Sr[B2(SO4)3(S2O7)]: A Borosulfate with an Unprecedented Chain Structure Comprising Disulfate Groups.

Unconventional borosulfates containing S-O-S bridges are still rare. Sr[B2(SO4)3(S2O7)] was synthesized solvothermally in oleum (65% SO3) and crystallizes in a new structure type in space group P21/n (Z = 4, a = 747.0(2) pm, b = 1533.4(4) pm, c = 1222.0(3) pm, β = 93.293(10)°). The structure features loop-branched vierer double chains, in which two terminal sulfate tetrahedra are condensed to a disulfate group. The resulting ratio between boron and sulfur of 2:5 was not yet found in borosulfate chemistry. The presence of S-O-S bridges was confirmed by FT-IR spectroscopy. Temperature-programmed X-ray powder diffraction in addition to thermogravimetric analysis revealed a transformation from chains containing S-O-S bridges in Sr[B2(SO4)3(S2O7)] to chains containing solely B-O-S bridges in Sr[B2(SO4)4] and to chains containing B-O-B bridges in Sr[B2O(SO4)3].