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

Abstract

Sodium yttrium fluoride (NaYF<sub>4</sub>) 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 NaYF<sub>4</sub> 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 Yb<sup>3+</sup> point defects within NaYF<sub>4</sub> 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 NaYF<sub>4</sub>.

Synthesis of NaYF4:Yb, Er up-conversion luminophore from nitrate flux

“…Bard et al [12] attempted to interpret this transformation as a manifestation of non-classical crystal growth by the oriented attachment of nanoparticles. This assumption should be considered untenable [49].…”

“…β-NaRF 4 phases, when codoped with Yb-Er, Yb-Tm, or Yb-Ho ions, are well known as effective up-conversion phosphors that convert IR radiation to the visible spectral range [1,2]. The number of publications devoted to these materials is huge, see for example [3][4][5][6][7][8][9][10][11][12][13]. The main areas of their potential application are biomedical applications and lighting sources [11][12][13].…”

“…β-NaRF 4 powders can be prepared by a broad variety of methods [11,[13][14][15][16]. Until now, several techniques have been employed for the synthesis of hexagonal NaRF 4 ' phases, both individual and co-doped ones, including solidstate synthesis [14,15,17], co-precipitation from water [18][19][20][21][22] and organic solvents [23], hydrothermal [12,24,25], solvothermal [26][27][28][29][30], mechanochemical [31], ionic liquid-assisted [32,33] and molten salt/flux methods [18,[33][34][35][36][37][38][39][40][41][42][43], etc.…”

Hydrothermal Synthesis of Euhedral Co3O4 Nanocrystals via Nutrient-Assisted Topotactic Transformation of the Layered Co(OH)2 Precursor under Anoxic Conditions: Insights into Intricate Routes Leading to Spinel Phase Development and Shape Perfection

“…Prior formation of a mesocrystal, as a requisite midway step of the latter process, has been suggested previously. 76,77 In the case of the hydrothermal synthesis with a high concentration of the nitrate precursor, the OH − ligand exchange by nitrates in the precipitated β-Co(OH) 2 layers is quick, and within 0.5 h the Co 2 (NO 3 )(OH) 3 phase dominates the reaction mixture (Figure 5b). Subsequent oxidation of the divalent cobalt cations in the Co 2+ (NO…”