Electron diffraction in geology, petrography and space: an interview

Dr. Enrico Mugnaioli is one of the first developers and users of the Automated Diffraction Tomography (ADT) for the acquisition of 3D electron diffraction data from single nanocrystals. Being  mainly involved in the development and application of electron crystallographic methods for the structure characterization of nanocrystalline and beam-sensitive materials, with special focus on nanoparticles, porous materials, pharmaceutics and macromolecules, we are more than happy that he is presenting for our upcoming webinar (you can register here) and answered some of our questions about the technique and the upcoming ELDICO webinar.

You are one of the authors of the breakthrough paper ‘3D Electron Diffraction: The Nanocrystallography Revolution’. What is your take on the title and the importance of electron diffraction?

I was very happy when ACS Central Science proposed to us to write this review, because I strongly felt the need for a genuine overview about what we call 3D ED. Since the beginning, this method has been independently developed by various laboratories and everyone was using different, and sometimes misleading acronyms. I liked instead to underline the overall idea of what is common to all these approaches: collecting diffraction data with a ‘tomographic’ strategy and without orienting the sample. The review was also important for looking back at our first steps and listing together all the published papers where 3D ED was used. There were more than I expected.

You are presenting soon in our webinar on the value proposition of electron diffraction for geology and planetary sciences. A sneak-peek into the upcoming talk – how is ED revolutionizing these fields?

Crystallography is essential for mineralogy. Because rocks are made of minerals, I think the true understanding of all geologic issues must pass through mineralogy, and therefore through crystallography. Like for many other scientific disciplines, one of the most interesting frontiers for modern geology is connected with the possibility to understand nano-scaled phenomena, and it is here where electron diffraction joins the game. We can use the methodology for understanding what is going on in very small and critical systems, like nano-inclusions, high-pressure samples or micro-meteorites. For all these systems the only crystallographic option was powder X-ray, which is a less desirable option when you have many different components inside.

Do you have examples of long-unsolved structures where electron diffraction made the difference?

A quite long list indeed. One of the first structures solved by 3D ED was the mineral charoite-90, which was known since 1970. Charoite embodies all problems that X-ray powder cannot solve but ED can: charoite forms sub-micrometric crystals, inter-grows with other minerals, has long cell parameters, forms several polytypes with close geometry, is full of stacking faults. Moreover, charoite is also beam sensitive and is therefore hard to analyze with high-resolution TEM imaging. Indeed, charoite is not an isolated case: one of the last structures I solved was kaliophilite, a mineral first recognized in 1839. Even Italy did not exist at that time. 

What are challenges in performing ED on the present instrumentation available?

I think a very experienced expert can turn a TEM into a reasonable electron diffractometer, but it remains a tough job and it remains a full-blown electron microscope with all its complexities. The lack of dedicated instruments does hinder the desirable and necessary dissemination of this fascinating technique which provides so much more and deeper insight into the matter. I know many colleagues that would like to use 3D ED for solving their problems, without the need to invest their precious time and energy for instrumentation issues. I think the possibility to have a dedicated electron diffractometer will be very appreciated by all these people.

Where do you see electron diffraction in 10 years from now?

3D ED has surely been a revolution for electron diffraction. Nowadays ED it used for dealing with a much wider pool of problems and sample types. At the same time, it has been discovered by a far larger community of scientists. I think it has good chances to be a true breakthrough in the whole area of crystallography. A breakthrough that has not fully bloomed yet. I still see huge perspectives of growth.