A Voigtian as profile shape function in Rietveld refinement

A modification of the Rietveld refinement program is introduced by replacing the Gaussian profile function with a Voigtian. The performance of the program is tested by an application to simulated and measured diffraction patterns of NaTaO₃, and excellent results are obtained when the tails of the reflections are included at distances 10 to 20 times the half-width of the reflections. Comparison between Gaussian and Voigtian refinements show large differences in the thermal parameters. This is due to the high level of background that is assumed in the Gaussian refinement to compensate for the lack of tail overlap. Application of the Voigtian refinement to the measured pattern of Ni yields a thermal parameter that is close to the literature values, while the Gaussian analysis gives a value which is 35% too large. The isotropic temperature factors of the room-temperature structure of NaTaO₃, respectively, drop down to the values B(Ta) = 0.09(2), B(Na) = 1.21(3), B(O1) = 0.59(3) and B(O2) = 0.61(3) Å², when the Voigtian analysis with a proper background is carried out. The role of background in powder pattern refinement is discussed, and it is suggested that a calculation of the average TDS should be included in the refinement.