Assessment of the U<sub>3</sub>O<sub>7</sub> Crystal Structure by X‑ray and Electron Diffraction
dataset
posted on 2016-09-20, 14:24 authored by Gregory Leinders, Rémi Delville, Janne Pakarinen, Thomas Cardinaels, Koen Binnemans, Marc VerwerftPolycrystalline
U<sub>3</sub>O<sub>7</sub> powder was synthesized
by oxidation of UO<sub>2</sub> powder under controlled conditions
using in situ thermal analysis, and by heat treatment in a tubular
furnace. The O/U ratio of the U<sub>3</sub>O<sub>7</sub> phase was
measured as 2.34 ± 0.01. The crystal structure was assessed from
X-ray diffraction (XRD) and selected-area electron diffraction (SAED)
data. Similar to U<sub>4</sub>O<sub>9−ε</sub> (more precisely
U<sub>64</sub>O<sub>143</sub>), U<sub>3</sub>O<sub>7</sub> exhibits
a long-range ordered structure, which is closely related to the fluorite-type
arrangement of UO<sub>2</sub>. Cations remain arranged identical to
that in the fluorite structure, and excess anions form distorted cuboctahedral
oxygen clusters, which periodically replace the fluorite anion arrangement.
The structure can be described in an expanded unit cell containing
15 fluorite-like subcells (U<sub>15</sub>O<sub>35</sub>), and spanned
by basis vectors <b>A</b> = <b>a</b><sub><b>p</b></sub> – 2<b>b</b><sub><b>p</b></sub>, <b>B</b> = −2<b>a</b><sub><b>p</b></sub> + <b>b</b><sub><b>p</b></sub>, and <b>C</b> = 3<b>c</b><sub><b>p</b></sub> (lattice parameters of the subcell are <i>a</i><sub>p</sub> = <i>b</i><sub>p</sub> = 538.00
± 0.02 pm and <i>c</i><sub>p</sub> = 554.90 ±
0.02 pm; <i>c</i><sub>p</sub>/<i>a</i><sub>p</sub> = 1.031). The arrangement of cuboctahedra in U<sub>3</sub>O<sub>7</sub> results in a layered structure, which is different from the
well-known U<sub>4</sub>O<sub>9−ε</sub> crystal structure.
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