Photoluminescence Tuning via Cation Substitution in Oxonitridosilicate Phosphors: DFT Calculations, Different Site Occupations, and Luminescence Mechanisms

Abstract

Tuning and optimizing luminescent properties of oxonitridosilicates phosphors are important for white light-emitting diode (WLED) applications. To improve the color rendering index, correlated color temperature and thermal stability of layer-structured <i>M</i>Si₂O₂N₂:Eu (M = Sr, Ba) phosphors, cation substitutions have been used to adjust their luminescent properties. However, the underlying mechanisms are still unclear. In this research, a series of (Sr_1–<i>x</i>Ba_<i>x</i>)­Si₂O₂N₂:Eu (0 ≤ <i>x</i> ≤ 1) compounds were prepared by solid-state reaction, after which systematic emission variations were investigated. The crystal structures of (Sr_1–<i>x</i>Ba_<i>x</i>)­Si₂O₂N₂:Eu (0 ≤ <i>x</i> ≤ 1) are nominally divided into three sections, namely, Phase 1 (0 ≤ <i>x</i> ≤ 0.65), Phase 2 (0.65 < <i>x</i> < 0.80), and Phase 3 (0.80 ≤ <i>x</i> ≤ 1) based on the X-ray diffraction measurements. These experimental results are further confirmed by optimizing the crystal structure data with first-principle calculations. Continuous luminescence adjustments from green to yellow are observed in Phase 1 with gradual replacement of Sr²⁺ with Ba²⁺, and the abnormal redshift is clarified through extended X-ray absorption fine structure analysis. Sr­(Eu)–O/N bond length shrinkage in local structure causes the redshift emission, and the corresponding luminescence mechanism is proposed. Controllable luminescence in Phase 2 (from blue to white) and Phase 3 (from cyan to yellowish green) are observed. Based on the high-resolution transmission electron microscopy and selected area electron diffraction analysis, the two kinds of luminescence tuning are attributed to phase segregation. This study serves as a guide in developing oxonitride luminescent materials with controllable optical properties based on variations in local coordination environments through cation substitutions