Structure and optical properties of a new borate K2Ba4Ga4Li2B6O21

• A new borate, K2Ba4Ga4Li2B6O21, was successfully synthesized and grown.
• K2Ba4Ga4Li2B6O21 belongs to noncentrosymmetric hexagonal space group P6¯2c.
• The structure of K2Ba4Ga4Li2B6O21 crystal is closely related to K2Al2B2O7.
• The second-harmonic signal of K2Ba4Ga4Li2B6O21 is half as lager as that of KH2PO4.

A new borate crystal, K2Ba4Ga4Li2B6O21, was successfully synthesized and grown by the spontaneous crystallization with K2O–B2O3–LiF flux. It belongs to a hexagonal space group P6¯2c with cell parameters of a = 8.6544(4) Å, c = 16.9226(16) Å, and Z = 2. The fundamental building units in the structure are the planar BO3 triangle, tetrahedral GaO4 and LiO4 anionic groups. These anions jointed together by corner-sharing O atoms to form a [Ga2Li(BO3)3]2-∞2 layer. The neighboring layers are linked with apical O atoms of GaO4 and LiO4 groups in the c direction to build up a three-dimensional framework. The structure is closely related to that of the nonlinear optical crystal K2Al2B2O7 with two Ga and one Li atoms occupying the three Al sites and simultaneously a mixed K+/Ba2+ and a pure Ba2+ cation replacing the two K sites in an ordered fashion. Second-harmonic generation (SHG) test indicates that the SHG signal of K2Ba4Ga4Li2B6O21 crystal is about half as large as that of KDP standard. The UV–Vis–NIR diffuse reflectance spectra shows that the crystal has a high transmission ranging from 400 nm to 1200 nm with a UV cut-off edge at 226 nm.