Controllable synthesis and up-conversion properties of tetragonal BaYF5:Yb/Ln (Ln = Er, Tm, and Ho) nanocrystals

► A facile and effective hydrothermal route was used to fabricate fluorescent nanocrystals. ► Uniform nanocrystals with spherical morphology and excellent up-conversion properties were obtained. ► The possible formation scheme for the nanocrystals prepared at different conditions is presented.