Metadata in high-pressure crystallography. Kamil Dziubek
Channel: International Union of Crystallography
Date: 08/09/2015

The deposition of metadata related to specific techniques used for crystallographic experiments can be simplified by formulating guidelines for their preparation. One of the experimental techniques quickly gaining ground in the field of crystallographic research is high-pressure diffraction studies. They involve additional equipment for pressure generation, pressure calibration, etc. The high pressure cell can interfere with the primary or diffracted beam, which can contaminate the diffraction patterns and introduce errors in reflection intensities. The experimental details are vital for the evaluation and analysis of the data, and therefore the metadata are needed to be stored along with the raw diffraction images. The most essential descriptors concern: (1) orientation of the pressure cell with respect to the incident beam and the detector; (2) the sample preparation and shape, both for powder and single crystals; (3) a reference to the high-pressure vessel, and for unique equipment the dimensions of its relevant components, such as the anvil design, gasket thickness, chamber diameter, backing-plate type; (4) chemical composition of the cell parts, e.g. anvils, gasket and backing plates, pressure-transmitting medium; (5) the method of fixing the sample in the high-pressure chamber, if used; (6) the method of positioning the pressure cell during the data acquisition; (7) the pressure-measurement method. This information is indispensable for reproducing the results of structural refinements from the raw data or for attempting other methods of refinement. The pressure transmitting medium can dramatically change the sample compression, due to its possible interaction with the sample (such as penetration into the pores) or hydrostatic limit of the medium. The sample history can also affect the results. If the sample was recrystallized in situ in isothermal or isochoric conditions, from solution or melt, the details of the crystallization protocol should be provided. Simple edition rules and a checklist can considerably simplify the deposition of metadata and increase their informative value.