XRD peak migration and apparent shift of cell-edge lengths of nano-sized hematite, goethite and lepidocrocite

Abstract X-ray powder diffraction patterns were simulated for nano-sized hematite, goethite and lepidocrocite by three-dimensional integration in reciprocal space. The cell-edge lengths were refined together with the size parameters X and X e of the Thompson-Cox-Hastings function, which for orthorhombic structures was extended by a biaxial broadening parameter X o . Variations of the structure factors across broad peaks resulted in apparent peak shifts and concomitant shifts in celledge lengths, which were significantly correlated with the size parameters for hematite and partially correlated for goethite and lepidocrocite. Regression equations are given for correcting cell-edge lengths obtained from Rietveld fits for size-induced shifts.