Unraveling the superior role of dicarboxylic acids as surface chelators in Eu3+-doped yttrium fluorides: A systematic modulation of the crystal phases and morphologies for highly tuned optical performance

Highlights: • Dicarboxylic acids and pH: crucial to control the crystal phase and morphology. • A plausible mechanism is proposed to explain the chemical processes involved. • The optical response of Eu³⁺ can be highly tuned through the different samples. • Very long lifetimes (up to 12 ms) and high quantum efficiencies are reported. • The materials are suitable for advanced applications in photonics or bioanalytics. -- Abstract: Eu³⁺-doped YF₃ and KY₃F₁₀ phosphors have been prepared hydrothermally in a wide range of pH values without the use of surface chelators and adding oxalic or tartaric acid. We have proved, for the first time, the usefulness of dicarboxylic acids to modulate the evolution of the surface and crystal phases in the KF-YF₃ system. The morphologies and crystal structures of the materials displayed a critical dependence on the pH and the chelator employed, and a plausible mechanism to explain the differences among the distinct series of samples is proposed. As a result, the fluorides exhibited outstanding and tunable photoluminescence, with extremely high quantum efficiencies and very long lifetimes. The asymmetry ratio and Judd-Ofelt parameters calculations allowed us to establish a relationship between the optical performance of the compounds and their physicochemical properties. We feel that this study can arouse widespread interest within the materials engineering community, since similar procedures could be implemented to the extended family of complex yttrium/lanthanide fluorides for advanced applications in important fields such as bioanalytics, biomedics, or photonics. Indeed, the materials in the present work, with orangish-yellow colored emissions, could be very interesting for application in white light emitting diodes through their combination with blue chips.