Symmetry Switching of Negative Thermal Expansion by Chemical Control
The vast majority of materials expand on heating. Therefore the rare exceptions which contract with increasing temperature – a novel property known as negative thermal expansion (NTE) – are interesting, both from a scientific and a technological perspective.
It is a common problem in engineering that when two materials that expand differently when heated are coupled together, this can lead to failures if the system undergoes large temperature changes. By understanding, and better still: being able to control, the rate of expansion of a material with temperature, we can design new materials with tunable thermal expansion properties.
Controllable uni-axial NTE (NTE along one dimension) has recently been demonstrated over a large temperature range in a layered perovskite material, known as a Ruddlesden-Popper oxide, with chemical formula Ca3-xSrxMn2O7. By varying the proportions of calcium and strontium in the structure, researchers were able to exploit the competition between different crystal phases to tune the thermal expansion from pronounced NTE right through to positive thermal expansion.
The team, including members from Imperial College London, Oxford University, Diamond Light Source and institutions in Korea and the US, have synthesised the material and demonstrated the effect, measured its crystal structure, and performed computational simulations.
Chris Ablitt, (TSM-CDT Cohort 6) used density functional theory (DFT) calculations to show that a particular type of atomic vibration, which consists of the tilting of rigid octahedral units in the structure, could be driving the NTE. He also showed that, when varying the amount of strontium in the compound, the change in frequency of these vibrations correlates with the change in the rate of the experimentally measured thermal expansion.
These results pave the way to being able to understand NTE in perovskite materials and to design materials with controllable thermal expansion.
The work has been published in the Journal of the American Chemical Society [http://pubs.acs.org/doi/abs/10.1021/jacs.5b13192].