PhD Project in Crystallography

An EPSRC studentship will be available from autumn 2002 in the Chemical Crystallography Laboratory in Oxford. The work will be supervised by Dr David Watkin and Dr Richard Cooper, and includes collaboration with Dr Peter Main in York and Prof Paul Beurskens in Nijmegen.

Applications are invited from graduate students with a first or upper second class degree in chemistry or physics. The graduate is also required to have fair mathematical ability, IT skills and must meet EPSRC requirements. Training in practical and theoretical aspects of crystallography will be provided.
Course fees and a maintainence allowance will be paid by EPSRC.

The Research

An increasing number of examples are being found where the basic building block for crystals of molecular materials is not a single molecule, but groups of molecules - the so-called Z'>1 crystals. They are a poorly-understood aspect of crystal formation, and the X-ray crystal structure analysis of these materials is not always simple.

The aims of this project include growing suitable crystals, analysing them by a variety of techniques, and developing new software for processing the X-ray diffraction data, leading to a better understanding of the nature and origin of these materials.

• Experimental analysis of properties of Z'>1 crystals: In spite of their curious constitution, high Z' materials are often very robust, crystallising reproducibly under a range of conditions. The reluctance of apparently simple molecules to crystallise with Z'=1 has implications for anyone attempting to predict crystal structures. A better understanding of the physics of the systems will certainly help with the modelling and interpretation. To compliment the diffraction work, our research group has had a long and productive collaboration with the solid state nmr group in Oxford.
• Mathematical modelling: Theoretical modelling to explore deformations from the experimental crystal structure. In some Z'>1 crystals, the pseudo-symmetry is quite close to a true symmetry, though the diffraction effect is unambiguously non-symmetric. Attempts to model the symmetric and pseudo symmetric structures will lead to a better understanding of some of the finer effects controlling crystal formation.
• Extended solution of the phase problem: Standard methods for estimating phases lead to adequate values for the pseudo-symmetry averaged structure. If the structure is to be revealed in detail, this pseudo-symmetry has to be resolved. A novelly weighted Fourier syntheses might lead to well resolved structures suitable for least squares refinement.
• Design of new discriminators, which could be used to decide which of a number of alternative models is preferable. The figures of merit currently used by the leading crystallographic journals are poor indicators of the confidence that can be placed in a proposed structure.
• Analysis of the systematic and random errors in diffraction data: There is no detailed analysis published of the systematic errors affecting CCD diffractometers. It is commonly seen that systematic absences have a net positive value, even for well-characterised materials. This bias in the weak data could seriously undermine its usefulness.
• The use of weak reflections: For structures containing pseudo-translational symmetry there are whole classes of reflections that are systematically weak. Some of the weak reflections may contain information which could resolve ambiguities, but most of them contain little more than noise. The problem is to uncover the information.

The work will impact on the routine handling of pseudo-symmetric materials (estimated as at least 5% of all structures published, i.e. potentially anyone using X-ray structure analysis).
Longer-term impact may be made on the understanding of the complexity of the crystalline state, assisting work on crystal structure and polymorph prediction, and therefore possibly on structure analysis from powders.

The Laboratory

The Chemical Crystallography Laboratory is very well equipped with single crystal X-ray diffractometers and computers, and there is access to excellent powder and solid-state nmr equipment within the chemistry department.
The mathematical procedures developed during the project will be incorporated into CRYSTALS, a crystallographic refinement and analysis program developed and maintained by the laboratory, which is distributed without charge to non-profit organisations.

Applications, in the form of a letter and curriculum vitae with the names and addresses of two referees, who may be contacted prior to interview, should be sent (not emailed) to:

The closing date for applications is 31 August 2002.

More information

Further information is available from David Watkin. E-mail david.watkin@chemistry.oxford.ac.uk. or telephone +44/0 1865 270826.

The Graduate Studies Prospectus provides useful general information about studying for a higher degree at Oxford.