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

Abstract

Abstract New borosulfates were obtained by reaction of B 2 O 3 in oleum or concentrated H 2 SO 4 at temperatures between 300 and 360 °C. They are the first examples of borosulfates with directly connected boron polyhedra, that is, ditetrahedral B 2 O 7 or layers of trigonal‐planar BO 3 units. Cs 2 B 2 S 3 O 13 [ P 2 1 / 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 BO 4 and SO 4 tetrahedra. The BO 4 tetrahedra form dimers. SO 4 tetrahedra connect four B 2 O 7 dimers to rings containing 10 tetrahedra, which are condensed to a layer. A third SO 4 tetrahedron connects two BO 4 tetrahedra of the B 2 O 7 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 Rb 4 B 2 S 4 O 17 or Rb 4 [B 2 O(SO 4 ) 4 ] [ Pnna , Z = 4, a = 8.0415(9), b = 10.6466(11), c = 20.425(2) Å], there are isolated [B 2 O(SO 4 ) 4 ] units which consist of a central B 2 O 7 unit. Besides one B–O–B bridge, two SO 4 tetrahedra connect the BO 4 tetrahedra. Additionally, each BO 4 tetrahedron has a terminal SO 4 unit. In the structure of A 3 HB 4 S 2 O 14 or A 3 H(SO 4 ) 2 (B 2 O 3 ) 2 [ P 6 3 / 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 SO 4 tetrahedra are isolated. Boron forms planar layers of corner‐sharing trigonal‐planar BO 3 units. There are two different types of BO 3 units in a ratio of 3:1. The first type forms six‐membered boroxol rings, B 3 O 3 , which are connected to the layers by the second type of BO 3 units. The layers are separated by isolated SO 4 2– 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 B 2 O 3 with trigonal‐planar BO 3 units and boroxol rings are supposed to exist as structure elements in vitreous boron oxide.