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

Automated summary

A new polar polymorph borate, beta-Ca-2[B5O8(OH)](2)[B(OH)(3)]center dot H2O (beta-CBO), was synthesized through hydrothermal reaction, showing that alkaline-earth metal borate could have a polar polymorph under mild conditions. Compared to the previously synthesized alpha-CBO, beta-CBO exhibits a larger second-harmonic generation (SHG) response due to its larger dipole moments.