GSAS Tutorial - Introduction

Instructions in Using GSAS Rietveld Software for Quantitative X-ray Diffraction Analysis of Portland Clinker and Cement

Leslie Struble (University of Illinois) and Paul Stutzman (National Institute of Standards and Technology)

Version 1 (March 2015)

Introduction

This document is an instructional guide in the use of Rietveld software for quantitative phase analysis of portland cement and related materials using powder X-ray diffraction (XRD). It presumes that the user is familiar with portland cement composition and with XRD analysis and has a basic understanding of crystallography and of Rietveld analysis. It provides step-by-step instructions with associated XRD data in the use of Rietveld software for quantitative phase analysis of portland cement.

Rietveld in this context refers to a technique for characterizing crystalline materials by refining neutron or X-ray diffraction patterns. A theoretical pattern is computed based on the crystal structure parameters of all phases in the material, parameters used to model peak shape for each phase, and instrument parameters (specimen height offset, for example). This theoretical pattern is refined until it best fits the measured pattern, using a least squares approach. As noted by Young^[1], this approach might better be called a "whole-pattern-fitting structure refinement", but the name Rietveld is simpler and acknowledges the importance of Rietveld's work in the development of the technique. Additional information may be found in Young^[2] and Argonne X-ray Science Division tutorial^[3], among others. Young^[1] gives an overview of the technique that briefly explains its underlying mathematics and the various refinable parameters. The technique may be used to refine the crystal structure parameters in a single phase or simple mixture or to determine the mass fraction of each phase in a complex mixture. The latter is the focus of these instructions.

There are several programs available for Rietveld refinement. The program known as Generalized Structure Analysis System (GSAS)¹ was selected for these instructions because it is well documented, it is in the public domain, and it is regularly updated. Once the user becomes familiar with GSAS (with the help of these instructions), it should be a relatively straight forward task to apply these procedures to any other program. GSAS was written by Larson and Von Dreele and the appropriate reference to use is^[4]. The accompanying graphical user interface, EXPGUI, was written by Toby^[5].

The Rietveld refinement requires detailed crystal structure information for all phases in the sample. The parameters include the space group and unit cell parameters and the position in the unit cell of all atoms in the phase. If any phase is not included in the refinement, the mass fraction results are biased even for the phases included. For this reason, use of the technique must be preceded by careful qualitative phase analysis.

These instructions make use of crystal structure parameters for phases in portland cement and related materials that were compiled previously. The user of these instructions will undoubtedly accumulate new parameters as additional structures are published or refined.

This document is similar to instructions prepared by Cranswick and Swainson^[7] but is more focused and specific to cement materials. You may wish to refer to those more general instructions when you are getting started with GSAS. You may also wish to refer to the manual included with the GSAS download^[4], but it is rather out of date and does not describe the graphical user interface that is used here, so the instructions of Cranswick and Swainson^[7] are probably more useful.

The related program CMPR^[8] may also be used for displaying diffraction data (including the GSAS experiment file), converting XRD patterns from one format to another, manual- and auto-indexing, peak fitting and other tasks. No instructions are given here for this program.

These instructions are self-contained, in so far as they do not require the user to generate diffraction patterns. The instructions include links to measured XRD patterns and instrument parameters.

 

Acknowledgements

Support for Leslie Struble's work on these instructions was provided by the National Science Foundation (DMR 10-08109) and support for Paul Stutzman's work was provided by the Portland Cement Association. Graduate students who took CEE501 at UIUC in 2012 kindly allowed us to use some analysis results in the section titled Assessment and provided useful comments on these instructions.

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1. Certain commercial equipment, instruments, or materials (including suppliers and software) are identified to enhance understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
2. GSAS, EXPGUI, and CMPR are available from: http://www.aps.anl.gov/Xray_Science_Division/Powder_Diffraction_Crystallography/.
3. Jade is available from Materials Data Inc., http://www.materialsdata.com