Calibration of Diffractometers Ii: Internal Consistency and the Balance

The paper outlines recent results from experiments devoted to evaluate the instrumental performance of Bragg-Brentano geometry based diffractometers. Reproducibility, repeatability, and accuracy increased when calibration procedures are taken into account. The evaluation of internal consistency is required for a good quality calibration process, a good check of the optical alignment, and accurate preparation of specimen as well. The results discussed here refer in particular to the contribution given by the asymmetric component of peaks on the short wavelength side of the main diffraction lines. An appropriate evaluation of this asymmetric component increases the accuracy of data processing. INTRODUCTION In spite of high internal consistency of data, there is often some difficulty to reliably compare data deriving from diffraction measurements. Three aspects seem to have a great importance for data comparison: i) performance of instruments and the optional devices, ii) inhomogeneity of the experimental set up and its variation in time, iii) specimen characteristics, preparation, and positioning. A new point now shall be considered as one of the most critical one: the data processing. A good calibration procedure shall consider all the variables evaluating the residual systematic effects (not removable) after instrument alignment and may be expressed in terms of the dispersion characteristics of the diffraction angles ∆2θ vs. the expected Bragg angle (2θ). The “calibration curve” and/or “practical calibration curve”, representing optimum alignment, was proposed by Jenkins and Jenkins and Snyder [1,2]. Significant differences between experimental and expected positions of the diffraction lines may be revealed in due course of measurements because of the differences between the effective values of parameters and the nominal ones of each experimental item involved in the diffraction measurement. These differences have the potential to introduce artefacts and ambiguities in the data interpretation process. The evaluation of the residual systematic effects, still present after the instrument alignment, becomes important when data have to be compared with others taken before by us or taken by others elsewhere. This evaluation is carried out by calculating the effective values of the parameters involved during the measurements. The “correction curve” and the verification of the goodness of the correction are essential steps in the aim of rendering data truly comparable [3,4]. Copyright©JCPDS International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. 270