Origins of anisotropic thermal expansion in flexible materials release_zskepk4mbrgzrkjow6r4l7oweu

Abstract

A definition of the Grüneisen parameters for anisotropic materials is derived based on the response of phonon frequencies to uniaxial stress perturbations. This Grüneisen model relates the thermal expansion in a given direction (α_ii) to one element of the elastic compliance tensor, which corresponds to the Young's modulus in that direction (Y_ii). The model is tested through ab initio prediction of thermal expansion in zinc, graphite, and calcite using density functional perturbation theory, indicating that it could lead to increased accuracy for structurally complex systems. The direct dependence of α_ii on Y_ii suggests that materials which are flexible along their principal axes but rigid in other directions will generally display both positive and negative thermal expansion.
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