Measurements of centroid and peak shifts due to dispersion and the Lorentz factor at very high Bragg angles

Differential response errors due to dispersion and the Lorentz factor have tan³ θ dependence for the powder diffractometer and should thus become critical for Bragg angles greater than ~ 155°, 2θ. The magnitude of these errors is demonstrated by measuring centroids and peaks at Bragg angles up to 170°, 2θ using Seeman–Bohlin geometry. Lattice parameters calculated from a single high angle centroid fall up to 0.0008 Å below the values obtained by extrapolation using the data of many profiles. No significant deviation using peak positions was observed within the limits of experimental error. While these errors are considered to be significant for residual stress measurements they can be ignored for routine lattice parameter determinations.