MS23-P09 Riding the camel: the double-peaked rocking curve and its use in the processing of precession electron diffraction dataData collected by the precession electron diffraction (PED) technique can be successfully used for solution and also accurate refinement of unknown crystal structures. Especially efficient in this respect is the technique of PED tomography (PEDT), where a series of PED patterns are collected on a crystal progressively tilted by small steps around a rotation axis .
Such PEDT data have certain special characteristics that make it different from standard rotation data. The most prominent is the rocking curve of a reflection in PEDT data, i.e. the reflection intensity as a function of the tilt angle. While the rocking curve of non-precession data is a narrow single peaked function, the rocking curve of a reflection in PED is a broad function with two maxima. Typically, each reflection is measured several times. The precession angle is usually much larger than the angular width of the non-precession rocking curve of the reflections, and therefore the width and shape of the rocking curve is determined mostly by the precession angle and not so much by the crystal itself. Because of the specific shape of the curve, we call the plot of the precession rocking curve colloquially "the camel plot".
This feature can be exploited in several way during the analysis of the PEDT data. One of the applications is the refinement of the crystal orientation. The crystals are often not steady during the PEDT experiment, but they turn. Ignoring this rotation leads to distorted data sets and very inaccurate lattice parameters. Using the knowledge of the rocking curve shape allows an accurate refinement of the crystal orientation during the experiment and substantial improvement of the data.
The knowledge of the rocking curve can also be exploited in the intensity extraction. The typical approach to the determination of the total intensity of certain reflection is the integration of intensity accross all frames on which the reflection is measured. However, this provides inaccurate rresults for partially covered reflections and it is also sensitive to intensity variations due to dynamical diffraction. Knowng the rocking curve allows the total intensity to be determined more accurately by fitting the rocking curve profile to the reflection intesities.
Implementation of these features in the computer program PETS 2.0  and their application to a number of practical examples shows that "riding the camel" can lead to significantly improved data quality and sometimes make a difference between a solved and unsolved structure.References:
 Mugnaioli, E., Gorelik, T. & Kolb, U. (2009). Ultramicroscopy, 109, 758-765
 Palatinus, L., Brazda, P., Jelinek, M., Hrda, J., Steciuk, G., Klementova, M. (2018). In preparationKeywords: precession electron diffraction, intensity extraction,