K2NaAlF6:Mn4+ red phosphor: room-temperature synthesis and electronic/vibronic structures

Abstract

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 (R_a = 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.