NaGdF4:Eu3+, NaEuF4, and NaGdF4 nanocrystals were synthesized in the high-boiling coordinating solvent N-(2-hydroxyethyl)-ethylenediamine (HEEDA). Phase pure nanomaterials, crystallizing either in the cubic α-phase or the hexagonal β-phase, were obtained by adjusting one reaction parameter only, i.e., the molar ratio between metal and fluoride ions in the synthesis. The hexagonal β-phase is formed, if this molar ratio is close to stoichiometric, whereas the cubic α-phase is obtained in the presence of excess metal ions. The optical properties of the Eu3+ doped samples are different for the two crystal phases. The results indicate an increased number of oxygen impurities close to Eu3+ ions, if excess metal ions are used in the synthesis.