Evaluate potency
What is potency?
In a high-pressure X-ray diffraction experiment, the X-rays cannot access the sample from all directions. Instead, some of them are inevitably absorbed by the metal elements of a Diamond Anvil Cell or other equipment used to exert the pressure. As a result, only a fraction of reflections can be collected during an experiment, resulting in a loss of diffraction data.
However, since crystals tend to have internal symmetry, not all reflections need to be measured to provide complete information. If two reflections are symmetrically equivalent, any of them suffices to inform us about the underlying crystal structure. The percent of independent reflections which should be theoretically available in an experiment can be estimated based on the crystal orientation and the anvil cell geometry. We call this value Potency.
How can I calculate potency?
For a given crystal lattice parameters and orientation, the reflection-associated nodes are placed in the reciprocal space and deemed accessible or inaccessible based on the anvil cell geometry. The number of independent accessible reflections is then juxtaposed with a sum of accessible and inaccessible reflections within a certain resolution threshold in order to evaluate the potency. This procedure has been implemented in Python package hikari described in the hikari tab.
While hikari provides much wider capabilities, the potency of individual experimental set-up can be also evaluated here. In order to do this, provide unit cell parameters, crystal symmetry, and experimental conditions in the form above. After clicking "Submit", the job will be assigned an ID and added to the queue. To review the results, refresh the page after the job has been completed. You can also access previous jobs by providing their ID via the browse tab.
How do I use potency maps?
Potency maps present the attainable completeness as a function of crystal orientation, assuming fixed values of other parameters. As a result, they can be utilised in a variety of ways.
Before a high-pressure experiment is even performed, a single potency map can be used to predict an expected value of completeness. After the experiment has been performed, the completeness-to-potency ratio can be used to validate the data reduction process. Ratio values significantly lower then 1.0 suggest that the experimental strategy does not probe the whole reciprocal space, leaving some otherwise accessible reflections unmeasured. Ratio values higher than 1.0 can be observed whenever partially-absorbed or diamond-diffracted signals have been erroneously included during data reduction.
Multiple potency maps can be compared to evaluate experimental conditions rather than crystal orientation. For example, two maps generated assuming different incident radiation wavelengths can be contrasted to decide whether a benefits of using a shorter wavelength are worth the drawbacks of a lower beam intensity.
References
If you have found this page useful or you have utilised any potency evaluation tools in your work, please consider consulting attributing the back-end library "hikari" by citing the following paper: