Speaker: Dr. Arash Mostofi (Director, CDT in Theory and Simulation of Materials Imperial College London)
Date/Time: 19 October 2017; 15:00-16:00
Venue: Cassie Building Room 3.25
Using first-principles calculations we explain why pronounced uniaxial negative thermal expansion (NTE) is found in certain layered perovskites, but elusive in bulk ABO3 perovskites. We show that there are two key ingredients that determine the extent of NTE:
- the proximity to a phase transition driven by a soft a phonon mode with negative Gruneisen parameter
- the presence of large off-diagonal components of the elastic compliance tensor that predispose the material to the deformations required for NTE.
Whilst both ABO3 and layered perovskites often possess the former, it is the unusually high elastic compliance unique to layered perovskites of certain symmetry that allows NTE to persist over a large temperature range. This fundamental insight opens up the possibility of high-throughput screening of NTE materials using the elastic tensor as a descriptor.