Two deep-ultraviolet nonlinear optical alkaline-earth metal borates based on different types of oxoboron clusters

Two non-centrosymmetric alkaline-earth metal borates, Ca₂[B₅O₈(OH)]₂[B(OH)₃]·H₂O (1) and Ba₂[B₆O₉(OH)₂][B₃O₃(OH)₄]·(OH)·4H₂O (2), have been synthesized under solvothermal conditions. Both of them exhibit layered structures constructed from two fundamental building blocks (FBBs), respectively. 1 features porous double-layers made by the acentric single A-layer and B-layer in a face-to-face arrangement. Ca-O layers gather the B-O double-layers together resulting in the chiral structure of 1. Compound 2 contains two different FBBs, which is rare in borate. Hexaborate B₆ clusters are responsible for the expansion of inorganic B₆-layers while the isolated triborate B₃ clusters are arranged in supermolecular B₃-layers. Ba1-B₆-layers and Ba2-B₃-layers are alternately arranged in the same direction generating the NCS structure of 2. Both 1 and 2 are phase-matchable materials with second-harmonic generation (SHG) responses of ∼1.3 and ∼0.8 times that of KDP (KH₂PO₄), respectively. UV-vis diffuse reflectance spectra reveal that they have short cut-off edges lower than 200 nm. These characteristics make them potential deep-UV NLO materials. The calculations reveal that the SHG efficiency mainly originates from the synergistic effect of the distorted MO_n (M = Ca, Ba; n = 7, 10, 11) polyhedra and the asymmetric π-conjugated of BO₃ units.