Development of methodologies for X-ray diffraction at ultra low temperatures

Abstract

The Xiphos is a new diffractometer that has been been designed and built to determine the solid-state structures of materials at ultra low temperatures. As one of very few single-crystal X-ray diffractometers capable of collecting diffraction data from crystals cooled to 2 K, the Xiphos has an enormous potential for improving our understanding of the relationship between the solid-state structure and the physical properties of many materials which show interesting and valuable behaviour at ultra low temperatures. However, there are some challenges to collecting good quality data due to the beryllium vacuum shrouds surrounding the crystal which are necessary components of the cryostat used for reaching these low temperatures.

A new program called Masquerade is described that helps to overcome these challenges by generating ‘masks’ describing the position of the beryllium scattering from the vacuum shrouds. These masks can be used during data integration to prohibit measuring the diffracted intensities of X-ray reflections which are contaminated by beryllium scattering. A new protocol for data collection is also presented to recover the reflection intensities that are missing as a result of the masks. Results are presented showing a marked improvement in the accuracy of the reflection intensities which can be obtained from diffraction patterns collected with the Xiphos when the data have been contaminated by beryllium scattering from the vacuum shrouds.

Several additional computer programs have been developed which enhance the data collection process by monitoring the sample temperature and offering the ability to monitor the status of the Xiphos and the progress of a diffraction experiment remotely. The development and use of these programs is described herein.

To show the Xiphos and Masquerade in action, structural studies at a range of temperatures between 2 K and 160 K are presented on charge-transfer complexes which exhibit superconductivity below 7 K, in addition to studies at ultra low temperatures of spin-crossover complexes, organic radicals and single molecular magnets. The relationship between the solid-state structure and interesting physical properties can be better understood by determining the structure at the same temperatures at which the properties are exhibited; a process that can now be undertaken routinely in a university laboratory with the Xiphos.