The trivalent lanthanides have been broadly utilized as emitting centers in persistent luminescence (PersL) materials due to their wide emitting spectral range, which thus attract considerable attention over decades. However, the origin of the trivalent lanthanides’ PersL is still an open question, hindering the development of excellent PersL phosphors and their broad applications. Here, the PersL of 12 kinds of the trivalent lanthanides with the exception of La³⁺, Lu³⁺, and Pm³⁺ is reported, and a mechanism of the PersL of the trivalent lanthanides in wide bandgap hosts is proposed. According to the mechanism, the excitons in wide bandgap materials transfer their recombination energy to the trivalent lanthanides that bind the excitons, followed by the generation of PersL. During the PersL process, the trivalent lanthanides as isoelectronic traps bind excitons, and the binding ability is not only related to the inherent arrangement of the 4f electrons of the trivalent lanthanides, but also to the extrinsic ligand field including anion coordination and cation substitution. Our work is believed to be a guidance for designing high-performance PersL phosphors.

Under 971 nm excitation, bright green and red emissions from Yb³⁺/Er³⁺ co-doped Ba₅Gd₈Zn₄O₂₁ phosphor can be observed, especially the intense red emission in highly doped samples. The experimental results indicate that Ba₅Gd₈Zn₄O₂₁:Yb³⁺, Er³⁺ emits stronger upconversion luminescence than NaYF₄:Yb³⁺, Er³⁺ under a low excitation power and a maximum upconversion power efficiency of 2.7% for Ba₅Gd₈Zn₄O₂₁:Yb³⁺, Er³⁺ was achieved. More significantly, to explain the red emission enhanced with the dopant concentration, this paper presents a possible cross-relaxation process and demonstrates it based on the rate equation description and temporal evolution. In view of the strong upconversion luminescence, colour tunable ability and stable chemical nature, Yb³⁺/Er³⁺ co-doped Ba₅Gd₈Zn₄O₂₁ phosphor could be an excellent candidate for efficient upconversion luminescence generation.

Developing X-ray scintillators that are water-dispersible, compatible with polymeric matrices, and processable to flexible substrates is an important challenge. Herein, Tb³⁺-doped Na₅Lu₉F₃₂ is introduced as an X-ray scintillating material with steady-state X-ray light yields of 15,800 photons MeV⁻¹, which is generated as nanocrystals on halloysite nanotubes. The obtained product exhibits good water-dispersibility and highly sensitive luminescence to X-rays. It is deposited onto a polyurethane foam to afford a composite foam material with dose-dependent radioluminescence. Moreover, the product is dispersed into polymer matrixes in aqueous solution to prepare rigid or flexible scintillator screen for X-ray imaging. As a third example, it is incorporated multilayer hydrogels for information camouflage and multilevel encryption. Encrypted information can be recognized only by X-ray irradiation, while the false information is read out under UV light. Altogether, we demonstrate that the water-dispersible scintillators are highly promising for aqueous processing of radioluminescent, X-ray imaging, and information encrypting materials.