Synthesis, structural characterization and intense far-red luminescence of Mn4+ -activated SrLaMgTa1-yAlyO6 oxide phosphors

The transition metal Mn⁴⁺ activated phosphors have attracted increasing attention due to the potential uses in phosphor-converted lighting emitting diodes (LEDs). Herein, the far-red emitting SrLaMgTa_1-yAl_yO₆:Mn⁴⁺ (y = 0−0.15) oxide phosphors were successfully synthesized by the high-temperature solid state reaction, in which the Mn⁴⁺ acted as an activator. The monoclinic double perovskite crystal structure, chemical composition, and 4+ state of activator Mn were confirmed by means of X-ray diffraction Rietveld refinement, scanning electron microscopy elemental mapping, and X-ray photoelectron spectroscopy. The optical properties were characterized with photoluminescence excitation and emission spectra, temperature-dependent emission spectra, and electroluminescence spectra. The excitation spectra are interweaved with the ligand-to-metal charge transfer band of Mn−O and intrinsic transitions (⁴A_2g→⁴T_1g, ⁴A_2g→²T_2g, and ⁴A_2g→⁴T_2g) of Mn⁴⁺, locating at the ultraviolet and blue light region. The emission spectra mainly contain a dominant far-red emission band from 650 to 775 nm with peaks at 695 and 708 nm, which are ascribed to the ²E_g→⁴A_2g transition of Mn⁴⁺. Moreover, the cationic substitution strategy with tiny Al³⁺ occupying Ta⁵⁺ octahedral site, promotes the improvement of luminescence intensity and optical thermal stability. The quantum yield of optimal phosphor reaches 88.2 % and the fluorescence intensity at 373 K (100 °C) retains 83.4 % in respect to that at ambient temperature, implying the phosphor with intense and thermally stable luminescence. The phosphor is packaged with 365 nm chip to fabricate an LED device, and the electroluminescence result is quite consistent with the photoluminescence, matching well with the absorption spectrum of the phytochrome. The Mn⁴⁺ activated oxide phosphors with intense far-red emission are promising for plant cultivation lighting.