An acentric calcium borate Ca₂[B₅O₉]·(OH)·H₂O: Synthesis, structure, and nonliner optical property

A novel noncentrosymmetric calcium borate, Ca₂[B₅O₉]·(OH)·H₂O (1), was synthesized under solvothermal condition using mixed solvents of pyridine and H₂O. Compound 1 crystallizes in the monoclinic space group Cc. Its structure contains [B₅O₁₂] units and features a three-dimensional (3D) pcu net with nine-membered ring (9-MR) channels along the b-axis, where the Ca²⁺ cations, OH^- ions, and H₂O molecules are located. Each Ca polyhedron shares three edges and one vertex with four neighbors to form a 3D dia Ca-O network. The pcu B-O net and dia Ca-O net are further interpenetrated to give the final denser net. The second harmonic generation (SHG) measurement shows that compound 1 is a type I phase-matchable material with a strong SHG response of ∼3 times that of KH₂PO₄. In addition, it exhibits a wide transparency range with a short UV cutoff edge below 200 nm. These results reveal that the compound is a potential deep-UV nonlinear optical material. The Vienna ab initio theoretical studies indicate the good SHG response is derived from the synergistic effect of the conjugated systems of BO₃ groups and distorted CaO₉ polyhedra.