Abstract

Although most X-ray powder diffraction (XRPD) measurements are performed under ambient conditions, demand is strong for insight into the crystallographic, microstructure and chemical changes that occur when materials are exposed to one or more external variables. In many industrial and research applications, ambient-condition measurements do not accurately represent a material's state within a dynamic process.

For example, in mineral processing, both the overall phase composition and the characteristics of individual phases in a sub-sample measured under ambient conditions may differ substantially from those in the process stream. Because the stream's behaviour depends on the crystallographic forms of the phases present, process conditions should be developed using measurements that closely reproduce the actual operating environment. Similarly, in materials science, observing how phases interact when a material is subjected to an electric current or magnetic field can guide the design of materials with improved performance. Because a diffraction pattern is derived directly from the phases present, it is an ideal probe to monitor the crystallographic changes taking place.

In situ measurements involve many experimental challenges, including:

• Creating a sample environment that accurately reproduces process conditions, either by using commercially available equipment or building a purpose-designed system;
• Allowing the X-ray or neutron beam to enter and leave the sample cell, particularly when operating conditions such as high pressure require the use of thick cell walls;
• Devising data-collection strategies that support analysis of the parameters of interest and enable reaction kinetics to be followed; and
• Analysing complex datasets that may contain hundreds or thousands of individual diffraction patterns and require the development of new correction and analysis models.

During data analysis, the analytical approach needs to be selected according to both the constraints of the experiment and the information to be extracted. For example, visually examining the complete sequence of diffraction patterns may be sufficient to identify phases as the external variable changes. However, detailed whole-pattern analysis over a broad 2θ range is preferable when information on crystal-structure changes, phase microstructure, or reaction kinetics is required.

This talk will examine some of the key considerations in planning and conducting in situ diffraction measurements. It will also show why careful data analysis is essential for extracting the greatest possible insight from the results. Historical studies along with some recent examples, primarily from mineralogical applications, will be used to illustrate the major points.