Search for silica polymorphs denser than stishovite

本文作者和合作者在1988年发现了一种假设的二氧化硅多晶型，具有二氯化肼结构，密度比辉石高约6%。然而，针长石向预测的新多晶型转变的压力太高，在地球的地幔中是不显著的。通过假设Si-O体系的经验和非经验两体势的分子动力学计算，研究了几乎所有已知的AX_2和A_2X化合物在300 K下高达200 GPa的压缩行为。结果表明：在压力作用下，二氧化硅的基本结构有四种类型：1)点状结构（即硅辉石），2)α - pbo_2型，3)二氯化肼型和4)正交ZrO_2型。其中，已研究型（二氯化肼型）的密度最高。由此可见，金红石型和cacl_2型二氧化硅（后者是金红石型的高压改性）只在地球内部深处稳定。因此，该项目未能发现任何奇迹结构。在研究过程中，在金红石型二氧化硅中观察到显著的声子软化。在- 150gpa时（该值主要取决于假设的电位参数），声子模式TA的频率降至零，仍然保持四方对称，而正交CaCl_2结构在- 160gpa时变得稳定。分子动力学、晶格动力学和根据金兹堡-朗道模型建立的模型对这种特殊行为的机理进行了深入的研究。

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.