The structure of the hexagonal phase of NaYF4 shows two crystallograph
ic sites of C3h symmetry for Nd3+ ions incorporated in this host and a
detailed spectroscopic study of NaYF4:Nd3+ leads to the energy level
scheme of Nd3+ ions in each site. The lifetimes of the 4F3/2, 2P3/2, 4
D3/2 and 2F(2)5/2 emitting levels are also measured.