Luminescence properties of nitrogen-rich Ca-SiAlON:Eu²⁺ phosphors prepared by freeze-drying assisted combustion synthesis

Nitrogen-rich Eu²⁺-doped Ca-α-SiAlON phosphors (Ca_{m/2−​​​​​​​x}Si_{12−​​​​​​​​​​​​​​m−​​​​​​​​​​​​​​n}Al_m+nO_nN_{16−​​​​​​​​​​​​​​n}:xEu) were synthesized by a freeze-drying assisted combustion synthesis (CS) route. Fast-synthesized products with high purity and uniform particle morphology were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The analysis of lattice parameters by comparison with empirical equations showed that the as-prepared phosphors had low oxygen content. A series of samples were prepared according to the stoichiometry of Ca_{m/2−​​​​​​​0.08}Si_{12−​​​​​​​​​​​​​​m}Al_mN₁₆:0.08Eu for further research. The influences of m value on the luminescence properties were investigated in detail. As m increased, a redshift phenomenon was observed in both the excitation and emission spectra. First-principle electronic structure calculations showed that the 3d energy level of Ca played an important role in the occurrence of the redshift phenomenon.