Abstract
The structural variations with pressure in α-cristobalite, a low-density polymorph of SiO2, have been studied through first-principles calculations using the projector-augmented-wave (PAW) method, with particular emphasis on its elastic and auxetic properties. We provide theoretical ab initio results for the volume compressibility and a complete set of independent elastic constants of cristobalite under hydrostatic pressures up to 10–15 GPa. Our calculated structural and elastic properties under pressure are in good agreement with the experimental data. In addition, the corresponding results of the molecular-dynamics simulations with the interatomic potential are also presented for comparison. The dominant mechanism of compression is the reduction of the Si–O–Si angles within the α-cristobalite structure, whereas the SiO4 tetrahedron undergoes only a slight distortion. α-Cristobalite is more compressible than other SiO2 polymorphs as shown by their volume compressibilities, because of its characteristic framework structure similar to re-entrant honeycombs. With increasing pressure, the rotational motions of the rigid SiO4 tetrahedra play an important role in the compressive behavior of cristobalite. The present simulations confirm that the system undergoes transformation from auxetic to non-auxetic under hydrostatic pressure of ca. 2 GPa, while retaining strong elastic anisotropy.
