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 Xe of the Thompson-Cox-Hastings function, which for orthorhombic structures was extended by a biaxial broadening parameter Xo. Variations of the structure
factors across broad peaks resulted in apparent peak shifts and concomitant shifts in cell-edge 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.