title: Highly distorted uranyl ion coordination and one/two-dimensional structural relationship in the Ba2[UO2(TO4)2] (T = P, As) system: an experimental and computational study.
abstract: Uranium compounds α-Ba2[UO2(PO4)2] (1), β-Ba2[UO2(PO4)2] (2), and Ba2[UO2(AsO4)2] (3) were synthesized by H3BO3/B2O3 flux reactions, though boron is not incorporated into the structures. Phases 1 and 2 are topologically identical, but 1 is heavily distorted with respect to 2. An unusual UO7 pentagonal bipyramid occurs in 1, exhibiting a highly distorted equatorial configuration and significant bending of the uranyl group, due to edge-sharing with one neighboring PO4(3-) tetrahedron. Compound 2 contains more normal square bipyramids that share corners with four neighboring PO4(3-) tetrahedra, but the uranyl cation UO2(2+) is tilted relative to the equatorial plane. Experimental evidence as well as density functional theory (DFT) calculations suggest that 1 is more stable than 2. In theory, 1 and 2 can interconvert by forming/releasing the shared edge between the uranyl polyhedron and the phosphate tetrahedron. Similar fundamental building blocks in β-Ba2[UO2(PO4)2] and Ba2[UO2(AsO4)2] indicate a possible evolution of uranyl-based structures from chain to layer type and formation of an accretional series.
authors.affiliation: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences , 510640 Guangzhou, China.
authors.forename: Shijun
authors.initials: S
authors.lastname: Wu
authors.forename: Piotr M
authors.initials: PM
authors.lastname: Kowalski
authors.forename: Na
authors.initials: N
authors.lastname: Yu
authors.forename: Thomas
authors.initials: T
authors.lastname: Malcherek
authors.forename: Wulf
authors.initials: W
authors.lastname: Depmeier
authors.forename: Dirk
authors.initials: D
authors.lastname: Bosbach
authors.forename: Shuao
authors.initials: S
authors.lastname: Wang
authors.forename: Evgeny V
authors.initials: EV
authors.lastname: Suleimanov
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Albrecht-Schmitt
authors.forename: Evgeny V
authors.initials: EV
authors.lastname: Alekseev