3 Terms and a Global Look

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3 Terms and a Global Look

We have some difficulty finding a concise name or definition for the field we are currently entering. There are indeed sizeable powder diffraction communities already involved with, what could loosely be described as, "static" analysis of materials, involving:

• phase identification
• structure refinement
• composition analysis of mixed phases
• studies of stress and strain within powders

However we are not looking here particularly at these aspects; rather we are interested in what can be learned about materials "in action"; this aspect has been one of the exciting trends of the last 2 decades, brought about mainly by the increase in powerful X-ray and neutron radiation sources and by the developments in detector technology. Previously many processes had to be studied by "before and after" methods in which the structure/composition of materials could only be examined in a coarse way: to get even an inkling of what was happening during the process one had to stop that process, take the material out as it were and assume the process had not been disturbed. We are now in a position to realise that those assumptions were often very wrong: true rates of reaction were often much faster or slower than that perceived by the conventional wisdom, sometimes with totally unexpected composition profiles (unknown intermediate phases etc.). It can be said that in many cases our understanding of materials and chemical processes has been completely changed by the opportunity to "watch" processes in-situ using powder diffraction.

Three terms, time-resolved, in-situ and non-ambient keep cropping up in the present context (a fourth, dynamic diffraction, is also used but is covered by the other 3). By these terms we mean:

• time-resolved diffraction: A set up in which many successive powder diffraction patterns are collected from a sample over a period of time. A single pattern collection may often involve several steps such as receiving a signal of readiness to start data collection; the actual data collection; moving the system to the next collection point. The time taken for these is often referred to as the time resolution or time step. Decades ago, realistic time resolutions would have been measured in units of an hour, whereas today spectacular time resolutions have been claimed in very specialised situations down to femto-seconds. In the studies quoted here we will come across examples of time resolutions that are in the region of seconds or less.
• in-situ study: This term carries the sense that a material is being studied using powder diffraction while simultaneously the sample material is being persuaded to behave in the manner of interest: this might for example involve growth/crystallisation of the material or the application of pressure, heat, corrosion, gaseous attack and so on. The essential idea is that we are using diffraction to study the material while it is actually performing. It implies two obvious requirements: a sample environment enclosure and a diffraction set-up compatible with this.
• non-ambient conditions: This implies sample conditions other than those of ordinary room temperature (e.g. 20-30°C) and gaseous pressure (ordinary air at a pressure of 1 atmosphere [1 bar]).

To perform a time-resolved, in-situ, non-ambient powder diffraction study (a mouthful!) requires harmonisation of all the components of the set-up. The various items are illustrated schematically below:

However these components are not independent; for example, the type of detector used bears upon the size of the exit window which in turn affects the performance of the sample environment enclosure; the speed of the changes under study in the sample puts demands on the radiation source and the speed of the detector, and so on. A "plug in" approach is not always possible and for this reason we need to look at all the components in this section.

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