NaRb3B6O9(OH)3(HCO3): A Borate-Bicarbonate Nonlinear Optical Material

A noncentrosymmetric mixed alkali metal borate–bicarbonate, NaRb3B6O9(OH)3(HCO3), was synthesized and characterized. The compound crystallizes in the monoclinic space group P21 (No. 4) with a = 8.988(3) A, b = 8.889(2) A, c = 10.068(4) A, and β = 114.6(4)°. The structure features a combination of chains of boron–oxygen [B6O9 (OH)3]3– groups and isolated HCO3– groups, with charge compensation provided by Rb+ and Na+ cations. It exhibits a second harmonic generation response of about 0.5 × KDP. The UV–vis–NIR absorption spectrum indicated a transparency of about 40% at 200 nm. The IR spectrum confirms the coordination environments of anionic groups, and thermogravimetric measurements indicate the material is thermally stable up to approximately 320 °C. Additionally, first-principles calculations were performed in order to gain insight into the role of boron–oxygen and HCO3– groups with respect to the band structure and NLO properties.