Ultrabroadband red luminescence of Mn⁴⁺ in MgAl₂O₄ peaking at 651 nm

Blue light pumped red luminescence with broadband and high photon-energy emission is highly desired for phosphor-converted white light-emitting diodes (pc-wLEDs), to achieve a high color rendering index and high luminous efficacy. Mn⁴⁺-doped red-emitting phosphors generally exhibit sharp vibronic emissions associated with the parity- and spin-forbidden ²E_g→⁴A_2g transitions. In this paper, two abnormal luminescence behaviors were observed for Mn⁴⁺ in the MgAl₂O₄:Mn⁴⁺ spinel phosphor with a short wavelength emission band peaking at 651 nm. Firstly, the Mn^{4+ 2}E_g→⁴A_2g transition exhibits ultrabroadband luminescence in MgAl₂O₄ and the large full-width at half-maximum (FWHM) is dependent both on the calcination temperature and on the partial substitution of Al³⁺ with Ga³⁺. Secondly, the thermal quenching behavior of the Mn^{4+ 2}E_g→⁴A_2g luminescence in MgAl₂O₄ shows a dependence on its thermal treatment and preparation method. The Rietveld refinement and Raman results demonstrate that the variation in the FWHM of the luminescence spectra is a sum effect of structural ordering (i.e., isotropic displacement decrease of constituent atoms) and the Mg ↔ Al anti-site disorder. A model for the observed varying thermal quenching of luminescence was tentatively proposed. The intrinsic thermal quenching temperature of Mn⁴⁺ luminescence in MgAl₂O₄ was found to be 390-400 K using the samples prepared by the co-precipitation and molten salt methods. The present work gives a novel perspective to understand the luminescence spectra of Mn^{4+ 2}E_g→⁴A_2g transition.