From Enhancement to Elimination of Auxeticity by Inserting Multilayers of Diatomic Molecules into a Hard-Sphere Crystal

Predicting how atomic-scale structural changes influence a system’s elastic properties is challenging in general. In this study, we examined how the reduction in the atomic degrees of freedom within layer inclusions embedded in a hard-sphere crystal affects its auxetic behaviour. This reduction was achieved by randomly pairing the neighbouring hard spheres into diatomic molecules called dumbbells. Specifically, we explored the effects of periodically introducing one-, two-, and three-layer inclusions of dumbbells on the crystal’s elastic properties. Our results show that inclusions of adjacent layers filled with dumbbells notably alter the elastic behaviour compared to systems with similar inclusions composed of spheres. We observed that by rigidly connecting neighbouring spheres to form diatomic molecules within inclusion layers, two opposing effects can be achieved. First, at slightly smaller diameters of the spheres forming dumbbells, we found an enhancement of auxetic properties in comparison with the system containing only hard spheres. Second, for systems with large sphere diameters forming dumbbells, we observed that the crystal’s auxetic properties were almost eliminated.