Philippe Boullay

Determination of a crystalline structure by electron microscopy techniques

The ability to experimentally define the different microstructures actually involved in a phase transformation is an important issue if one wants to validate some theoretical works on ``phase transitions in crystals''. Some features of EM techniques in terms of symmetry and structure determination are reminded.

The dynamic conditions of the scattering of electrons by the atoms in the crystal allow to obtain unique informations on the symmetry elements present in this crystal. The point group can be obtained and this even for nanometric scale particles.

High Resolution Electron Microscopy (HREM) is also an unique tool to access structural information at the microscopic scale. The use of the image mode is illustrated by an example extract from the work previously realized by the author on the oxide $Ba_xMnO_2$ ($x\approx$0.25) having an incommensurate structure - see ref.(1) for details.

The different steps of the structure determination are developped:

$\bullet$ use of the analytical capabilities of the Transmission Electron Microscope (TEM): chemical composition evaluation.

$\bullet$ collect of Electron Diffraction (ED) patterns: reconstitution of a 3D map of the reciprocal space and interpretation in terms of symmetry.

$\bullet$ interpretation of HREM images in the aim of structure modelisation: intuitive approach and image simulation technique.

$\bullet$ study of local phenomena: interpretation of the structural disorders existing in the example.

Lastly the author points out the question of the accuracy of the structure determination using EM techniques:

$\bullet$ enhancement of the accuracy by coupling EM and other techniques (e.g. X-Ray Powder Diffraction, ...) when the image interpretation leads to a qualitative non accurate structural model.

$\bullet$ recent developments in the field of ``Electron Crystallography'' are mentionned. Comparisons with X-Ray and Neutron Diffraction are drawn.

(1) Ph. Boullay, M. Hervieu and B. Raveau, J. Solid State Chem. 132, (1997) 239.