Order and disorder in solid oxygen - Neutron scattering investigations

A precise diffraction analysis on large single crystals of solid hydrogen shows that the disorder of molecular axis of the molecules can be interpreted in terms of localized forbidden orientations, or orientational holes, with a wide disorder among the other orientations. An important contribution of the translation-rotation coupling must be taken into account for the 6c molecules. Analysis of diffuse scattering by both an approximate analytical model and molecular dynamics simulation shows an important contribution to the orientational correlations between 6c-6c and 2a-6c molecules. Magnetic disorder can be investigated by neutron polarization analysis of the paramagnetic scattering. The 3D long range antiferromagnetic order of the a phase becomes a 2D short range helicoidal order in the beta phase, with a very short 0.5 nm correlation range. In the gamma phase, one observes, as in the liquid, a strong antiferromagnetic correlation, with a suggestion of a 1D order along the linear chains of 6c molecules. In all the measured phases, the variation of the scattering intensity with scattering vector q cannot be reproduced at high q by the known magnetic form factor. (Author)