A novel phenomenon about the luminescence intensification by the coupling of two phases in a complex was presented. The phosphors were synthesized through solid-state reaction and the highest luminescence was obtained by tailoring SrO and Al2O3 ratios in the SrO–Al2O3 system. The red emission originated from Mn4+, while Mn2+ was also identified from electron paramagnetic resonance spectra. The lattice vibration was observed from Raman spectra. Meanwhile, the dielectric properties were measured. The mechanism based on electromechanical coupling between Sr4Al14O25 and SrAl2O4 phases in the 3SrO · 5Al2O3 complex was proposed and discussed from the viewpoint of lattice vibrations and the dielectric field.