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Hexagonal sodium yttrium fluoride, NaYF4, is the most efficient host material to date for green (Yb3+/Er3+ doped) and blue (Yb3+/Tm3+ doped) upconversion (UC) phosphors, i.e., phosphors which emit visible light upon infrared (IR) excitation. The structure of the hexagonal phase gives rise to controversy about the cation sites and distribution. The X-ray diffraction patterns of our phosphors do not fit well with the crystal structure reported for NaNdF4 (space group P6̄, Z = 1.5). The Na:M ratio (M = Y, Nd, Er, Tm, Yb) of the hexagonal phase deviates significantly from 1:1, and it depends on M and the preparation temperature. It is proposed that the hexagonal phase is isostructural to the chlorides Na3xM2-xCl6 with M = La−Sm. This structure (space group P63/m, Z = 1) contains only one M3+ site which is partially occupied by Na+, and the formula Na3xM2-xF6 (x ≈ 0.45) accounts for the nonstoichiometry. The model was derived from powder X-ray diffraction on the green and blue phosphor materials as well as the pure Nd, Y, and Yb compounds. The light emission properties of the material crucially depend on the phase purity, doping ratio, Na:M ratio, and preparation temperature. Because earlier spectroscopic investigations generally suffered from impure materials which reduced the UC efficiency, a new, reproducible preparation route was developed. Its key features are an excess of NaF in the synthesis, i.e., a 2:1 ratio of Na:M, the use of HF gas for fluorination, and a process temperature lower than the melting point of the respective NaF/MF3 eutecticum. It yields the pure hexagonal phase without admixture of the cubic phase or other impurities except for minor inclusions of NaF. Upon IR excitation at 10 245 cm-1, the samples doped with 18% Yb + 2% Er and 25% Yb + 0.3% Tm showed the highest UC efficiencies for green and blue emission, respectively. Relative UC efficiencies were measured with a powder test setup in a standard procedure. The doping ratios and all steps of the synthesis were optimized with respect to the UC efficiency. The obtained phosphor materials show no degradation under high-power IR laser excitation.