Cubic nano-spheres and hexagonal microprisms of sodium yttrium tetrafluoride (NaYF4) have successfully been prepared separately via a mild hydrothermal route. The morphology and the structure of the NaYF4 crystals can be changed by adjusting the Y3+/F- ratio in the reaction solution. Cubic NaYF4 nano-spheres are synthesized when the Y3+/F-ratio is stoichiometric; while hexagonal NaYF4 microprisms appear as excess F- is present at 180 degrees C for more than 5 h. Ethylenediamine tetra-acetic acid (EDTA) plays an important role as a chelating ligand, and F- acts as both a reactant and a mineralizer during the synthesis process. A growth mechanism is discussed. (c) 2006 Elsevier B.V. All rights reserved.