Na₃B₆O₁₀(HCOO): an ultraviolet nonlinear optical sodium borate-formate

A new sodium borate-formate, Na₃B₆O₁₀(HCOO) (1), has been successfully synthesized in a surfactant-thermal system. Compound 1 crystallizes in the orthorhombic system, with the polar P2₁2₁2₁ space group and the parameters a = 7.6375(8) Å, b = 9.9345(9) Å, c = 12.7231(13) Å, V = 965.36(17) ų, and Z = 4. Compound 1 contains basic building units of (B₆O₁₃)⁸⁻, and each (B₆O₁₃)⁸⁻ unit connects with six other neighboring units and forms a novel 3D open-framework with 9R channels. The channels accommodate Na⁺ ions and HCOO⁻ units. The whole framework can be simplified as a pcu-type net. Compound 1 shows a moderate second-harmonic-generation (SHG) signal, a short cut-off edge in the ultraviolet (UV) region, and a suitable birefringence of 0.062 at 1064 nm. These characteristics reveal that compound 1 is a promising UV nonlinear optical crystal. Density functional theory calculations show that the B-O and HCOO⁻ units are responsible for the nonlinear optical properties of 1.