Disorder of (NH4)3H(SO4)2 in the high-temperature phase I

The highly disordered crystal structure of triammonium hydrogen disulfate, (NH4)3H(SO4)2, in the high-temperature phase I was studied using single-crystal neutron diffraction. It is known that the O atom involved in hydrogen bonding between neighbouring SO4 tetrahedra is disordered and takes a split-atom position, building a two-dimensional hydrogen-bond network in the (001) plane. The H atoms in these SO4–H—SO4 hydrogen bonds are disordered and hence refined with a split-atom model. Moreover, from the much larger anisotropic mean-square displacements of ammonium protons the NH_4^+ groups were refined with a reasonable split-atom model, and their motional behaviour was also analysed by rigid-body treatment. Finally, careful consideration was given to show possible supplementary proton migration between the ammonium protons and those of the hydrogen bonds in this high-temperature phase.