Understanding Negative Thermal Expansion in Layered Perovskites

Negative thermal expansion (NTE) is an unusual phenomenon where a material shrinks rather than expands with increasing temperature. We will present recent results showing that in layered perovskites there is a significant enhancement of elastic anisotropy due to symmetry breaking that results from the combined effect of layering and frozen rotations of oxygen octahedra. This feature, unique to layered perovskites of certain symmetry, is what allows uniaxial NTE to persist over a large temperature range [1]. Since the structure of this phase facilitates cooperative strains in-plane and along the layering axis without necessitating the deformation of stiff nearest neighbour bonds, it has been possible to derive a mathematical description of this mechanism in an idealised system using simple geometrical models [2]. This insight has allowed us to investigate how changing structural features, such as the layer thickness [3]; physical features, such as temperature [1]; and chemistry, by substitutional doping [4] may all be used to control the thermal expansion of the material.

[1] Ablitt, C. et al. npj Comput. Mater. 3, 44 (2017)
[2] Ablitt, C. el al. arXiv:physics/0402096 (2018)
[3] Ablitt, C. et al. Front. Chem. 6, 455 (2018)
[4] Senn, M. S. et al. J. Am. Chem. Soc. 138, 5479–5482 (2016)