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

Abstract

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.