Thermochemical Investigation of the Stability and Conversion of Nanocrystalline and High-Temperature Phases in Sodium Neodymium Fluorides

As important upconversion materials, sodium rare-earth fluorides (nominally NaREF₄ in composition but actually often harboring sodium deficiency, especially in nanophase materials) have been subjected to intensive studies, particularly in the synthesis and applications of nanocrystals. However, the mechanisms of the conversion between the two phases (α and β) of NaREF₄ nanocrystals during the synthesis are still controversial and lack thermodynamic investigations, which limit the rational design, synthesis, and processing of these materials. In this work, aiming at NaREF₄ with light rare-earth elements, the thermochemistry of the NaF–NdF₃ system, including the α and β phases in nanocrystalline/nanophase and bulk stoichiometric samples, is systematically studied by thermogravimetry and differential scanning calorimetry and high-temperature oxide melt solution calorimetry. With the help of compositional analysis and structural characterization, a strong Na deficiency is found in nanocrystals with small crystal sizes, which leads to the formation of cubic (α) crystallographic polymorphs at the nucleation stage, possibly because of the relative thermodynamic stability of the α phase compared to the β phase in such compositions. After converting to the hexagonal (β) structure, the crystal growth is accompanied by an increase of Na content in nanocrystals with increasing energetic stability until the formation of the stoichiometric compound (β-NaNdF₄). On the contrary, the stoichiometric α phase (α-NaNdF₄) is metastable at room temperature but is the intermediate phase as the temperature increases. We show that the α → β phase conversion in aqueous solution synthesis is distinct from the β → α transition driven by temperature because of composition differences.