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

Abstract

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.

Thermochemistry of 3D and 2D Rare Earth Oxychlorides (REOCls)

“…This is because slow dissolution and the lack of precisely controlled reaction conditions could have introduced complexities, making this data less reliable. − Thermodynamic evaluations have been attempted on REOCl systems to estimate the formation enthalpies, but limited and ambiguous experimental data reduce the reliability of such studies and restrict them to compounds having the PbFCl-type structure. − In the present work, the formation enthalpies of REOCls were determined from light to heavy RE elements (La, Nd, Gd, Y, and Ho–Lu) by high-temperature oxide melt solution calorimetry. This calorimetric technique has been widely employed in oxide compounds and was recently applied to fluoride and oxyfluoride systems. ,, The methodology was extended and proven to be effective for chloride and oxychloride compounds in this work. Experimental thermodynamic data were used in Born–Haber cycles to calculate the lattice energies of REOCls, shedding light on the effects of RE ionic size on the energetics of 3D and 2D RE materials.…”

“…This calorimetric technique has been widely employed in oxide compounds and was recently applied to fluoride and oxyfluoride systems. 12,34,35 The methodology was extended and proven to be effective for chloride and oxychloride compounds in this work. Experimental thermodynamic data were used in Born−Haber cycles to calculate the lattice energies of REOCls, shedding light on the effects of RE ionic size on the energetics of 3D and 2D RE materials.…”