Interstitial Oxygen-Driven Far-Red/Near-Infrared Emission and Efficiency Enhancement via Heterovalent Cation Substitution in Ca₃WO₆ Phosphors

In this work, Ca₃WO₆ (CWO) phosphors were successfully synthesized using a high-temperature solid-state method, exhibiting an anomalous far-red/near-infrared (FR-NIR) emission centered at 685 nm. The origin of this FR-NIR emission is confirmed through Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), density functional theory (DFT) calculations, and heterovalent cationic substitution (Y³⁺/Na⁺ → Ca²⁺). These analyses indicate that interstitial oxygen (O_i) defects within the lattice are primarily responsible for the FR-NIR emission. The heterovalent substitution of Y³⁺ for Ca²⁺ increases the concentration of O_i, significantly enhancing the FR-NIR emission intensity. This results in an increase in the internal quantum efficiency (IQE) increasing from 12.8 to 90.9%, realizing an efficient FR-NIR emission from self-activated phosphors. Furthermore, CWO phosphors demonstrate a unique dual-band emission characterized by a blue emission at 435 nm and an FR-NIR emission at 685 nm. This dual-band emission endows CWO phosphors with multifunctional applications in plant growth lighting, nondestructive testing, and night vision fields.