Structural ordering and charge variation induced by cation substitution in (Sr,Ca)AlSiN₃:Eu phosphor

Nitride phosphors which are suitable for white light-emitting diode applications. In this study, the structure of phosphor has been modified through cation substitution to induce charge variation and a rearrangement of neighboring nitride clusters, and consequently enhance its luminescent behavior. Substitution of Ca²⁺ by Sr²⁺ cations expanded the lattice volume and the b-c plane, but shortened the distance between the layers along the a axis. Lattice distortion of the framework introduced high-coordination sites in the Sr/Eu centers and adequate space, thereby facilitating charge variation of activators under reduced atmosphere, as detected through X-ray absorption near-edge structure spectroscopy. As such, the photoluminescent intensity of the phosphors increased by more than 10% and a blue shift occurred. The microstructures of the samples were also analyzed using high-resolution transmission electron microscopy. Cation substitution induced a special change in the anion environment, as indicated in the solid-state Raman spectra. Moreover, a typical ordering variations in the SiN₄ and AlN₄ clusters are generated in the lattice. Meanwhile, neighbor sequence of (Si/Al)N₄ around the divalent centers were observed through solid-state nuclear magnetic resonance spectroscopy. The modified ordering distribution resulted in a rigid structure and improved the thermal quenching behavior. Thermal stability has been enhanced by 10% at 473 K when x = 0.9 in Sr_xCa_0.993-xAlSiN₃:Eu²⁺_0.007 compared with that at x = 0. This study promotes the research of neighbor sequence for selective tetrahedral sites such as Li, Mg, Al, Si coordinated by N atoms contact with cation sites.