A Mechanistic Understanding of Non-Classical Crystal Growth in Hydrothermally Synthesized Sodium Yttrium Fluoride Nanowires

Sodium yttrium fluoride (NaYF₄) is an upconverting material with many potential uses in chemistry, materials science, and biology that can be synthesized hydrothermally in both cubic (α) and hexagonal (β) crystallographic polymorphs. Understanding the mechanisms underlying the phase conversion between the cubic and hexagonal polymorphs is of great interest to help inform future efforts to synthesize atomically-precise quantum materials with well-defined sizes and morphologies. In this work, we use a combination of analytical transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), powder X-ray diffraction (XRD), in situ liquid cell TEM, atom probe tomography (APT), and extended x-ray absorption fine structure (EXAFS) measurements to show evidence suggesting that the hexagonal NaYF₄ nanowires form through a non-classical crystal growth mechanism involving the formation and subsequent oriented attachment of mesocrystals consisting of cubic (α) plase units. EXAFS spectroscopy also suggests that substitutional Yb³⁺ point defects within NaYF₄ are distributed evenly throughout the crystal lattice without clustering, and also that they may exhibit selective substitution into one of the two possible trivalent yttrium sites in the unit cell for hydrothermally synthesized NaYF₄.