Electron microscope diffraction characterization of the fibrous structure of poly (sulphur nitride) crystals

Abstract

The work reported here concerns the fibrous microstructure of macroscopic poly (sulphur nitride) crystals of current topicality in view of their metallic and superconducting properties. Following the outline of the synthesis by solid state polymerization, together with the achievement of the desired crystal habits appropriate for this study, methods for making the crystals accessible for transmission electron microscopy and diffraction are described. The principal features of the diffraction patterns were in accord with the crystal structure reported previously but additional ones were also observed which amongst others corresponded to a doubling of the unit cell and to twinnings, including a so far unreported twin mode on (¯1 0 1). The reflections were highly streaked along the layer lines and, significantly, the streaks were observed as a set of regularly spaced spots flanking the main reflections. This indicates the existence of a periodicity perpendicular to the chains beyond that of the atomic lattice. The spacing corresponds to ∼200 Å as obtained from the closest spot separation which, in turn, indicates a representative “fibre” diameter of this magnitude. The latter broadly corresponds to the 250 Å obtained as an upper limit of the “wire” diameter deduced from conductivity measurements by Civiak et al. The transmission image confirmed the existence of a longitudinal periodicity but this was of variable magnitude with a minimum of 25 Å. The latter is at least consistent with a fibre diameter of 30 Å previously observed on bromine-treated samples. Consistency with the diffraction pattern requires an integral relation between the range of periodicities seen, of which the 25 Å should be the basic and the ∼200 Å a representative maximum unit. The origin of the fibrous structure in terms of the solid state polymerization is discussed with two possible mechanisms in view: (1) phase separation during simultaneous polymerization and crystallization, and (2) twinning occurring during polymerization from solid solution. However, neither of these would account for the periodicities observed without invoking additional factors raised in the discussion.