Auxeticity Modifications and Unit Cell Doubling in Yukawa Fcc Crystals with [001]-Nanochannels Filled by Hard Spheres

In recent years, the investigation of auxetic materials is receiving more and more attention due to their wide range of applications which follow enhancing indentation resistance, toughness, shear resistance, and other advantages of such materials. This work reports results of studies of models of auxetic metamaterials with nanoinclusions. Yukawa crystals with nanoinclusions in the form of nanochannels (NCs) in the [001] crystallographic direction, filled by hard spheres, were simulated by Monte Carlo in a wide range of pressures to determine their elastic properties. Particular attention has been devoted to the Poisson's ratio (PR). It has been found that depending on the NCs' type and pressure, the value of PR can vary from -0.302(12) to 1.083(14). The microscopic structures of the crystals were also examined in detail. A solid-solid phase transition in a host-guest system (the Yukawa crystal with hard spheres) was observed. Interestingly, this phase transition generates a unit cell doubling along the NCs. To localize this phase transition, apart from studies of the structure, the PR as a sensitive indicator of the phase transition was applied. In addition, it was found that the studied Yukawa systems with nanoinclusions for certain pressure values are completely non-auxetic, despite both the Yukawa and hard sphere crystals without inclusions are partially auxetic at the same conditions. This indicates that the presence of [001] NCs in the system not only can enhance auxeticity in comparison to the system without NCs but also, at some thermodynamic conditions, can lead to a completely non-auxetic behavior of the system which is partially auxetic without the NCs. Hence, one can use NCs to tune auxetic properties of crystals.