Deep-red-emitting SrLaLiTeO6: Mn4+ double perovskites: Correlation between Mn4+-O2? bonding and photoluminescence

Mn⁴⁺-activated deep red-emitting SrLaLiTeO₆ phosphors are investigated for indoor plant growth LED applications for the first time. The phosphors crystallize in monoclinic (P2₁/n) symmetry is isostructural with SrLaLiTeO₆ host. B-site substitution of Mn⁴⁺ ions is confirmed from the redshift of high energy phonon modes in both Raman and IR spectra. The phosphor exhibited a far-red emission centered at 696 nm corresponding to the ²E_g → ⁴A_2g spin-forbidden transition of the Mn⁴⁺ ions. Approximate crystal field parameters depict the weak influence of neighboring ligand fields on Mn⁴⁺ ions and the least covalence of Mn⁴⁺-ligand bonding compared to other double perovskite phosphors. Moreover, the phosphors exhibit excellent thermal stability with an activation energy of 0.23 eV. Phosphor parameters including CCT, color purity, and quantum yield are evaluated and their values meet the requirements of a red-emitting phosphor for LED applications. Furthermore, the PL emission spectrum of SrLaLiTeO₆: Mn⁴⁺ matches with the absorption spectrum of plant phytochromes denoting the prospects of this phosphor for indoor plant growth LED applications.