Direct space methods of crystal structure determination from powder diffraction applied to intermetallic compounds

Abstract

Direct space methods of structure determination from powder diffraction of non-molecular compounds (inorganics, extended solids, intermetallic compounds, etc.) are reviewed. They do not need powder pattern decomposition, and are based on a global optimization of a structural model to improve the agreement between the observed and calculated diffraction patterns. The success of the method depends very much on an appropriate modeling of the structure from building blocks. Modeling from larger building blocks improves the convergence of the global optimization algorithm by a factor of up to ten. The correctness of the building block (its rigidity, deformation, bonding distances, and ligand identity) must be examined carefully. Dynamical Occupancy Correction implemented in the direct space program FOX has shown to be useful when merging excess atoms, and even larger building blocks like coordination polyhedra. It also allows to join smaller blocks into larger ones in the case when the connectivity is not a-priori evident from the structural model. Available computer programs working in direct space are listed.