Uranium compounds α-Ba 2 [UO 2 (PO 4 ) 2 ] (1), β-Ba 2 [UO 2 (PO 4 ) 2 ] (2), and Ba 2 [UO 2 (AsO 4 ) 2 ] (3) were synthesized by H 3 BO 3 /B 2 O 3 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 UO 7 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 PO 4 3− tetrahedron. Compound 2 contains more normal square bipyramids that share corners with four neighboring PO 4 3− tetrahedra, but the uranyl cation UO 2 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-Ba 2 [UO 2 (PO 4 ) 2 ] and Ba 2 [UO 2 (AsO 4 ) 2 ] indicate a possible evolution of uranyl-based structures from chain to layer type and formation of an accretional series.