Order and disorder in solid oxygen: Neutron scattering investigations

Abstract

Solid oxygen exists at zero pressure in three crystallographic phases, denoted α, β, γ. Various results have been obtained by neutron scattering in the two disordered β and у phases. Plastic γ-O₂ has a A15 structure with 8 molecules per unit cell. The molecules, located at 2α and 6c sites, exhibit two different types of disorder of molecular axes: a spherical-like disorder for the 6c molecules, and a disc-like disorder for the 6c molecules. A precise diffraction analysis on large single crystals 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 α phase becomes a 2D short range helicoidal order in the β phase, with a very short 0.5 nm correlation range. In the γ phase, one observes, as in the liquid, a strong antiferromagnetic correlation, with a suggestion of a ID 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. This suggests the need for a better description of the л orbital and the theoretical magnetic form factor of the molecule.