Orientational disorder in perovskite like structures of Li2X(OD) (X=Cl, Br) and LiBr center dot D2O

The crystal structures of Li2X(OD) (X = Cl, Br) and LiBr.D2O were found closely related to the perovskite structure type of Li3BrO by means of single crystal X-ray structure determinations, temperature-dependent X-ray and neutron powder diffraction measurements. For Li2Cl(OD) and LiBr.D2O a first order transition from orthorhombic to cubic symmetry was detected. In the case of Li2Br(OD) a phase transition without symmetry change was observed by neutron powder diffraction. Solid state H-2-NMR spectroscopy indicates a dynamical orientational disordering of the molecules OD- and D2O in the high-temperature (HT) modifications. Temperature-dependent neutron powder diffraction experiments show a nonspherical deuteron distribution in these phases which can be described with split models. Jump processes of the deuterons in the HT phases are probable. Corresponding accumulation of deuterium density in the HT phases of Li2Cl(OD) and LiBr.D2O was found but is rather different from the orientation of D in OD- for Li2Br(OD). These results can be explained according to the different acceptor/donor strength of hydrogen bonds in these compounds.