Hexagonal Sodium Yttrium Fluoride Based Green and Blue Emitting Upconversion Phosphors

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…