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.

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.