Search for silica polymorphs denser than stishovite

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A hypothetical polymorph of silica with the hydrazinium dichloride structure and denser than stishovite by ca. 6 %, was discovered by the present author and co-investigators in 1988. The transition pressure of stishovite to the predicted new polymorph is, however, too high to be significant within the Earth's mantle.A systematic survey in search for other polymorphs has been carried out by following way : By means of the molecular dynamics calculation assuming empirical and non-empirial two-body potentials for the Si-O system, behaviours under compression up to 200 GPa have been investigated on virtually all known AX_2 and A_2X compounds at 300 K. The results show that the following four types are fundamental structures for silica under pressures : 1) the retile type(known as stishovite), 2)the alpha-PbO_2 type, 3)the hydrazinium dichloride type and 4)the orthorhombic ZrO_2 type. Among them, the already studied type(the hydrazinium dichloride type)showed the highest density. If follows that the rutile type and the CaCl_2-type silica(the latter being the high pressure modification of the rutile type)is only stable within deep interior of the Earth. So the project failed to discover any miracle structure(s).During the course of the study, remarkable phonon softing was observed in the rutile type silica. At -l5O GPa(the value critically depending on assumed potential parameters)the frequency of acoustic phonon mode TA fall to zero, still holding a tetragonal symmetry, and orthorhombic CaCl_2 structure becomes stable at - 160 GPa. The mechanism of this peculiar behaviour has been intersively studied by molecular dynamics, lattice dynamics and by modeling according to the Ginzburg-Landau model.