Synthesis and structure of hydrogen sulfates of the type M(HSO4)(H2SO4) (M=Rb,Cs and NH4)

From the binary systems M(2)SO(4)/H2SO4 (M = Rb, Cs, NH4), three new hydrogen sulfates of the type M(HSO4)(H2SO4) could be synthesized and structural characterized. The rubidium and caesium compounds are isotypic whereas NH4(HSO4)(H2SO4) is topologically very similar to both. All three compounds crystallize with nearly identical cell parameters [Rb: a = 7.382(1), b = 12.440(2), c = 7.861(2), beta = 93.03(3); Cs: a = 7.604(1), b = 12.689(2), c = 8.092(2), beta = 92.44(3); NH4: a = 7.521(3), b = 12.541(5), c = 7.749(3), beta = 92.74(3)], in the monoclinic space group P2(1)/c. There exist two kinds of SO4-tetrahedra: HSO4- anions (S1) and H2SO4-molecules (S2). The HSO4- anions form hydrogen bridged zig-zag chains. In the case of the Rb and Cs compounds, the H2SO4 molecules connect these chains forming double layers. The metal atoms are coordinated by 9 O-atoms with M-O-distances of 2.97-3.39 Angstrom (Rb) and 3.13-3.51 Angstrom (Cs). In the ammonium compound additional hydrogen bonds are formed originating from the NH4+ cation. This finally leads to the formation of S2-NH4+ chains (parallel to the S1 chains) as well as to a three-dimensional connection of both kinds of chains.