Synchrotron X-ray Diffraction in Mineralogy and Materials Chemistry. Possibilities and Applications

Resumen

The average crystal structure of minerals and materials is needed in order to explain/predict their properties. Furthermore, the microstructure of the samples may allow explaining the performances of materials in working conditions. Synchrotron diffraction is an invaluable technique for characterizing the structures and microstructures of minerals and inorganic compounds. Here, we will show examples of synchrotron studies for characterizing both polycrystalline materials with powder diffraction techniques and single crystals with single crystal diffraction techniques. Powder diffraction is adequate for carrying out very demanding experiments including the accurate quantitative mineralogical analysis at very high temperatures up to 1500 ºC. In this case, samples must be contained within Pt tubes and a very energetic wavelength must be used, λ=0.30Å, in order to penetrate the Pt capillaries. Furthermore, the border between powder diffraction and single crystal diffraction is becoming very diffuse. Polycrystal samples are being already studied by single crystal-type diffraction by indexing the reflection arising from different grains with the appropriate algorithms. Powders may be also studied by single-crystal diffraction by selecting a single microcrystal since the intensity in some third generation synchrotron source beamlines allows obtaining useful data from submicrometric individual grains. This technique is exemplified by two cases: i) the study of a kaolinite microcrystal; and ii) the structures of metal-organic-framework materials from tiny crystals. On the other hand, powder diffraction type data may be obtained from micro-size beams which is best suited for the study of materials or minerals which are hierarchically structured. In this case, intensities may be misleading as they arise from non-randomly oriented powders but the positions of the peaks allow identifying the phases from their powder diffraction patterns. This technique will be illustrated by two examples of gothic paintings studied by synchrotron microdiffraction where the different layers were analyzed and their mineralogy identified.