Instrumental Resolution Functions
Resources for describing the peak profile in different Rietveld refinement software packages
Table of Contents
Obtaining the instrumental profile
The instrumental profile function or instrumental resolution function (IRF) is a critical starting point for modeling methods that rely on refinement against the observed powder diffraction pattern. In principle, the profile parameters can be fitted to any measurement, however, it is usually desired to obtain information about the microstructure of the sample, and thus the effects of the instrument must be deconvoluted from the pattern. For a fundamental discussion on this topic, we refer the reader to the excellent resource by Dinnebier & Scardi—X-ray powder diffraction in education. Part I. Bragg peak profiles.
Typically, the IRF is determined through the measurement and fitting against a line shape reference standard where the microstructure parameters are either known or assumed to be negligible, such as lanthanum hexaboride (LaB6). In this case, the effects of crystallite size and microstrain broadening are assumed to be negligible, and thus all broadening of the observed pattern assumed to come from the instrument.
A quick comparison
As a quick test, we can check the instrumental resolution function by he trends in peak widths measured from a diffraction standard, in this case case measured by the integral breadth of the Bragg reflections from lanthanum hexaboride (LaB6). We can compare to data measured with a similar setup at DESY P02.1. The data collected at ESRF ID31 by Momentum Transfer has a higher resolution, despite the higher energy and shorter detector distance, due to the optics configuration and detector used.
Reference data are collected and provided for every beamtime
All measurements provided by Momentum Transfer are provided by default with measurements performed on several reference standards. The goal of the profile refinement is to both determine the IRF as well as standardize the data against any instrumental aberrations to the peak positions in 2θ such as from imperfections in 2D image calibration or parallax effects. Although our setup is highly consistent, it is best practice to re-refine the IRF against the reference measurement provided on the same date as the target measurement to ensure to highest level on consistency. We provide this analysis and standardization of the data by default for every beamtime as performed through Rietveld refinement using TOPAS v7, which can be found in the supporting attachments to your measurement requests in our platform https://app.momentum-transfer.com/.
However, if you use another software package for Rietveld refinement, then it is important to determine the IRF using the same software tool, as IRF functions and their implementations can vary between different tools.
Example Instrumental Resolution Setups
IRF setups determined using different Rietveld refinement software packages.
Note: the following files can be considered as a starting configuration. They should be re-refined to the high-resolution powder diffraction measurement for LaB6 provided with your measurement request as measured on the same date as your samples.
TOPAS v7
(determined for data measured on 27th May 2025)
GSAS-II
(determined for data measured on 10th Dec. 2024)
FullProf
(determined for data measured on 15th April 2025)
Profex
(demonstration using data from Momentum Transfer)