Correction for preferred orientation in Rietveld refinement

A new version of the Rietveld total-powder-pattern refinement method is introduced. The profile function is true Voigtian and the Gaussian and Lorentzian components include broadening due to instrumental effects, particle size and strain and wavelength dispersion. The code is applicable to neutron, synchrotron and conventional X-ray data when the incident radiation is essentially of one wavelength. The effects of preferred orientation are included by expanding the orientation distribution in spherical harmonics. The harmonics are picked according to the Laue class, and the coefficients are the parameters being refined simultaneously with other parameters. X-ray measurements on Ni, Mg and NaNO3 are used as test data. The actual preferred orientation corrections are known for Ni and Mg from earlier measurements of radial pole figures. The corrections from the refinement agree very closely with the measured values, and different samples yield consistent results when preferred orientation is included in the model. Only a few terms of the harmonic expansion are needed for an adequate fit. The largest correction factors exceed 2, and in such cases the refinements without allowance for preferred orientation fail totally.