Symmetrical hydrogen bond in SO₄-H-SO₄ dimer in K₃H(SO₄)₂ by neutron diffraction

The hydrogen bond in K₃H(SO₄)₂ was investigated using a neutron single crystal diffraction technique at room temperature and 2.3 K. The title compound crystallizes in the monoclinic space group A12/a1 (No. 15) and SO₄ dimers are linked by symmetrical hydrogen bonds. Down to 2.3 K no evidence for the structural phase transition was observed, although the mineral letovicite (NH₄)₃H(SO₄)₂ shows various phase transitions at low temperatures owing to the proton ordering process. At room temperature, the nuclear density of the proton shows an elongation along two oxygen atoms with a distance between these two oxygen atoms of 2.504 A. At 2.3 K, the proton density becomes more symmetrical, which is unusual in the hydrogen bonded systems reported thus far. The distance between two oxygen atoms is then 2.472 A. Based on an analysis of the anisotropic displacement parameters, the hydrogen bond in the title compound seems to be a centered hydrogen bond or an intermediate form between a cigar-like shape and a disk-like shape. (author)