Crystallography

In the absence of single crystals it is important to establish methods by which the entire crystal structure can be solved ab-initio from powder X-ray or neutron diffraction. We are involved in the development of a powerful new method by which this can be achieved. The method uses a simulated annealing approach to minimise the difference between observed and calculated powder diffraction patterns. It is the direct descendant of the Rietveld technique, which is well established in powder diffraction for refining crystal structures. This area of research has far reaching benefits beyond crystallography itself and can be used to solve the structures of many important compounds, e.g. new drugs.

The Simulated Annealing Method of Ab-Initio Structure Determination from Powder Diffraction Data

We developed a method capable of solving ab-initio complex crystal structures which contain individual atoms and/or highly flexible molecular fragments. The method employs a global optimisation technique and a versatile description of flexible molecular moieties in terms of stereochemical descriptors (bond lengths, bond angles and torsion angles) The power of the method was first demonstrated by solving the previously unknown crystal structure of (CH2CH2O)3:LiN(SO2CF3)2 (Andreev YG, Lightfoot P, Bruce PG. "Structure of the polymer electrolyte poly(ethylene oxide)3: LiN(SO2CF3)2 determined by powder diffraction using a powerful Monte Carlo approach" Chem. Comm.(18): 2169-2170 SEP 21 1996):

A detailed description of the simulated annealing method can be found in Andreev YG, Lightfoot P, Bruce PG. A general Monte Carlo approach to structure solution from powder-diffraction data: Application to poly(ethylene oxide)3:LiN(SO2CF3)2 J. Appl. Crystallogr. 30: 294-305 Part 3 JUN 1 1997.

The method has revolutionised the field of structure determination from powders and a number of complex crystal structures have been solved since its introduction. The structure of the most complex solid to be solved by powder diffraction (CH2CH2O)6:LiAsF6, with 50 atoms belonging to three separate moieties in the asymmetric unit and 79 variables (including 15 torsion angles) has been determined. (MacGlashan GS, Andreev YG, Bruce PG. Structure of the polymer electrolyte poly(ethylene oxide)6:LiAsF6 Nature 398 792-794 29 April 1999 


Using the Debye Equation to Establish Structure of Nanoparticles from Powder Diffraction Data

We are developing a methodology to extract detailed structural information of nanoparticular objects (size, shape, strains, mean-square displacements, etc) using powder diffraction data, when traditional refinement (Rietveld method) fails. Using this approach we have established that the ordered regions within TiO2-B nanotubes are fragmented within the tubes and consist of segments of deformed bulk structure of TiO2-B (Y. G. Andreev , P. G. Bruce Demonstrating Structural Deformation in an Inorganic Nanotube. J. Am. Chem. Soc. 130, 2008, 9931-9934).

The shape and the size of TiO2-B nanoparticles were established using refinement procedure based on the Debye equation and global optimisation by simulated annealing. Atomic modelling and molecular dynamics simulations revealed that the underlying reason for the observed shape and other structural features of the particles was hydroxylation of the surface during hydrothermal synthesis (Y. G. Andreev, P. M. Panchmatia, Z. Liu, S. C. Parker, M. S. Islam and P. G. Bruce The Shape of TiO2 -B Nanoparticles J. Am. Chem. Soc. 136, 2014, 6306-6312).