BaCa(BO)F: π-conjugation of BO in the planar pentagonal layer achieving large second harmonic generation of -borate.

The nonlinear optical (NLO) crystals that can expand the wavelength of the laser to the deep-ultraviolet (DUV) region by the cascaded second harmonic generation (SHG) are of current research interest. It is well known that borates are the most ideal material class for the design of new DUV NLO crystals owing to the presence of good NLO genes, , BO or BO groups. However, the NLO -borates with the BO dimers as the sole basic building units are still rarely reported owing to their small SHG responses.