Direct space methods of crystal structure determination from powder
diffraction applied to intermetallic compounds
Chem.
Met. Alloys 1
(2008) 120-127
https://doi.org/10.30970/cma1.0041
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 a proper modeling of the structure from building blocks. The
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 like 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 allows also joining
smaller blocks into larger ones in the case when the connectivity is not a
priori evident from the structural model. The available computer programs
working in direct space are listed.
Solving a structure ab-initio in
direct space implies describing the structure through a number (N) of Degrees
of Freedom (DoF): translation and rotation of the
molecule or polyhedron, and internal DoF like torsion
angles, bond length and bond angles. These parameters must then be randomly
changed in order to find the minimal cost (usually the best agreement between
the calculated and experimental powder pattern). This corresponds to exploring a N-dimensional hypersurface until
the global minimum is found. The surface represented here corresponds to a 2D
cut of the hypersurface corresponding to the
variation of one torsion angle and one translation.
Keywords
Structure solution / Powder diffraction / Inorganic
compound / Simulated annealing / Genetic algorithm