One of the most common errors in diffraction patterns is the so-called "specimen displacement error", especially if they are measured in reflection using a powder diffractometer with divergent-beam geometry. You can check and (if necessary) correct for this error using the "Correct specimen displacement" command in the "Pattern" menu.
In a first step, Match! tries to find pairs of peaks based on the fact that e.g. d(100) = 2.0 * d(200). If the quotient d(peak 1) / d(peak 2) is close to 2.0, Match! will calculate the corresponding parameter T = -s/R (s = effective specimen displacement, R = radius) and add it to a histogram. When all pairs have been checked, the histogram will be displayed in order to let the user select the value of T used in the correction. Match! will propose a value for the correction based on the analysis of the histogram, indicated by a red vertical line.
The heigher a bar in the histogram, the more peak pairs have been found for the corresponding correction value. Hence, the user should generally select the correction according to the heighest bar as close to 0.0 as possible. A value is either selected by double-clicking at the corresponding position in the histogram or by entering the value manually in the input field. Finally, please press "OK" to perform the actual correction on the 2θ-values.
Depending on the geometry used in the diffraction experiment, Match! either uses the formula for Bragg-Brentano geometry (Δ2θ = -2.0 * T * cos(θ)) or the one for asymmetry geometry (Δ2θ = (T / sin(Ω)) * sin(2θ)).
All data views (diffraction pattern, candidate list, peak list etc.) will instantly reflect the change of the peak positions. If the peak list contains entries, their FoM-values will also be updated immediately.
As an alternative to the automatic correction, you can also correct the specimen displacement directly in the pattern graphics using the mouse: In order to do so, please make sure that the mouse cursor is located within the pattern graphics, then press <Ctrl+Alt+Shift>. The mouse cursor will change to . While keeping the Ctrl, Alt and Shift buttons pressed, turn the mouse wheel to modify the specimen displacement parameter. The effect will take place in the pattern graphics straight away.