From [B₆O₁₃]⁸⁻to [GaB₅O₁₃]⁸⁻to [Ga{B₅O₉(OH)}{BO(OH)₂}]²⁻: synthesis, structure and nonlinear optical properties of new metal borates

Three new metal borates, namely K_0.5[B₆O₁₀]·H₂O·1.5H₃O (1), [GaB₅O₁₀]·H₂en (2) and NaCs[Ga{B₅O₉(OH)}{BO(OH)₂}] (3), have been synthesized under solvothermal conditions. Compound1features an intricate 3D framework composed of [B₆O₁₃]⁸⁻clusters, resulting in a 6-connected achiralpcunet. After introducing the organic gallium, one of the BO₄groups of the [B₆O₁₃]⁸⁻cluster is substituted by a GaO₄polyhedron to form the acentric [GaB₅O₁₃]⁸⁻cluster in2. It also displays a high second-harmonic generation (SHG) response about approximately 2.6 times that of KH₂PO₄(KDP), which is higher than those of alumino-borates (ABOs) containing the acentric [AlB₅O₁₃]⁸⁻cluster (0.3-2.1 times that of KDP). Dipole moment calculations have been carried out to confirm these results. In the process of trying to synthesize a new compound with a denser structure than2, we have unexpectedly obtained3crystallized in the centrosymmetric (CS) space group, which shows a 1D structure constructed from the [Ga{B₅O₉(OH)}{BO(OH)₂}]²⁻cluster. This kind of low-dimensional structure is rare in gallo-borates (GBOs) as well as ABOs.