Structure–Photoluminescence Relationship in\nCe<sup>3+</sup>-Doped K<sub>5</sub>Y(P<sub>2</sub>O<sub>7</sub>)<sub>2</sub> and Its Sensitization to Dy<sup>3+</sup> with Enhanced W

From\na fundamental point of view, the structure–photoluminescence\nrelationship is vitally important for developing efficient phosphors\nand rationally improving their performances. In this work, Rietveld\nrefinements on high-resolution powder X-ray diffraction (XRD) data\nwere conducted to verify the Ce<sup>3+</sup> distribution in K<sub>5</sub>Y(P<sub>2</sub>O<sub>7</sub>)<sub>2</sub> (KYPO). Ce<sup>3+</sup> ions are located at both M1 and M2 sites, with occupation factors\nof 0.31(2) and 0.19(2), respectively. Importantly, the crystal field\nstrength (CFS) is stronger at the former site due to the larger bond\nvalence sum. Correspondingly, the Ce<sup>3+</sup> emission can be\ndeconvoluted into four Gaussian-type bands centered at ∼353,\n∼383, ∼343, and ∼369 nm, where the former two\nbelong to Ce<sup>3+</sup> at the M1 site and the latter two are assigned\nto Ce<sup>3+</sup> at the M2 site. Ce<sup>3+</sup> in KYPO is a highly\nefficient activator, with optimal internal and external quantum efficiencies\nof 98.9 and 74.6%, respectively, and was thus utilized to sensitize\nthe Dy<sup>3+</sup> emission. Indeed, energy transfer from Ce<sup>3+</sup> to Dy<sup>3+</sup> was confirmed by fluorescent decay curves\nfor Ce<sup>3+</sup>/Dy<sup>3+</sup> codoped KYPO, and the mechanism\nwas supposed to be dominated by dipole–dipole interactions.\nIndeed, it can be pumped by a 310 nm LED chip and emits a bright white\nlight. Ce<sup>3+</sup> photoluminescence exhibits high resistance\nto thermal quenching, and it can be further enhanced by codoping Dy<sup>3+</sup>, i.e. the emission intensity remains 96.3% at 423 K.