Site Occupancies, Luminescence, and Thermometric Properties of LiY₉(SiO₄)₆O₂:Ce³⁺ Phosphors

In this work, we report the tunable emission properties of Ce³⁺ in an apatite-type LiY₉(SiO₄)₆O₂ compound via adjusting the doping concentration or temperature. The occupancies of Ce³⁺ ions at two different sites (Wyckoff 6h and 4f sites) in LiY₉(SiO₄)₆O₂ have been determined by Rietveld refinements. Two kinds of Ce³⁺ f–d transitions have been studied in detail and then assigned to certain sites. The effects of temperature and doping concentration on Ce³⁺ luminescence properties have been systematically investigated. It is found that the Ce³⁺ ions prefer occupying Wyckoff 6h sites and the energy transfer between Ce³⁺ at two sites becomes more efficient with an increase in doping concentration. In addition, the charge-transfer vibronic exciton (CTVE) induced by the existence of free oxygen ion plays an important role in the thermal quenching of Ce³⁺ at 6h sites. Because of the tunable emissions from cyan to blue with increasing temperature, the phosphors LiY₉(SiO₄)₆O₂:Ce³⁺ are endowed with possible thermometric applications

High Light Yield of Sr₈(Si₄O₁₂)Cl₈:Eu²⁺ under X‑ray Excitation and Its Temperature-Dependent Luminescence Characteristics

In this work, we first investigate the relationship between temperature and lattice parameters by means of Rietveld refinement and then demonstrate its impact on the luminescence peak position of Eu²⁺ in Sr₈(Si₄O₁₂)­Cl₈. It is found that with increases in temperature, lattice expansion takes place without significant distortion of the coordination around Eu²⁺. As a result, the crystal field splitting of the Eu²⁺ 5d state decreases. At the same time, with the experimental data of the full width at half-maximum of Eu²⁺ emission at different temperatures and the infrared spectrum, the effective phonon frequency is evaluated and the main vibration motions are determined using first-principles calculation. Due to the high light yield under X-ray excitation and the excellent thermal stability of luminescence intensity and decay, a further optimized sample Sr_7.7Eu_0.3(Si₄O₁₂)­Cl₈ could be a potential scintillation material