Thermally Stable and Deep Red Luminescence of Sr_1-xBa_x[Mg₂Al₂N₄]:Eu²⁺ (x = 0-1) Phosphors for Solid State and Agricultural Lighting Applications

The systematic substitution of Ba in the Sr site of Sr[Mg₂Al₂N₄]:Eu²⁺ generates a deep-red-emitting phosphor with enhanced thermal luminescence properties. Gas pressure sintering (GPS) of all-nitride starting materials in Molybdenum (Mo) crucibles yields pure-phase red-orange-colored phosphors. Peaks in the synchrotron X-ray diffraction (SXRD) data show a systematic shift toward smaller angles due to the introduction of the larger Ba cation in the same crystal structure. The photoluminescence property reveals that Ba substitution shifts the original emission wavelength of Sr[Mg₂Al₂N₄]:Eu²⁺ (625 nm) toward a690 nm for Ba[Mg₂Al₂N₄]:Eu²⁺. Thermal stability measurement of Sr_1-xBa_x[Mg₂Al₂N₄] indicates a systematic increase in stability from x = 0 to x = 1. X-ray absorption near-edge spectroscopy (XANES) results demonstrate the coexistence of Eu²⁺ and Eu³⁺. The red-shift and the enhanced thermal stability reveals that the distance of the emitting 5d level to the conduction band of Ba[Mg₂Al₂N₄]:Eu²⁺ is large. The ionic size mismatch of Eu occupying a Ba site reduces the symmetry, thereby further splitting the degenerate emitting 5d level and lowering the energy of the emitting center. The development of deep-red phosphors emitting at 670-690 nm (x = 0.8-1.0) offers possible candidates for plant lighting applications.