Ba6Zn6(B3O6)6(B6O12): Barium Zinc Borate Contains π-Conjugated [B3O6]3- Anions and [B6O12]6- Anion with Edge-Sharing BO4 Tetrahedra.

A novel barium zinc borate contains π-conjugated [B3O6]3- anions and [B6O12]6- anion with edge-sharing BO4 tetrahedra, Ba6Zn6(B3O6)6(B6O12), has been successfully synthesized via a high-temperature solution reaction. In its structure, the isolated planar π-conjugated B3O6 groups are interconnected by ZnO4 tetrahedra via corner sharing to construct a [Zn3(B3O6)3]3- single layer parallel to the ab plane with the large Zn3B6 9-member rings. Two adjacent [Zn3(B3O6)3]3- single layers are interconnected by [B6O12]6- anions into a two-dimensional [Zn6(B3O6)6(B6O12)]12- double layer with 1D tunnels of Zn4B8 12-member rings along the a-axis. Neighboring such double layers are packed in an A-B-A-B... fashion along the c axis, and the Ba2+ ions act as counterbalance cations filling in the voids of double layers. All of the planar π-conjugated [B3O6]3- groups in Ba6Zn6(B3O6)6(B6O12) are in approximately parallel arrangement, producing large optical anisotropy and birefringence. The UV-vis-NIR absorption spectrum manifests that the UV cutoff edge for the title compound is below 200 nm. Ba6Zn6(B3O6)6(B6O12) possesses the largest birefringence (0.115@1064 nm) among the zincoborates reported. Its thermal stability, infrared spectrum, and theoretical calculations were also performed.