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

Abstract

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. Compound 1 features an intricate 3D framework composed of [B₆O₁₃]⁸⁻ clusters, resulting in a 6-connected achiral pcu net. After introducing the organic gallium, one of the BO₄ groups of the [B₆O₁₃]⁸⁻ cluster is substituted by a GaO₄ polyhedron to form the acentric [GaB₅O₁₃]⁸⁻ cluster in 2. 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 than 2, we have unexpectedly obtained 3 crystallized 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.