Why Some Noncentrosymmetric Borates Do Not Make Good Nonlinear Optical Materials: A Case Study with K3B5O8(OH)2

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, K 3 B 5 O 8 (OH) 2, was discovered with a short absorption edge below 200 nm. The title compound crystallizes in a noncentrosymmetric, polar space group, Fdd 2 (point group mm 2), with the following cell parameters: a = 13.736(9) Å, b = 19.317(12) Å, c = 7.606(5) Å. The structure of K 3 B 5 O 8 (OH) 2 features a 3D framework composed of [B 5 O 8 (OH) 2 ] 3– 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 2 was observed. Symmetry dictates that the χ 14 component of the macroscopic NLO susceptibility is equal to zero in mm 2, which prevents the maximal component of the microscopic NLO susceptibility for [B 5 O 8 (OH) 2 ] 3– units (χ 14 ) 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 [B 5 O 10 ] units that crystallize in point groups with nonzero χ 14, such as 222. These findings provide insight into understanding the relationship between crystal structure and SHG efficiency in [B 5 O 10 ]-based compounds and discovering other borate-based DUV materials.