Search for silica polymorphs denser than stishovite

一种假设的二氧化硅多晶型物，具有二氯化肼结构，密度比硅镁石大。6%，是作者和合作研究者在1988年发现的。然而，斯铁橄榄石向预测的新多晶型体的转变压力太高，在地球地幔中不具有重要意义。通过对Si-O系统采用经验和非经验两体势的分子动力学计算，在300 K下，对几乎所有已知的AX_2和A_2X化合物在200 GPa压力下的压缩行为进行了研究。结果表明，压力下SiO_2的基本结构有四种类型：1）锐钛矿型（又称方石英），2）α-PbO_2型，3）二氯化肼型，4）正交ZrO_2型。其中，已经研究过的类型（二氯化肼型）密度最高。因此，金红石型和CaCl_2型二氧化硅（后者是金红石型的高压改性）只在地球深部稳定。在研究过程中，在金红石型二氧化硅中观察到了明显的声子软化现象。在-150 GPa（该值与假定的势参数密切相关）下，声子模TA的频率降为零，仍保持四角对称性，正交CaCl_2结构在-160 GPa下稳定。这种特殊行为的机制已被深入研究的分子动力学，晶格动力学和根据Ginzburg-Landau模型建模。

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.