Exploring a new structure family: alkali borosulfates Na5[B(SO4)4], A3[B(SO4)3] (A = K, Rb), Li[B(SO4)2], and Li[B(S2O7)2].

New alkali borosulfates were obtained by precipitation from oleum, solid-state reactions, or thermal decomposition. The crystal structures were characterized with single-crystal data. They are all based on corner-linked BO4 and SO4 tetrahedra with varying coordination of the alkali cations. According to the ratio of BO4 and SO4 tetrahedra, different frameworks are observed, i.e., noncondensed complex anions (1:4), one-dimensional chains (1:3), or three-dimensional (3D) networks (1:2). This is in analogy to silicates, where the ratio Si/O relates to the dimensionality also. For Na5[B(SO4)4], which exists in two different polymorphs, there are noncondensed pentameric units. Na5[B(SO4)4]-I: space group Pca21, a = 10.730(2) Å, b = 13.891(3) Å, c = 18.197(4) Å. Na5[B(SO4)4]-II: space group P212121, a = 8.624(2) Å, b = 9.275(2) Å, c = 16.671(3) Å. A3[B(SO4)3] (A = K, Rb) are isotypic with Ba3[B(PO4)3] adopting space group Ibca [K3[B(SO4)3], a = 7.074(4) Å, b = 14.266(9) Å, c = 22.578(14) Å; Rb3[B(SO4)3], a = 7.2759(5) Å, b = 14.7936(11) Å, c = 22.637(2) Å] with vierer chains of BO4tetrahedra based on two bridging and two terminal SO4 tetrahedra. Li[B(SO4)2] [space group Pc, a = 7.6353(15) Å, b = 9.342(2) Å, c = 8.432(2) Å, and β = 92.55(2)°] comprises a 3D network that is closely related to β-tridymite. Li[B(S2O7)2] [space group P212121, a = 10.862(2) Å, b = 10.877(2) Å, c = 17.769(4) Å] represents the first example of a disulfate complex with noncondensed [B(S2O7)2](-) units. Vibrational spectra were recorded from all compounds, and the thermal behavior was also investigated.