Why Some Noncentrosymmetric Borates Do Not Make Good Nonlinear Optical Materials: A Case Study with KBO(OH).

Synthesis of deep-ultraviolet (DUV) transparent materials, especially those with noncentrosymmetric structures, remains a great challenge in the solid-state chemistry community. A new DUV transparent borate, KBO(OH), was discovered with a short absorption edge below 200 nm. The title compound crystallizes in a noncentrosymmetric, polar space group, Fdd2 (point group mm2), with the following cell parameters: a = 13.736(9) Å, b = 19.317(12) Å, c = 7.606(5) Å. The structure of KBO(OH) features a 3D framework composed of [BO(OH)] basic building units that are linked by contacts with K cations and O-H···O hydrogen bonds. Second harmonic generation (SHG) measurements were performed, and an SHG efficiency of 0.5 × SiO was observed. Symmetry dictates that the χ component of the macroscopic NLO susceptibility is equal to zero in mm2, which prevents the maximal component of the microscopic NLO susceptibility for [BO(OH)] units (χ) from contributing to the macroscopic NLO susceptibility of the crystal and therefore limits the SHG efficiency. In contrast, large SHG effects can be observed from compounds containing [BO] units that crystallize in point groups with nonzero χ, such as 222. These findings provide insight into understanding the relationship between crystal structure and SHG efficiency in [BO]-based compounds and discovering other borate-based DUV materials.