Ba₄Ca(B₂O₅)₂F₂: π-conjugation of B₂O₅ in the planar pentagonal layer achieving large second harmonic generation of...

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).