Assessment of the U 3 O 7 Crystal Structure by X‑ray and Electron Diffraction Gregory Leinders,* ,†,‡ Re ́ mi Delville, † Janne Pakarinen, † Thomas Cardinaels, †,‡ Koen Binnemans, ‡ and Marc Verwerft † † Belgian Nuclear Research Centre (SCK·CEN), Institute for Nuclear Materials Science, Boeretang 200, B-2400 Mol, Belgium ‡ Department of Chemistry, KU Leuven, Celestijnenlaan 200F, P.O. Box 2404, B-3001 Heverlee, Belgium * S Supporting Information ABSTRACT: Polycrystalline U 3 O 7 powder was synthesized by oxidation of UO 2 powder under controlled conditions using in situ thermal analysis, and by heat treatment in a tubular furnace. The O/U ratio of the U 3 O 7 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 4 O 9-ε (more precisely U 64 O 143 ), U 3 O 7 exhibits a long-range ordered structure, which is closely related to the fluorite-type arrangement of UO 2 . 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 15 O 35 ), and spanned by basis vectors A = a p - 2b p , B = -2a p + b p , and C =3c p (lattice parameters of the subcell are a p = b p = 538.00 ± 0.02 pm and c p = 554.90 ± 0.02 pm; c p /a p = 1.031). The arrangement of cuboctahedra in U 3 O 7 results in a layered structure, which is different from the well-known U 4 O 9-ε crystal structure. 1. INTRODUCTION Oxidation of UO 2 in dry air at temperatures above about 200 °C results in formation of U 3 O 8 (O/U = 2.667), the thermodynamically more stable oxide of uranium. 1 The formation of U 3 O 8 from UO 2 is associated with a volume increase of about 36%. This transformation is an important threat for the integrity of storage containers for UO 2 , especially when considering long-term storage and final repository of irradiated nuclear fuels. 1-4 The oxidation behavior of UO 2 at ambient to medium temperatures up to 300 °C has been investigated already for many decades, and novel insights continue to be obtained. 5-14 A wide variety of intermediate oxides can be formed by oxidation of UO 2 under different conditions. 1,15 Compounds with an O/U ratio between 2 and 2.5 have structures in which the cation arrangement remains closely related to the original fluorite-type UO 2 structure, the most notable change being a deviation from cubic symmetry with increasing oxidation. 16 In the broad hyperstoichimetric range of compositions, commonly referred to as UO 2+x (O/U < 2.234), the excess oxygen develops randomly distributed defects, and the structure can be described as defective cubic fluorite. 17-20 At the composition O/U = 2.234 (“U 4 O 9 ”), an ordered superstructure develops, which also has cubic symmetry. 21,22 The compound thus formed is usually assigned the nominal formula U 4 O 9 ; however, it should be recognized that this does not correspond exactly with the structural composition U 64 O 143 . 23 Throughout the text, the notation U 4 O 9-ε , where ε = 0.0625, is used to refer to this compound. When the O/U ratio exceeds 2.234 the symmetry is lowered, 24 but the atomic arrangement remains closely related to the fluorite arrangement until U 3 O 8 is formed, which has a different crystal structure. 25,26 The compounds intermediate to U 4 O 9-ε and U 3 O 8 have been assigned tentative formulas on the basis of thermogravimetric data (e.g., U 3 O 7 , U 2 O 5 ). 27,28 Uranium oxides with an O/U ratio close to 2.333 are commonly referred to as U 3 O 7 , 29 despite possible variations in composition. Their crystal structure is characterized by a distortion of the fluorite-type cubic structure to tetragonal symmetry. In the absence of detailed crystallographic information, the accepted criterion for identification has been the axial ratio c/a. A few early studies reported the existence of two main polymorphs: α-U 3 O 7 (c/a ≈ 0.986) and β-U 3 O 7 (c/a ≈ 1.031). 28,30 However, more recent studies have questioned the existence of α-U 3 O 7 , suggesting that early workers failed to differentiate it from the cubic U 4 O 9-ε phase, which is formed at the earlier stage of oxidation. 13,31 Variations in axial ratio (1 < c/a ≤ 1.031) are regularly reported, but never exceed c/a ≈ 1.031, that is, the value for β-U 3 O 7 . 9,13,31-35 In what follows, the notation U 3 O 7 will be used to refer to this state. Received: August 10, 2016 Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.6b01941 Inorg. Chem. XXXX, XXX, XXX-XXX