Cryogenic Gas Temperature Characterization and Calibration

The automation of rapid full collections of single crystal X-ray diffraction data at various temperatures would be a powerful tool in the investigation thermal based phase transitions including spin crossover and anomalous thermal expansion properties. For these types of studies the ability to rapidly and accurately change the sample temperature would be essential. Monitoring tools were developed to log the gas temperature of the installed cryogenic cooling devices. Of particular interest is the speed of temperature change, the time taken to stabilize at a specified temperature and extent of overshoot. Additionally, automation tools for full collections at multiple temperatures have been developed. Compounds with accurately known phase transitions at temperatures of interest were crystallized and used as in situ calibrants.

Figure 1: Phase transition of Rochelles salt (ammonium potassium tartrate dihydrate) from 106 – 113K.

Phosphor Correction

High resolution data collected at the Australian Synchrotron were shown to lead to models with unusual thermal ellipsoids and in some cases non-positive definite (NPD) errors on refinement. Analysis of the issue with the kind assistance of Prof. George Sheldrick and Prof. Kay Deidrichs, led to the need for a sensor thickness correction factor to be added. An approximate value of 0.010 mm was added to the XDS software input file and has yielded a great improvement to the quality of high resolution structures. It would be of benefit to have an empirically measured value for this correction.