Reorientational disorder of anions in the hydrogen sulfides of sodium and potassium (NaDS and KDS) investigated by neutron diffraction between T=4 K and T=470 K

High resolution neutron powder diffraction experiments on NaDS and KDS were carried out as a function of temperature between T=4 K and T=470 K at ROTA X (ISIS, UK). The probability density function (pdf) of the atoms was studi ed in detail, in particular that of deuterium. Several forms of temperature dependent reorientational disorder of the anions are the reason for struct ural polymorphism of these compounds. A monoclinic low (LTM), a rhombohedra l middle (MTM), and a cubic high temperature (HTM) modification are the pol ymorphs. For the structures of the different modifications the parameters w ere refined by the use of split-atom models, Fourier synthesis, and/or cubi c harmonics. As a main result of this study the anisotropy of the pdf of de uterium in NaDS and KDS was evaluated. It is due to large amplitudes of lib rations of the anions. (C) 2000 American Institute of Physics. [S0021-9606( 00)50640-0].