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

Abstract

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.