Neutron Investigation of Reorientations in Organic Molecular Compounds

Abstract The application of the Incoherent Quasielastic Neutron Scattering (IQNS) technique to the study of the dynamic disorder in molecular crystals is illustrated by a series of recent experiments. First, some basic principles of incoherent neutron scattering from organic hydrogenous compounds are outlined. The Elastic Incoherent Structure Factor (EISF), a fundamental quantity directly related to the spatial average distribution of the scatterers, is introduced. An illustration of the isotropic rotational model is provided by the case of norbornane. A comparison is made with a related compound, norbornadiene, the latter providing evidence of a strong translation—rotation coupling. The case of deformable molecules is illustrated with different analyses of the dynamics of alkyl chains in various situations. The flexibility of the chains is analysed for tetracarboxylate complexes in liquid-crystal phases, decylammonium and alkylene diammonium molecules in perovskite-type layered crystalline structures, and nonadecane in its rotator phase. Some studies related to alkanes in inclusion compounds are also covered. The final part of the paper is devoted to recent studies of glassy and metastable phases, and also to critical phenomena. We report on the dynamics of the cyanoadamantane molecules in the quenched glassy phase and on the modifications observed by aging of the sample. In the case of p-terphenyl, long-range correlations appear when approaching the phase transition on cooling and contribute to the scattered intensity. We conclude with a description of the rich polymorphism of methoxybenzylidenebutylaniline, where the quenched degrees of freedom are progressively relaxed in a succession of metastable phases.