Comparative evaluation of the March and generalized spherical harmonic preferred orientation models using X-ray diffraction data for molybdite and calcite powders

Preferred crystallographic orientation, i.e. texture in crystalline materials powder diffraction data, can cause serious systematic errors in phase composition analysis and also in crystal structure determination. The March model [Dollase (1986). J. Appl. Cryst. 19, 267-272] has been used widely in Rietveld refinement for correcting powder diffraction intensities with respect to the effects of preferred orientation. In the present study, a comparative evaluation of the March model and the generalized spherical harmonic [Von Dreele (1997). J. Appl. Cryst. 30, 517-525] description for preferred orientation was performed with X-ray powder diffraction data for molybdite (MoO₃) and calcite (CaCO₃) powders uniaxially pressed at five different pressures. Additional molybdite and calcite powders, to which 50% by weight silica gel had been added, were prepared to extend the range of preferred orientations considered. The patterns were analyzed initially assuming random orientation of the crystallites and subsequently the March model was used to correct the preferred orientation. The refinement results were compared with parallel refinements conducted with the generalized spherical harmonic [Sitepu (2002). J. Appl. Cryst. 35, 274-277]. The results obtained show that the generalized spherical harmonic description generally provided superior figures-of-merit compared with the March model results.