ELECTRON DIFFRACTION TOMOGRAPHY IN MINERALOGY

Electron diffraction tomography (EDT) provides 3D information about the reciprocal space of studied material. It was originally developed for data collection with the aim of structure solution as an analogue of rotation method used in x-ray crystallography. The EDT experiment is quite simple. Starting with a random crystal orientation, the crystal is sequentially tilted in a range (usually 90-120deg), and a diffraction pattern is recorded at every tilt step (usually 1deg). It can be done manually or automatically, possibly with limited input needed from the user to check the sample position during tilting. Sampling of the reciprocal space obtained by EDT is much finer than collecting oriented zone-axis patterns, but one degree step is still quite coarse. This undersampling problem can be solved in two different ways: by precession EDT (PEDT) [1] and rotation EDT (REDT) [2].

The 3D information on reciprocal space can be used in many ways, from simple phase identification to determination of orientation relationship between precipitate and matrix to more complex tasks such as structure solution and refinement. At present, complex structures with hundreds of atoms in the unit cell can be solved. The initial model for the refinement is typically optimized using the kinematical approximation for the calculation of model diffracted intensities. This approximation is quite inaccurate for electron diffraction and leads to high figures of merit and inaccurate results with unrealistically low standard uncertainties. The obvious remedy to the problem is the use of dynamical diffraction theory to calculate the model intensities in structure refinement. This technique can be used not only on oriented zone-axis patterns [3] but also on non-oriented patterns acquired by PEDT [4]. Potential of the technique will be demonstrated on samples of widenmannite and orthopyroxene.

[1] E. Mugnaioli et al., Ultramicroscopy 109 (2009) 758-765.

[2] D. Zhang et al., Z. Kristallogr. 225 (2010) 94-102.

[3] L. Palatinus et al., Acta Cryst. A69 (2013) 171–188.

[4] L. Palatinus et al., Acta Cryst. A71 (2015) 235-244.