Hydrothermal synthesis of the intense green photoluminescence of hexagonal phase NaYF4:Yb3+/Er3+ microcrystals

► β-NaYF4:Yb3+/Er3+ microcrystals are synthesized via a facile hydrothermal method. ► RNaF/Ln and pH value significantly affect crystal structure and morphology. ► EDTA contributes to the formation of β-NaYF4:Yb3+/Er3+ microprisms. ► The introduction of EDTA negatively affects the UC efficiency. ► The UC efficiency is related to crystallization degree, morphology, and size.