Anharmonicity of potentials of atoms in potassium hydrogensulfide (KDS) determined by neutron single-crystal diffraction

Abstract

Potassium hydrogensulfide (KHS) is an ionic compound with an anionic molecular group HS^-. The fast reorientational disorder of the anions was determined for the ambient temperature modification [R\bar 3m; Jeffrey (1974). Can. J. Phys. 52, 2370–2378]. Single crystals are available now as protonated or deuterated specimens. With neutron single-crystal diffraction at room temperature, a considerable anharmonicity of the atom potential of the H or D atoms was observed. Even the thermal motions of K and S atoms show small deviations from an isotropic probability density function, which can be modelled using anharmonic temperature factors. The temperature factors of the atoms were expanded into a Gram–Charlier series [Kuhs (1992). Acta Cryst. A48, 80–98] in order to evaluate the anharmonicity quantitatively. Parameters up to a fourth-order approximation are relevant for the D atoms. Results from neutron single-crystal diffraction are compared with split-atom models extracted from neutron powder diffraction patterns of fully deuterated samples.