Molecular modelling of the deformation mechanisms acting in auxetic silica

Abstract

Molecular mechanics simulations have been performed to undertake a systematic investigation into the structure and mechanical properties of α-cristobalite undergoing uniaxial loading along each of the 3 mutually orthogonal principal directions and also hydrostatic pressure loading. Simulations were performed using both the BKS and Burchart force-fields. The simulations indicate that pressure loading and uniaxial loading along the x3 direction leads to uniform variation of the four independent Si-O-Si intertetrahedral angles, indicative of cooperative tetrahedral rotation about tetrahedral axes which transform the α-cristobalite structure into the ‘idealised’ β-cristobalite structure. Uniaxial loading along either of the transverse directions (x1 and x2) leads to a divergence of the intertetrahedral angles, consistent with tetrahedral rotation about the tetrahedral axes which transform the idealised β-cristobalite structure into the ‘ordered’ β-cristobalite structure. The data also indicate that a phase transition to one of the proposed β phases may be induced by a negative hydrostatic pressure or tensile stress along x3. The phase transition is accompanied by a change in sign of some of the Poisson’s ratios (i.e. from positive to negative). A negative hydrostatic pressure is also predicted to lead to conversion of initially positive to negative Poisson’s ratio values (within the same phase).