Preparation and Luminescence Property Study of Red-Emitting Na[sub.3.6]Y[sub.1.8][sub.3]:Eu[sup.3+],Li[sup.+]/K[sup.+] Phosphors with Excellent Thermal Stability for Light-Conversion Application

A series of red-emitting phosphors, Na[sub.3.6]Y[sub.1.8−x](PO[sub.4])[sub.3]:xEu[sup.3+], have been synthesized by a high-temperature solid-phase method. The impact of the partial Li[sup.+]/K[sup.+] ion substitution on the crystal structure and photoluminescence (PL) performance of Na[sub.3.6]Y[sub.1.05](PO[sub.4])[sub.3]:0.75Eu[sup.3+] phosphor have been investigated. Various techniques have been used for characterization of the as-obtained materials. X-ray diffraction (XRD) analysis was utilized to confirm the composites of these samples, and the morphology and element distribution were examined by scanning electron microscope (SEM) and transmission electron microscope (TEM). This study found that the developed Na[sub.3.6]Y[sub.1.8−x](PO[sub.4])[sub.3]:xEu[sup.3+] phosphors exhibited a prominent emission peak at ~620 nm when excited at 393 nm, which corresponded to [sup.5]D[sub.0] → [sup.7]F[sub.2] transitions of Eu[sup.3+] ions. Furthermore, the robust emission peak at ~705 nm ([sup.5]D[sub.0] → [sup.7]F[sub.4]) of these phosphors enables a better match with plant pigment absorption. Beyond that, the partial substitution of Li[sup.+]/K[sup.+] ions probably changed the crystal structure, and reduces the symmetry around Eu[sup.3+], leading to significantly enhanced luminous intensities by 23.24% and 18.29%, with the highest quantum yields (QYs) reaching 99.85% and 96.29%, respectively. Additionally, the prepared phosphors show non-thermal quenching and superior thermal stability at elevated temperatures from 298 to 473 K. These findings and results suggest that Li⁺/K⁺-substituted Na[sub.3.6]Y[sub.1.05](PO₄)₃:0.75Eu[sup.3]⁺ phosphors can serve as promising red-emitting phosphors for plant lighting applications.