β‐CsB9O14: A Triple‐Layered Borate with Edge‐Sharing BO4 Tetrahedra Exhibiting a Short Cutoff Edge and a Large Birefringence

The first triple-layered borate with edge-sharing BO4 tetrahedra, β-CsB9 O14 was obtained under vacuum-sealed condition. It represents a new structure type and enriches the structural diversity of borates. Moreover, β-CsB9 O14 exhibits a short UV cutoff edge and a large birefringence, indicating that it could be regarded as a DUV birefringent crystal.