Structure solution from powders

The main obstacle in indexing unit cells from powder diffraction patterns and subsequently solving crystal structures is posed by the projection of 3-dimensional onto one-dimensional reciprocal space. A large part of valuable information needs to be recovered from this projection. The situation deteriorates when more than one phase are present in the sample, leading to overlap of Bragg peaks. In worst cases more than one novel unknown phase are present in the same sample. Such samples however present real-case working materials for many applications. Our laboratory focuses on solving crystal structures from such samples using the direct space approach, which does not require accurate intensities to be extracted prior to structure solution, and the software FOX, developed here by Vincent Favre-Nicolin and Radovan Cerny.

The computer program FOX (for Linux, MacOS X and windows) was written for the ab initio crystal structure solution from diffraction data (mostly powder diffraction data). Its most interesting features for ab initio structure determination are:

  • A versatile description of the crystal contents: either isolated atoms , molecules described using a bond length, bond angles and dihedral angles, and polyhedra for inorganic compounds. You can describe your structure by using any combination of groups of atoms, using a chemist’s or crystallographer knowledge about the connectivity in your sample to constrain possible solutions.
  • An automatic correction for special positions and shared atoms between polyhedra, suitable for global optimization algorithms.
  • The ability to use simultaneously multiple powder patterns (X-rays, neutrons), as well as single crystal data (e.g. extracted from a powder pattern).
  • Smart global optimization algorithms which can get out of false minima.
  • A graphical interface (see the screenshots) with a 3D crystal structure view, with live updates during the optimization process.

FOXBanner

RĂ©alisation : Sur Mesure concept