Microwave-Assisted Solvothermal Synthesis of Upconverting and Downshifting Rare-Earth-Doped LiYF₄ Microparticles

Growing attention toward optically active materials has prompted the development of novel synthesis methods for a more reliable and efficient access to these systems. In this regard, microwave-assisted approaches provide unique advantages over traditional solvothermal methods reliant on convectional heating: namely, significantly shorter reaction durations, more rigid reaction conditions, and thus a higher degree of reproducibility. Reported herein for the first time is a rapid synthesis of rare-earth (RE³⁺)-doped LiYF₄ upconverting and downshifting microparticles with well-defined bipyramidal morphology and good size dispersion via a microwave-assisted solvothermal process. The suggested material growth mechanism identifies a suitable Li⁺ to RE³⁺ ion ratio, an abundance of pH-sensitive acetate surface-capping ligands, and an appropriate reaction temperature/time profile as crucial for enabling a phase transformation of an intermediary yttrium ammonium fluoride phase into LiYF₄ and subsequent particle ripening. The versatility of the reported method is highlighted by its extension toward the synthesis of other state of the art M(RE)F₄ (M = alkali metal) optical materials: RE³⁺-doped LiYbF₄ microparticles and β-NaGdF₄ and α-NaYF₄ nanoparticles. All of the obtained Yb³⁺/Er³⁺- and Yb³⁺/Tm³⁺-codoped M(RE)F₄ materials exhibited characteristic upconversion emission, while downshifting capabilities were induced through Ce³⁺/Tb³⁺ codoping of LiYF₄. Further attention was devoted to single-particle optical characterization via hyperspectral imaging of Yb³⁺/Er³⁺- and Yb³⁺/Tm³⁺-codoped LiYF₄ microparticles to explore the spatial variability of upconversion emission within individual particles.