Anomalous elastic properties in stishovite
Auxetics are materials which have a negative Poisson's ratio, that is, upon uniaxial tensile loading, they also expand in a direction perpendicular to the applied force. Here, we analyze the elastic constants of stishovite, a high pressure silica polymorph which is known to be a significant constituent of the earth's mantle, and show that it exhibits a negative Poisson's ratio when stressed in a range of directions in the (100), (010) and (001) planes under specific ambient pressure ranges. We explain this behaviour through mechanisms involving rotations and distortions of the constituent octahedra. These findings have important practical implications since stishovite is one of the hardest known oxides, and has proven to be important to various fields ranging from seismology to materials science.