Abstract:
Since their beginning in the middle of the past century, Direct Methods (DM) have played a central role in the determination of crystal structures. Due to the increasing computing power, structure solution strategies and computing algorithms have continuously adapted to take full advantage of this change. The principal stages in the evolution of (automated) DM will be illustrated with the help of Patterson-function direct methods (PFDM) [1].
This especial sort of DM solves the unknown crystal structures by direct interpretation of the corresponding origin-free modulus functions in terms of the structure factor phases (F). The optimization of the F phases is achieved by maximizing the so-called SM modulus sum function through an improved algorithm actively using strong and weak intensities without needing cumbersome weighting schemes [2].
This algorithm has been implemented in the XLENS software [3] and can work either in reciprocal space (SM-TF) or also alternating reciprocal and real spaces (SM-FFT) [4]. Recently, the latter SM-FFT algorithm has been improved by exploiting the similarity between the r2 and |r| density functions which allows handling larger structures (up to 400 atoms in the asymmetrical unit) more efficiently [5].
Some examples on the application of PFDM to the solution of inorganic and organic crystal structures from powder diffraction data [6], from 3D electron diffraction data [7] and from synchrotron through-the-substrate microdiffraction data [8] (extracted with the D2dplot software [9]) will be given.
[1] J. Rius, IUCrJ 1 (2014) 291. [2] J. Rius, Acta Cryst. A49 (1993) 506. [3] J. Rius, XLENS® software: Available at departments.icmab.es/crystallography/software. [4] J. Rius, A. Crespi, X.Torrelles, Acta Cryst. A63 (2007) 131. [5] J. Rius, Acta Cryst A (accepted). [6] J. Rius, Acta Cryst. A67 (2011) 63. [7] J. Rius, E. Mugnaioli, O. Vallcorba, U. Kolb, Acta Cryst. A69 (2013) 396. [8] J. Rius et al., IUCrJ 2 (2015) 452. [9] O. Vallcorba, J. Rius, J. Appl.Cryst. 52 (2019) 478.
Bio:
Professor Jordi Rius studied Geology at the University of Barcelona and obtained his doctorate degree in Natural Sciences (Mineralogy) at the University of Marburg in 1980 on the determination of superstructures from X-ray diffraction data. He is Research Professor at the ICMAB since 2003.
His principal research field in CSIC has been the development of new crystal structure solution algorithms. Initially, these were based on Patterson search methods (ROTSEARCH software) but later they evolve to a new type of direct methods (generically called Patterson-function direct methods, PFDM) which are implemented in the XLENS® software. In combination with powder diffraction data, XLENS solved some notoriously difficult crystal structures of relevant materials such as the cementitious phase in hydrated calcium aluminate cement or the blue aerinite pigment used in Romanesque wall paintings. Two recent advances in powder diffraction have been the application of PFDM to clusters of overlapped reflections as well as a new direct-space structure solution strategy for solving molecular compounds (TALP software). Recently, he worked in the application of PFDM (δ- recycling algorithm) to 3D electron diffraction data, and currently, is developing the synchrotron through-the-substrate (tts) microdiffraction technique and its associated software.
He is in charge of the X-ray Diffraction Laboratory at the ICMAB.
Hosted by Carles Miravitlles, former ICMAB Director, and Xavier Obradors, current ICMAB Director
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