CaB₂ S₄ O₁₆ : A Borosulfate Exhibiting a New Structure Type with Phyllosilicate Analogue Topology

The reaction of Ca(CO₃ ) with H₃ BO₃ in oleum (20 % SO₃ ) yielded colorless single-crystals of CaB₂ S₄ O₁₆ (monoclinic, P2₁ /c, a=5.5188(2), b=15.1288(6), c=13.2660(6) Å, β=92.88(1)°, V=1106.22(8) ų ). X-ray single-crystal structure analysis revealed a phyllosilicate-analogue anionic sub-structure, forming 2D infinite anionic layers, which exhibit an unprecedented arrangement of condensed twelve-membered (zwölfer) and four-membered (vierer) rings of corner-shared (SO₄ ) and (BO₄ ) tetrahedra. Charge compensation is achieved by Ca²⁺ cations, residing exclusively above the centers of the twelve-membered rings. DFT investigations on the solid-state structure corroborate the experimental findings and allow for a detailed valuation of charge distribution within the anionic network and an assignment of vibrational frequencies.