Effective Correction of Peak Asymmetry: Rietveld Refinement of High-Resolution Synchrotron Powder Data of Li_1.8(Hf_1.2Fe_0.8)(PO₄)₃

Rietveld refinement of powder diffraction data is a very popular and powerful technique. Several effects in the diffraction patterns, such as anisotropic peak broadening, preferred orientation or extinction, are properly taken into account; however, the asymmetry due to axial divergence remains problematic. Recently, a seminal paper by Finger, Cox & Jephcoat [J. Appl. Cryst. (1994), 27, 892–900] proved that this effect can be treated with parameters related to the diffractometer optic. Several refinements of asymmetric profiles obtained with synchrotron and laboratory X-ray and neutron radiation are reported. Furthermore, the refinement of the structure of Li_1.8(Hf_1.2Fe_0.8)(PO₄)₃ with synchrotron data (λ ≃ 0.4 Å) taken on the world's highest resolution diffractometer (at BM16, ESRF) is presented. The fit of the pattern which has very asymmetric peaks is excellent, as indicated by the low value of R_wp = 8.1% and the very flat difference curve. It can be said that asymmetry due to axial divergence is no longer a problem.