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revised on 21.01.2020 and posted on 22.01.2020by Alex Bard, Xuezhe Zhou, Xiaojing Xia, Guomin Zhu, Matthew Lim, Seung Min Kim, Matthew Johnson, Justin Kollman, Matthew Marcus, Steven Spurgeon, Daniel Perea, Arun Devaraj, Jaehun Chun, James De Yoreo, Peter Pauzauskie
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.