Electron Nano-Crystallography: Precession Electron Diffraction in an Aberration-Free Environment

The vast majority of important materials are crystalline in nature. As such it is important that techniques are available which enable the crystal structure to be determined for new materials. X-ray and neutron methods are now very sophisticated and accurate but often fail to solve structures because the material is multi-phased, the crystal of interest has too small a volume or there is too much disorder at the lengthscale of the x-ray/neutron beam. Electron microscopy offers a way to record diffraction data from nm-sized regions overcoming the limitations of x-ray and neutron techniques. However electrons interact strongly with the crystal and the resultant diffraction intensities cannot in general be used directly to determine the crystal structure. Precession electron diffraction is a method by which the electron beam in scanned in a hollow cone above the specimen and then de-scanned below to give rise to diffracted intensities which are integrated through the Bragg condition; this geometry is equivalent to precessing the crystal about a fixed beam. The diffracted precession intensities are less prone to dynamical effects and may be used to solve crystal structures. In broad terms, the larger is the scan, or precession, angle, the smaller the effects of dynamical scattering on the diffracted intensities. In this proposal we plan to implement precession on an aberration-corrected electron microscope so that large precession angles can be used whilst retaining ultra-small nanoscale beams. On conventional instruments the spherical aberration of the probe-forming lens gives rise to very large probes for all but the smallest of precession angles.Precession will be optimised to generate the smallest probe for the largest precession angle. We will evaluate two ways of achieving this. The first requires dynamical control of the aberrations, changing the correction optics as the beam precesses. In theory this gives a very small beam but will be difficult to implement in practice. The second, probably preferred, option is to choose a lens defocus that enables a pass-band to match the precession angle and limit the distortion in the overall beam as it scans around the precession cone. Here the correction is fixed and far easier to implement in practice. We then plan to determine universal curves for the optimal precession angle for particular material types using a large number of simulations from many different structure types to establish trends and similarities. Running concurrently will be a development of precession theory using electron channelling and the concept of scattering strength from atomic strings. Normalizing data is crucial to successful structure solution and this will be investigated thoroughly. How best to incorporate high order (HOLZ) data, which contain high spatial frequency information will also be studied. The PI recently published a paper in Acta Cryst A detailing a new method to solve crystal structures using electron data combining 'charge-flipping' and phase symmetry. We plan to extend this to combine with elements of more conventional 'direct methods', a more statistical approach to structure analysis. By combining diffraction patterns recorded about a tilt axis it should be possible to solve crystal structures directly in 3D. This combination of tomographic acquisition and precession diffraction should prove to be a powerful tool. These methods can be applied to many crystal systems. In the proposal, we focus on three: The first in bismuth manganite, a multiferroic (ferroelectric and ferromagnetic) material, whose behaviour depends upon the crystal structure and oxygen sub-stoichiometry. The second is rhenium oxide with a negative thermal expansion dependent on the crystallography. Lastly, we plan to study metal-organic frameworks and how precession can be optimised to study this fascinating class of new materials, which have promise for catalysis, fuel cell technology and gas storage.