The role of polar, lamdba (λ)-shaped building units in noncentrosymmetric inorganic structures

A methodology for the design of polar, inorganic structures is demonstrated here with the packing of lambda (λ)-shaped basic building units (BBUs). Noncentrosymmetric (NCS) solids with interesting physical properties can be created with BBUs that lack an inversion center and are likely to pack into a polar configuration; previous methods to construct these solids have used NCS octahedra as BBUs. Using this methodology to synthesize NCS solids, one must increase the coordination of the NCS octahedra with maintenance of the noncentrosymmetry of the bulk. The first step in this progression from an NCS octahedron to an inorganic NCS solid is the formation of a bimetallic BBU. This step is exemplified with the compound CuVOF ₄(H ₂O) ₇: this compound, presented here, crystallizes in an NCS structure with ordered, isolated [Cu(H ₂O) ₅] ²⁺ cations and [VOF ₄(H ₂O)] ^2- anions into λ-shaped, bimetallic BBUs to form CuVOF ₄(H ₂O) ₆· H ₂O, owing to the Jahn-Teller distortion of Cu ²⁺. Conversely, the centrosymmetric heterotypes with the same formula MVOF ₄(H ₂O) ₇ (M ^II = Co, Ni, and Zn) exhibit ordered, isolated [VOF ₄(H ₂O)] ^2- and [M(H ₂O) ₆] ²⁺ ionic species in a hydrogen bond network. CuVOF ₄(H ₂O) ₇ exhibits a net polar moment while the heterotypes do not; this demonstrates that λ-shaped BBUs give a greater probability for and, in this case, lead to NCS structures.