Hydrothermal synthesis and upconversion luminescence properties of β-NaGdF4:Yb3+/Tm3+ and β-NaGdF4:Yb3+/Ho3+ submicron crystals with regular morphologies

► β-NaGdF4 of hexagonal prisms, spindles, and spheres was synthesized. ► Regular hexagonal prisms have intense UCL. ► Gd3+:6PJ → 8S7/2 transition was found. ► Different spectral distributions for different morphologies were reported.