Uranium compounds alpha-Ba-2[UO2(PO4)(2)] (1), beta-Ba-2[UO2(PO4)(2)] (2), and Ba-2[UO2(AsO4)(2)] (3) were synthesized by H3BO3/B2O3 flux reactions, though boron is not incorporated into the structures. Phases I 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 PO43- tetrahedron. Compound 2 contains more normal square bipyramids that share corners with four neighboring PO43- tetrahedra, but the uranyl cation UO22+ is tilted relative to the equatorial plane. Experimental evidence as well as density functional theory (DFT) calculations suggest that I 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 beta-Ba-2[UO2(PO4)(2)] and Ba-2[UO2(AsO4)(2)] indicate a possible evolution of uranyl-based structures from chain to layer type and formation of an accretional series.