The real background and peak asymmetry in diffraction on nanocrystalline metals

What is it about?

A fundamental problem in interpretation of diffraction from a multi-phase material is to separate peaks originating from only one phase from the background intensity. To this end our Team made a spectacular step - we have developed method to calculate the 'true background' of the peak, based on exact physical criteria. The method is applicable if so called microstrains are not important factors in the peak broadening - the situation often met in catalytic studies. Currently the method was tested on supported metal nanocrystals. The developed method enables in depth insight into the structure of nanocrystals and their surface. Monitoring the structural evolution during physico-chemical process- e.g. catalytic reaction at the surface of metal catalyst, may provide evidence of reversible elongations of nanocrystals along some directions. They result from a stronger bonding of some crystal faces with the adsorbing reagents. The crystallite responds by increasing surface of that faces, changing its shape and minimizing the energy. Such changes we have observed e.g. for nanocrystalline gold.

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Why is it important?

The 'background problem' is at the basis of PXRD analysis. All material estimates deeply depend on it. If microstrain plays no important role in the diffraction pattern, the proposed analysis is the objective approach. The paper explains why the diffraction peaks from nanometals are usually asymmetric and how one can recover from the peak shape the lattice constant - size dependency and the column size distribution.

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