THEORETICAL-STUDY OF A 5-COORDINATED SILICA POLYMORPH

Theoretical calculations are performed to study transformations in sil ica as a function of nonhydrostatic stress. Molecular-dynamics calcula tions reveal a crystalline-to-crystalline transition from alpha-quartz to a phase with five-coordinated silicon (Si-V) at high pressure in t he presence of deviatoric stress. The phase, which appears for specifi c orientations of the stress tensor relative to the crystallographic a xes of quartz, is a crystalline polymorph of silica with five-coordina ted silicon. The structure possesses P3(2)21 space-group symmetry. Fir st-principles calculations within the local-density approximation, as well as molecular dynamics and energy minimization with interatomic po tentials, find this phase to be mechanically and energetically stable with respect to quartz at high pressure. The calculated x-ray diffract ion pattern and vibrational properties of the phase are reported. Upon decompression, the Si-V phase reverts to alpha-quartz through an inte rmediate four-coordinated phase and an unusual isosymmetrical phase tr ansformation. The results suggest the importance of application of non hydrostatic stress conditions in the design and synthesis of novel mat erials.