DOE OSTI2020
UTa2O7 crystallizes in the orthorhombic Imma space group. The structure is three-dimensional. U4+ is bonded to eight O2- atoms to form distorted UO8 hexagonal bipyramids that share edges with two equivalent UO8 hexagonal bipyramids and edges with six TaO6 octahedra. There are a spread of U–O bond distances ranging from 2.08–2.66 Å. There are two inequivalent Ta5+ sites. In the first Ta5+ site, Ta5+ is bonded to six O2- atoms to form TaO6 octahedra that share corners with six TaO6 octahedra and edges with two equivalent UO8 hexagonal bipyramids. The corner-sharing octahedra tilt angles range from 35–44°. There is two shorter (1.96 Å) and four longer (2.02 Å) Ta–O bond length. In the second Ta5+ site, Ta5+ is bonded to six O2- atoms to form TaO6 octahedra that share corners with six TaO6 octahedra and edges with four equivalent UO8 hexagonal bipyramids. The corner-sharing octahedral tilt angles are 44°. All Ta–O bond lengths are 1.99 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a bent 150 degrees geometry to two equivalent Ta5+ atoms. In the second O2- site, O2- is bonded in a 3-coordinate geometry to one U4+ and two Ta5+ atoms. In the third O2- site, O2- is bonded in a 2-coordinate geometry to two equivalent U4+ and two equivalent Ta5+ atoms. In the fourth O2- site, O2- is bonded in a bent 120 degrees geometry to two equivalent U4+ atoms.