Structure and thermodynamic stability of UTa3O10, a U(v)-bearing compound

Xiaofeng Guo; Christian Lipp; Eitan Tiferet; Antonio Lanzirotti; Matthew Newville; Mark H Engelhard; Di Wu; Eugene S Ilton; Stephen R Sutton; Hongwu Xu; Peter C Burns; Alexandra Navrotsky

doi:10.1039/c6dt02843h

Structure and thermodynamic stability of UTa₃O₁₀, a U(v)-bearing compound

Dalton Trans. 2016 Nov 29;45(47):18892-18899. doi: 10.1039/c6dt02843h.

Authors

Affiliations

¹ Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA and Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, California 95616, USA. anavrotsky@ucdavis.edu.
² Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, USA.
³ Nuclear Research Center - Negev Be'er-Sheva 84190, Israel Institution, Israel.
⁴ Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, USA.
⁵ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
⁶ The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99163, USA.
⁷ Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
⁸ Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, USA and Department of Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, USA.
⁹ Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
¹⁰ Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, USA and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, USA.
¹¹ Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, California 95616, USA. anavrotsky@ucdavis.edu.

PMID: 27722670
DOI: 10.1039/c6dt02843h

Abstract

Heating a mixture of uranyl(vi) nitrate and tantalum(v) oxide in the molar ratio of 2 : 3 to 1400 °C resulted in the formation of a new compound, UTa₃O₁₀. The honey colored to yellow brown crystals of UTa₃O₁₀ crystallize in an orthorhombic structure with the space group Fddd (no. 70), lattice parameters a = 7.3947(1), b = 12.7599(2), c = 15.8156(2) Å, and Z = 8. Vertex sharing [TaO₆]^7- octahedra of two crystallographically distinct Ta cations form a three dimensional tantalate framework. Within this framework, six membered rings of [TaO₆]^7- octahedra are formed within the (001) plane. The center of these rings is occupied by the uranyl cations [UO₂]⁺, with an oxidation state of +5 for uranium. The pentavalence of U and Ta was confirmed by X-ray photoelectron spectroscopy and X-ray adsorption spectroscopy. The enthalpy of formation of UTa₃O₁₀ from Ta₂O₅, β-U₃O₇, and U₃O₈ has been determined to be 13.1 ± 18.1 kJ mol^-1 using high temperature oxide melt solution calorimetry with sodium molybdate as the solvent at 700 °C. The close to zero enthalpy of formation of UTa₃O₁₀ can be explained by closely balanced structural stabilizing and destabilizing factors, which may also apply to other UM₃O₁₀ compounds.