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Uni-axial Negative Thermal Expansion in Layered Perovskites

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Using first-principles calculations we explain why pronounced uniaxial negative thermal expansion (NTE) is found in certain layered perovskites, such as Ruddlesden-Popper oxides, but elusive in bulk ABO3 perovskites. We show that there are two key ingredients that determine the extent of NTE : (1) the proximity to a phase transition driven by a soft a phonon mode with negative Gruneisen parameter; and (2) 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. We show that the anisotropy in compliance in Ruddlesden-Popper oxides increases as a function of the density of interface layers in the material and describe a model for the atomic mechanism that is responsible for this feature. These insights open up the possibility of high-throughput screening of NTE materials using the elastic tensor as a descriptor.

This talk is part of the Theory of Condensed Matter series.

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