Powder Diffraction


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Thanks to the participants of the Synchrotron Powder Diffraction School 2016.

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INTRODUCTION


X-ray powder diffraction (XRPD) allows rapid non-destructive analysis of multi-component mixtures and of materials not available in single crystals and the study of industrial compounds in the same microcrystalline form as the final product1. Furthermore, non-ambient XRPD analysis is often more successful than using single crystals owing to the difficulties in preserving the quality of a single crystal during the phase transformations.

Since a powder is formed by a very large number of microcrystals, ideally all possible crystal orientations are measured simultaneously and the three-dimensional reciprocal lattice is projected onto a one-dimensional space. The reduction of the entire reciprocal lattice into one dimension limits the data volume, simplifies the data collection strategy, and reduces the overall measurement time even for small and weakly scattering samples, opening opportunities for time resolved studies. However, these advantages are often at the expense of the ease of analysis and interpretation of the data.

Crystal structure determination is especially complicated by the overlap of reflections in a powder diffraction pattern. For this reason XRPD has been traditionally used only to analyze the phase composition of samples of known crystal structure (fingerprinting) or to follow the dependence of the cell parameters on external conditions (e.g. temperature, pressure). Structural solution with powder data has, however, greatly improved in the last 10 to 15 years. Developments in instrumentation, computer technology and powder diffraction experimental techniques [e.g. anisotropic thermal expansion2 and texture3 methods] and methodologies [e.g. global optimization techniques4; resolution bias algorithm5; charge flipping6] implemented to strengthen the power of direct methods have all contributed to this success. The advent of synchrotron sources has caused powder diffraction methods to enter a new era of development7. The collimation and monochromaticity of the X-ray beam allow for an improvement in the angular resolution of the acquired patterns compared with conventional laboratory sources, whereas the high brilliance of the sources reduces measurement times by several orders of magnitude, allowing the study of the dynamics of samples on the time scale of fractions of a second.

Microstructure analysis as well as any analysis on nanomaterials or non-crystalline matter benefits as well from the use of synchrotron radiation. Tunable energy, high photon flux, beam parallelism, wide angular range, make it possible to collect very high-quality patterns from tiny sample quantities and/or very poorly scattering phases (including medium-small molecules in non-ordered state, e.g. solution). Subsequent analysis using either Bragg profile analysis8,9 or Total Scattering methods10 hugely benefits from the improved signal quality.

The improvements in the radiation source must be accompanied by improved performances of radiation detectors. This is why the Paul Scherrer Institut has invested remarkable resources to support an outstanding detector-development group, lead by Bernd Schmitt, which is delivering to PSI and worldwide outstanding detectors. The MYTHEN11 1D microstrip and the PILATUS12 2D pixel detectors, in operation since 2001 at the Materials Science and the Protein Crystallography beamlines are among the best achievements of the PSI detector group. MYTHEN and PILATUS are now commercialized by the spin-off company DECTRIS.
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5. Altomare, A., Cuocci, C., Giacovazzo, C., Maggi, S., Moliterni, A. & Rizzi, R. (2009). Acta Cryst. A65, 183–189.
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10. Diffraction at the Nanoscale: Nanocrystals, Defective & Amorphous Materials, Ed. by A. Guagliardi & N. Masciocchi, Insubria Press:Como, ISBN:9788895362359
11. A. Bergamaschi, A. Cervellino, R. Dinapoli, F. Gozzo, B. Heinrich, I. Johnson, P. Kraft, A. Mozzanica, B. Schmitt and X. Shi, Nucl. Instrum. Methods Phys. Res. Sect. A (2009). 604, 136-139.
12. C.M. Schlepuetz, R. Herger, P.R. Willmott, B.D. Patterson, O. Bunk, C. Broennimann, B. Henrich, G. Huelsen, and E.F. Eikenberry, Acta Crystal. (2005). A 61, 418.



N.Casati, reviewed on February, 2015