Sr₂[B₅O₈(OH)]₂ ⋅ [B(OH)₃] ⋅ H₂O: A Strontium Borate That Shows Deep-Ultraviolet-Transparent Nonlinear Optical Properties

A new noncentrosymmetric strontium borate, P1-Sr₂[B₅O₈(OH)]₂ ⋅ [B(OH)₃] ⋅ H₂O (1), has been synthesized under the hydrothermal condition. The P1-Sr₂[B₅O₈(OH)]₂ ⋅ [B(OH)₃] ⋅ H₂O shows a layered B-O network with 9-ring windows in the ab plane. Sr²⁺ cations, H₃BO₃, and H₂O molecules are located in the voids of layers and interlayers, respectively. The P1-Sr₂[B₅O₈(OH)]₂ ⋅ [B(OH)₃] ⋅ H₂O is the first synthetic phase of veatchite, while the other three polymorphs are found in different natural minerals. This strontium borate is a potential deep-ultraviolet-transparent nonlinear-optical (NLO) crystal whose second-harmonic-generation (SHG) intensity is 1.7 times that of KH₂PO₄ (KDP) and is phase-matchable. The short wavelength cutoff edge of compound 1 is below 190 nm. Density functional theory (DFT) calculations show that the B-O units are responsible for the nonlinear optical property.