Non-Classical Hydrothermal Synthesis of Hexagonal Sodium Yttrium Fluoride Nanowires

25 October 2019, Version 1

Abstract

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 and cryogenic 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 oriented attachment of grains of the cubic (α) phase. EXAFS spectroscopy also suggests that substitutional Yb3+ point defects within NaYF4 are distributed evenly throughout the crystal lattice without clustering, and also that they selectively substitute into one of the two possible trivalent yttrium sites in the unit cell.

Keywords

sodium yttrium fluoride
UCNP
NaYF4
sodium yttrium fluoride nanocrystals
transmission electron microcopy
x-ray diffraction
EXAFS
crystal growth
Phase Conversion
oriented attachment
Non-classical crystal growth
Liquid cell TEM
In situ TEM
Hydrothermal Synthesis

Supplementary materials

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