title: Disorder of (NH4)3H(SO4)2 in the high-temperature phase I.
abstract: The highly disordered crystal structure of triammonium hydrogen disulfate, (NH(4))(3)H(SO(4))(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 SO(4) 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 SO(4)-H-SO(4) 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.
authors.affiliation: RWTH Aachen University, Institut für Kristallographie, Jägerstrasse 17-19, 52056 Aachen Germany. sohn@xtal.rwth-aachen.de
authors.forename: Yoo Jung
authors.initials: YJ
authors.lastname: Sohn
authors.forename: Karine M
authors.initials: KM
authors.lastname: Sparta
authors.forename: Martin
authors.initials: M
authors.lastname: Meven
authors.forename: Gernot
authors.initials: G
authors.lastname: Heger