How are Centrosymmetric and Noncentrosymmetric Structures Achieved in Uranyl Borates?

Four uranyl borates, UO₂B₂O₄ (UBO-1), α-(UO₂)₂[B₉O₁₄(OH)₄] (UBO-2), β-(UO₂)₂[B₉O₁₄(OH)₄] (UBO-3), and (UO₂)₂[B₁₃O₂₀(OH)₃]·1.25H₂O (UBO-4), have been prepared from boric acid fluxes at 190 °C. UBO-3 and UBO-4 are centrosymmetric, whereas UBO-1 and UBO-2 are noncentrosymmetric (chiral and polar). These uranyl borates possess layered structures constructed from UO₈ hexagonal bipyramids, BO₃ triangles, and BO₄ tetrahedra. In the case of UBO-4, clusters of BO₃ triangles link the layers together to form open slabs with a thickness of almost 2 nm. The ability of uranyl borates to use very similar layers to yield both centrosymmetric and noncentrosymmetric layers is detailed in this work.