Two deep-ultraviolet nonlinear optical barium borates framework: Alkali metal enhances the second-harmonic generation response

Two non-centrosymmetric barium borates with acs-type net, namely Ba₃ [B₉O₁₆][B(OH)₄]·2H₂O (1) and NaBa₃(OH)[B₉O₁₆][B(OH)₄] (2) have been synthesized under hydrothermal conditions. Compound 1 is type-I phase matchable with second-harmonic generation (SHG) response of approximately 1.6 times than that of KDP (KH₂PO₄). After Na⁺ cation has been successfully introduced into the oxoboron cluster backbone, compound 2 displays a larger SHG response about approximately 2.0 times of KDP. The calculations show that the large local dipole moments of distorted NaO₇ polyhedrons make 2 exhibits a relatively larger SHG response. The ultraviolet–visible (UV–Vis) diffuse reflectance spectra show that both 1 and 2 have a wide transparency range with UV cutoff edge below 200 ​nm. These properties reveal that the introduction of different size and charge alkali metal on the basis of alkaline earth metal borate is beneficial to obtain a promising deep-ultraviolet nonlinear optical (NLO) material with a larger SHG response.