Polar polymorphism: β-Ca₂[B₅O₈(OH)]₂[B(OH)₃]·H₂O—synthesis, structure and nonlinear optical property

The hydrothermal reaction of H₃BO₃, CaCO₃ and H₂O led to a new polar polymorphs borate, β-Ca₂[B₅O₈(OH)]₂[B(OH)₃]·H₂O (β-CBO). It represents the first example of alkaline-earth metal borate with the polymorph being polar one under mild hydrothermal reaction. Compared with compound α-CBO (monoclinic system) which we have synthesized before, β-CBO crystallizes in the lower triclinic system with polar space group P1. The arrangement of β-CBO is –AAAA– sequence, while α-CBO is –ABAB– sequence. Further, β-CBO is type-I phase matchable with second-harmonic generation (SHG) responses of ~1.8 times of KH₂PO₄ (KDP), which is larger than α-CBO (~1.3 times of KDP). The dipole moments calculations show that the larger dipole moments make β-CBO exhibit a relatively larger SHG response.