Intense deep-red zero phonon line emission of Mn⁴⁺ in double perovskite La₄Ti₃O₁₂

Phosphors that emit in the deep-red spectral region are critical for plant cultivation light-emitting diodes. Herein, ultrabroadband deep-red luminescence of Mn⁴⁺ in La₄Ti₃O₁₂ was studied, which showed intense zero phonon line emission. The double-perovskite structural La₄Ti₃O₁₂ simultaneously contains two Ti⁴⁺ sites forming slightly- and highly-distorted TiO₆ octahedra, respectively. The influence of octahedral distortion on the Mn⁴⁺ emission energy in the two distinct Ti⁴⁺ sites was studied both experimentally and theoretically. The spectral measurements indicated that Mn⁴⁺ in La₄Ti₃O₁₂ showed intense zero phonon line emission (ZPL) at deep-red 710-740 nm under excitation of 400 nm charging the O^2- → Mn⁴⁺ charge transfer transition. The splitting of the ZPL of the Mn^{4+ 2}E_g → ⁴A_2g transition as well as the intensity of ZPL relative to the vibronic phonon sideband emissions were found to be greatly influenced by the degree of octahedral distortion. The crystal-field strength and Racah parameters of Mn⁴⁺ in each Ti⁴⁺ site were also estimated. The Mn^{4+ 2}E_g → ⁴A_2g luminescence exhibited severe thermal quenching, which was explained by the low-lying ⁴T_2g level and charge-transfer state.