Time-Resolved X-ray Powder Diffraction Using a Large-Area CCD-Based Detector and Rietveld Refinement: Solid-State Polymerization of S₂N₂ to (SN)_x

A kinetic study of the solid-state polymerization of disulfur dinitride (S₂N₂) to polysulfur nitride [(SN)_x] has been performed, combining monochromatic high-energy (λ = 0.3263 Å) synchrotron radiation X-ray powder diffraction, a large-area (ø = 220 mm) CCD-based X-ray image-intensifier detector and Rietveld refinement. Recently developed techniques for detector calibration and reduction of two-dimensional images to one-dimensional diffraction patterns have been employed for data processing/analysis. Good fits were obtained after Rietveld refinement [R_p = 8.4%, wR_p = 9.4%, sin(θ_max)/λ = 0.585 Å⁻¹] of diffraction patterns of S₂N₂ from images with 2 s exposure time. The solid-state polymerization of S₂N₂ to (SN)_x, was followed at a maximum rate of two diffraction images per minute. Scale factors and cell parameters for S₂N₂ and β-(SN), as functions of time were readily obtained after Rietveld refinement of the diffraction patterns obtained from each individual image throughout the polymerization. The polymerization was preceded by a lattice distortion of S₂N₂, and at 50% conversion the a axis had decreased by about 1% and the c axis had increased about 1%. The polymerization yielded not only the expected polymer β-(SN)_x, but also a small amount of a compound that could be another phase of (SN)_x.