Sunlight-excitable orange or red persistent oxide phosphors with excellent performance are still in great need. Herein, an intense orange-red Sr_3−xBa_xSiO₅:Eu²⁺,Dy³⁺ persistent luminescence phosphor was successfully developed by a two-step design strategy. The XRD patterns, photoluminescence excitation and emission spectra, and the thermoluminescence spectra were investigated in detail. By adding non-equivalent trivalent rare earth co-dopants to introduce foreign trapping centers, the persistent luminescence performance of Eu²⁺ in Sr₃SiO₅ was significantly modified. The yellow persistent emission intensity of Eu²⁺ was greatly enhanced by a factor of 4.5 in Sr₃SiO₅:Eu²⁺,Nd³⁺ compared with the previously reported Sr₃SiO₅:Eu²⁺, Dy³⁺. Furthermore, Sr ions were replaced with equivalent Ba to give Sr_3−xBa_xSiO₅:Eu²⁺,Dy³⁺ phosphor, which shows yellow-to-orange-red tunable persistent emissions from λ=570 to 591 nm as x is increased from 0 to 0.6. Additionally, the persistent emission intensity of Eu²⁺ is significantly improved by a factor of 2.7 in Sr_3−xBa_xSiO₅:Eu²⁺,Dy³⁺ (x=0.2) compared with Sr₃SiO₅:Eu²⁺,Dy³⁺. A possible mechanism for enhanced and tunable persistent luminescence behavior of Eu²⁺ in Sr_3−xBa_xSiO₅:Eu²⁺,RE³⁺ (RE=rare earth) is also proposed and discussed.

A variety of hexagonal sodium yttrium fluoride (NYF) crystals with tunable shapes have been grown, and their up-conversion (UC) emission has been greatly improved after post-treatment in an aqueous solution of NH₄HF₂ and NaF. The enhancement has been attributed to topotactic ion insertion of sodium cations into the channels of Na_3xY_2–xF₆. As sodium-ion insertion continues to occur and the sodium content rises, the up-conversion emission intensity and green-to-red ratio increase up to the saturation level. However, the X-ray diffraction patterns and SEM images suggest that both the crystallographic structure and geometric morphology remain unchanged during this topotactic sodium ion insertion. This strategy is very different to previous reports on the use of ion exchange in the preparation of sodium yttrium fluoride crystals and improvement of UC emission. The results of this work demonstrate that sodium vacancy in the hexagonal Na_3xY_2–xF₆ crystal is the key to the clear reduction in the up-conversion photoluminescence properties of the crystals, but the topotactic sodium ion insertion can revive the up-conversion efficiency greatly.

Invited for the cover of this issue is the group of Jing Wang and Mingmei Wu at Sun Yat-Sen University. The cover image shows that the up-conversion emission of hexagonal sodium yttrium fluoride crystals can be greatly enhanced by adding more sodium cations into the channels.

Porous ZnGa₂O₄ prisms assembled by highly oriented nanoparticles have been fabricated by an in situ chemical conversion approach. We report, for the first time, that a solid α-Ga₂O₃ precursor can be directly converted into ZnGa₂O₄ rather than through the intermediate GaOOH. Based on a detailed study of the evolution of ZnGa₂O₄ prisms, a growth mechanism is proposed for the in situ conversion reaction. During this conversion process, the precursor morphology can be highly retained, which is attributed to the similar atomic arrangements of the Ga and O atoms and excellent matching of the lattice spacing between the α-Ga₂O₃ and ZnGa₂O₄ prisms. The direct reaction between the precursor α-Ga₂O₃ and Zn²⁺ ions is more efficient than that between the byproduct GaOOH and Zn²⁺ ions. Moreover, the photoluminescent color of the ZnGa₂O₄ phosphor can be tuned by doping with Mn²⁺ ions. Efficient energy transfer (ET) from the host lattice to the Mn²⁺ centers is observed, whereas ET from the defects to the Mn²⁺ ions is prohibited. The fabricated ZnGa₂O₄ products have potential in the field of display applications.