Tetragonal zircon-type codoped Yb, Ln–GdVO4 (Ln=Tm, Ho, Er) upconverting nanocrystals with square and rectangular sections were prepared through an efficient low-temperature hydrothermal synthesis. Further processing that combined annealing at 600 °C followed by coating of the surface with a uniform 5 nm-shell of SiO2 resulted in a significant improvement of the intensity of the upconverted emitted visible light following near-infrared (∼980 nm) diode laser excitation with respect to raw hydrothermal nanocrystals. (450 characters)