show the great promise of K2NaAlF6:Mn4+ as a red phosphor for highly efficient warm WLEDs and wide-gamut displays.