Ba4Ca(B2O5)2F2: π-conjugation of B2O5 in the planar pentagonal layer achieving large second harmonic generation of pyro-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, e.g., BO3 or B3O6 groups. However, the NLO pyro-borates with the B2O5 dimers as the sole basic building units are still rarely reported owing to their small SHG responses. In this communication, by constructing a planar pentagonal [Ca(B2O5)]∞ layer, the NLO pyro-borate Ba4Ca(B2O5)2F2 with a large SHG response (∼2.2 × KDP, or ∼7 × α-Li4B2O5) and a DUV transparent window has been designed and synthesized. The first-principles calculations show that the large SHG response of Ba4Ca(B2O5)2F2 mainly originates from the better π-conjugation of the coplanar B2O5 dimers in the [Ca(B2O5)]∞ layer. In addition, the planar pentagonal pattern in the [Ca(B2O5)]∞ layer provides an ideal template for designing the new DUV NLO crystals, apart from those in known DUV borates, e.g., the [Be2BO3F2]∞ layer in KBe2BO3F2 (KBBF).