Ostwald Ripening, Particle Size Focusing, and Decomposition of Sub-10 nm NaREF₄ (RE = La, Ce, Pr, Nd) Nanocrystals

NaREF 4 nanocrystals of the lighter rare earth elements RE = La, Ce, Pr, Nd are more difficult to prepare than those of the heavier lanthanides because the materials are sensitive to decomposition. Here, we present a procedure for the synthesis of sub-10 nm particles of β-NaLaF 4, β-NaCeF 4, β-NaPrF 4, and β-NaNdF 4, as well as of α-NaCeF 4, α-NaPrF 4, and α-NaNdF 4 . These particles were used to study the decomposition reaction taking place in the standard oleic acid/octadecene solvent. Ostwald ripening of the β-phase particles in this solvent leads to the expected particle growth and broadening of the particle size distribution only in the case of RE = Ce, Pr, Nd, whereas particles of NaLaF 4 decompose to LaF 3 . Ostwald ripening of the α-phase particles in oleic acid/octadecene yields two products: larger particles of the hexagonal β-phase with a very narrow size distribution and smaller particles of the rare earth trifluoride REF 3 . The yield of the nearly monodisperse β-NaREF 4 particles decreases from Nd to Ce and becomes negligible for the latter. X-ray fluorescence analysis reveals that the decomposition into the REF 3 phase is caused by a strong sodium deficiency of the cubic α-NaREF 4 nanocrystals, which is observed even if these particles are prepared in the presence of a 6-fold excess of sodium ions. We show that larger particles of phase-pure β-NaCeF 4, β-NaPrF 4, and β-NaNdF 4 can nevertheless be prepared by Ostwald ripening of the corresponding cubic α-NaREF 4 particles, if the sodium deficiency of the latter is compensated for during their conversion to the β-phase.