[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(H₂O)]{Sr[B(OH)₃]}[B₅O₈(OH)]₂ (SBOH) (1) was successfully synthesized under hydrothermal conditions. 1 displays a fascinating 2D double-layer architecture which is consisted of the [Sr(H₂O)]²⁺, {Sr[B(OH)₃]}²⁺ cations and [B₅O₈(OH)]^2- anions (FBU: 2□3Δ: <Δ2□>-<2Δ□>) 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 × K₂Al₂B₂O₇, 5.8 × LiNaB₄O₇, and 1.16 × CsLiB₆O₁₀. The calculations for dipole moments revealed that its SHG response stemmed from the synergistic action of the Bφ₃ (φ = O/OH) (0.967-3.797 D), BO₄ (1.435-5.592 D), SrO₁₀ (5.521 D), and SrO₁₁ (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.