Crystal structure determination of Sr2Fe2O5 using conventional and in situ electron diffraction tomography

Date: 24 January 2020

Venue: UAntwerp, Campus Groenenborger, Room U.408 - Groenenborgerlaan 171 - 2020 Antwerp (Wilrijk) (route: UAntwerpen, Campus Groenenborger)

Time: 11:30 AM - 12:30 PM

Organization / co-organization: EMAT

Short description: Friday Lecture by Maria Batuk, EMAT

About the lecture

In the quest for sustainable energy sources, many applications feature perovskites as materials with high potential, especially the La1-xSrxFe1-yCoyO3-δ compositions, for example solid oxide and proton conducting fuel cells, thermochemical energy storage, chemical looping and water splitting. All these applications are based on reduction and oxidation (redox), however, the information on the evolution of these materials during this redox reaction is very scarce. XRD in situ can only study these materials in powder form, as the single crystals needed for single crystal X ray diffraction are too large to be relevant for these applications.  Therefore, we are investigating the possibility to study these in situ reactions in oxidizing and reducing gas environments with electron diffraction instead.

In my talk, I will first demonstrate the results on the ex situ electron diffraction study of the pristine Sr2Fe2O5 brownmillerite material, and afterwards show our first results from in situ electron diffraction in oxidizing environment upon heating. We chose this material as a test case since it is the parent material of that best performing perovskite La1-xSrxFe1-yCoyO3-δ, and the only one in the series of which the intermediate phases are known from alternate techniques.

We used electron diffraction tomography with and without precession to refine the crystal structure of Sr2Fe2O5. So far, the structure refinement from single crystal data using any radiation source was hindered by the severe twinning and extensive stacking faults in the crystals. With electron diffraction data we could circumvent these hurdles and successfully refine the structure.

Next, using the Climate holder from Denssolutions, we followed the phase transition from brownmillerite Sr2Fe2O5 to perovskite SrFeO3 and back, and observed several intermediate structures. Due to the single-tilt design of the environmental holders combined with the complexity of these structures, in-zone electron diffraction and high resolution imaging on random crystallites are unrealistic, but electron diffraction tomography does not require in zone orientation and could thus be applied to gather structural data on the different phases.