X-ray diffraction study of diffuse scattering in incommensurate rubidium-IV

Abstract

X-ray diffraction data have been collected from single crystals of incommensurate $\mathrm{Rb}\text{\ensuremath{-}}\mathrm{IV}$ in order to study the sheets of diffuse scattering arising from the one-dimensional (1D) guest chains. Discontinuities in both the relative intensity and width of the diffuse sheets are observed on pressure decrease below $16.7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the pressure at which the interchain correlations begin to decrease rapidly. These changes in the diffuse scattering are accompanied by the progressive disappearance of the Bragg peaks from the guest chains. By modeling the diffuse scattering as coming from either a perturbed 1D lattice or a 1D harmonic liquid, we have determined the pressure dependence of the intrachain spacing of the guest atoms and the root-mean-square (RMS) fluctuation of the guest atoms from their unperturbed sites. We have also made estimates of the sound velocity along the guest-atom chains, and their effective Debye temperature.