nanocrystals (0 ≤ x ≤ 1): growth, size control and shell formation on β-NaCeF4:Tb core particles

Abstract

is an interesting shell material for β-NaREF₄ particles of the lighter lanthanides (RE = Ce, Pr, Nd), as variation of its strontium content x allows to vary its lattice parameters and match those of the core material. It crystallizes in the hexagonal gagarinite structure (NaCaYF₆) which is closely related to the hexagonal structure of β-NaREF₄, known from the upconversion host lattices β-NaYF₄ and β-NaGdF₄. Here, we studied the formation of nanocrystals over the entire range 0 ≤ x ≤ 1. Heating of a solution of the metal oleates in oleic acid/octadecene in the presence of NH₄F results in the formation of small (≈3 nm) particles as the first product. For x ≤ 0.2, these particles mainly consist of the hexagonal gagarinite phase whereas for x > 0.2 most particles crystallize in a cubic lattice resembling the structure of α-NaGdF₄. Further heating at 320 °C leads to dissolution of the cubic phase and formation of larger particles of the hexagonal phase. Both the time required for complete dissolution as well as the final size of the particles systematically grows with increasing strontium content x, showing that high values of x decrease the number of hexagonal seeds formed. The lattice parameters of the hexagonal phase are found to increase linearly with x and become, for high values of x, similar to those of the hexagonal phase of NaCeF₄, NaPrF₄, and NaNdF₄. Small particles of the cubic phase of NaGdSrF₆ are shown to be suitable precursors to form a gagarinite shell on β-NaCeF₄:25% Tb particles.