Borosulfates Cs2B2S3O13, Rb4B2S4O17, and A3HB4S2O14 (A = Rb, Cs) – Crystalline Approximants for Vitreous B2O3?

New borosulfates were obtained by reaction of B2O3 in oleum or concentrated H2SO4 at temperatures between 300 and 360 °C. They are the first examples of borosulfates with directly connected boron polyhedra, that is, ditetrahedral B2O7 or layers of trigonal‐planar BO3 units. Cs2B2S3O13 [P21/c, Z = 4, a = 14.765(3), b = 6.710(1), c = 12.528(3) Å, β = 104.60(3)°] contains layers of corner‐sharing BO4 and SO4 tetrahedra. The BO4 tetrahedra form dimers. SO4 tetrahedra connect four B2O7 dimers to rings containing 10 tetrahedra, which are condensed to a layer. A third SO4 tetrahedron connects two BO4 tetrahedra of the B2O7 unit. The layers are stacked in the [100] direction. One Cs+ cation is located within the layer of tetrahedra, the second one between the layers. In Rb4B2S4O17 or Rb4[B2O(SO4)4] [Pnna, Z = 4, a = 8.0415(9), b = 10.6466(11), c = 20.425(2) Å], there are isolated [B2O(SO4)4] units which consist of a central B2O7 unit. Besides one B–O–B bridge, two SO4 tetrahedra connect the BO4 tetrahedra. Additionally, each BO4 tetrahedron has a terminal SO4 unit. In the structure of A3HB4S2O14 or A3H(SO4)2(B2O3)2 [P63/m, Z = 2, A = Cs: a = 6.5648(2), c = 19.5669(5) Å; A = Rb: a = 6.502(6), c = 19.023(14) Å], the SO4 tetrahedra are isolated. Boron forms planar layers of corner‐sharing trigonal‐planar BO3 units. There are two different types of BO3 units in a ratio of 3:1. The first type forms six‐membered boroxol rings, B3O3, which are connected to the layers by the second type of BO3 units. The layers are separated by isolated SO42– tetrahedra and Cs+ cations. The “missing” proton could not be localized but was assumed to achieve electroneutrality and was confirmed by IR spectroscopy. Planar layers of B2O3 with trigonal‐planar BO3 units and boroxol rings are supposed to exist as structure elements in vitreous boron oxide.