Cubic energy materials such as thermoelectric or hybrid perovskite materials are often considered highly disordered. In GeTe and related IV-VI compounds, it is believed to provide the low thermal conductivity needed for thermoelectric applications.
Because conventional crystallography can’t distinguish between static disorder and atomic motion, researchers at Columbia Engineering and Université de Bourgogne have developed a new kind of “camera” that can see the local disorder.
Striking is a variable shutter speed. Because the disordered atomic clusters move, the dynamic disorder was obscured when the team used a slow shutter, but was visible when they used a fast shutter. The new technique, known as variable shutter PDF or vsPDF (for atomic pair distribution function), differs from conventional cameras in that it measures atomic positions with a shutter speed of about one picosecond, or a million (a trillion) times faster than normal camera shutters. It uses neutrons from a source at the Oak Ridge National Laboratory (ORNL), a facility of the US Department of Energy.
Simon Billinge, professor of materials science and applied physics and mathematics, said: “Only with this new vsPDF tool can we see this side of materials. It gives us a whole new way to unravel the complexity of what happens in complex materials, hidden effects that can enhance their properties. With this technique we can look at a material and see which atoms are in the dance and which are out.”
The researchers identified broken atomic symmetries in GeTe, a crucial material for thermoelectricity, which converts waste heat into energy, using the vsPDF technique (or electricity in cooling). The displacements and the dynamic changes and how quickly they fluctuated had never been seen before.
The team created a new hypothesis that explains how such local fluctuations may arise in GeTe and related materials as a result of vsPDF’s discoveries. Researchers can find new materials with these properties and apply external factors to influence the result, creating even better materials with a mechanistic understanding of the dance.
The next step is to make his technique easier for the research community and apply it to other systems with the dynamic disorder. At the moment the technique is not yet turn-key, but with further development it should become a much more standard measurement.
- Kimber, SAJ, Zhang, J., Liang, CH, et al. Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe. Wet. case. 22, 311-315 (2023). DOI: 10.1038/s41563-023-01483-7