Proton ordering in (NH₄)₃H(SO₄)₂ at low-temperature phase transitions

The proton ordering plays an important role in the structural phase transitions of (NH₄)₃H(SO₄)₂ at low temperature. Crystal structure analysis using neutron diffraction reveals a sequential ordering of the ammonium groups and of the proton in the (SO₄)H(SO₄) dimer with decreasing temperature, related to the different phase transitions.