[Sr(H 2 O)]{Sr[B(OH) 3 ]}[B 5 O 8 (OH)] 2 : A Deep-Ultraviolet Second-Order Nonlinear Optical Strontium Borate Constructed by Mixing π-Conjugated and Non-π-Conjugated Units

A new deep-ultraviolet (DUV) second-order nonlinear optical (NLO) strontium borate [Sr(H 2 O)]{Sr[B(OH) 3 ]}[B 5 O 8 (OH)] 2 (SBOH) ( 1 ) was successfully synthesized under hydrothermal conditions. 1 displays a fascinating 2D double-layer architecture which is consisted of the [Sr(H 2 O)] 2+, {Sr[B(OH) 3 ]} 2+ cations and [B 5 O 8 (OH)] 2– anions (FBU: 2□3Δ: −) containing π-conjugated/non-π-conjugated units. 1 was fully characterized by SCXRD/SEM/EDS/PXRD/IR/XPS tests, and its thermal stability, UV–vis–NIR diffuse reflectance spectrum, and second-order NLO performance were investigated in detail. The results showed that 1 possessed the wide transmission range (λ cutoff = 195 nm), large band gap ( E g = 6.36 eV), and enhanced SHG effect (1.16 × KDP, 105–150 μm) that was 2.9 × K 2 Al 2 B 2 O 7, 5.8 × LiNaB 4 O 7, and 1.16 × CsLiB 6 O 10 . The calculations for dipole moments revealed that its SHG response stemmed from the synergistic action of the Bφ 3 (φ = O/OH) (0.967–3.797 D), BO 4 (1.435–5.592 D), SrO 10 (5.521 D), and SrO 11 (2.783 D) polyhedral units. In addition, theoretical calculations (calculated E g: 6.097 eV, partial DOS/total DOS) were well performed. The PDOS/total DOS curves reflected that the top of the VB and the bottom of the CB were mainly occupied by the O 2p orbitals and the Sr 3d/B 2p orbitals, respectively. The studies indicated that 1 could serve as an enhanced DUV second-order NLO material.