Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan.
Proc Natl Acad Sci U S A. 2011 Jan 25;108(4):1252-5. doi: 10.1073/pnas.1013594108. Epub 2011 Jan 5.
Ultrahigh-pressure phase relationship of SiO(2) silica in multimegabar pressure condition is still quite unclear. Here, we report a theoretical prediction on a previously uncharacterized stable structure of silica with an unexpected hexagonal Fe(2)P-type form. This phase, more stable than the cotunnite-type structure, a previously postulated postpyrite phase, was discovered to stabilize at 640 GPa through a careful structure search by means of ab initio density functional computations over various structure models. This is the first evidential result of the pressure-induced phase transition to the Fe(2)P-type structure among all dioxide compounds. The crystal structure consists of closely packed, fairly regular SiO(9) tricapped trigonal prisms with a significantly compact lattice. Additional investigation further elucidates large effects of this phase change in SiO(2) on the stability of MgSiO(3) and CaSiO(3) at multimegabar pressures. A postperovskite phase of MgSiO(3) breaks down at 1.04 TPa along an assumed adiabat of super-Earths and yields Fe(2)P-type SiO(2) and CsCl (B2)-type MgO. CaSiO(3) perovskite, on the other hand, directly dissociates into SiO(2) and metallic CaO, skipping a postperovskite polymorph. Predicted ultrahigh-pressure and temperature phase diagrams of SiO(2), MgSiO(3), and CaSiO(3) indicate that the Fe(2)P-type SiO(2) could be one of the dominant components in the deep mantles of terrestrial exoplanets and the cores of gas giants.
在兆巴压力条件下，SiO2（二氧化硅）的超高压相态关系仍相当不清楚。在此，我们报告了一个关于二氧化硅以前未被描述的稳定结构的理论预测，其具有意想不到的六方 Fe2P 型形式。通过对各种结构模型进行从头算密度泛函计算的仔细结构搜索，发现这种相比先前推测的后黄铁矿相更稳定，在 640 GPa 下稳定存在。这是所有二氧化物化合物中第一个证明压力诱导向 Fe2P 型结构转变的证据。晶体结构由紧密堆积的、相当规则的 SiO9 三帽三角棱柱组成，晶格非常紧凑。进一步的研究进一步阐明了这种 SiO2 相变对多兆巴压力下 MgSiO3 和 CaSiO3 稳定性的巨大影响。假设超地球行星的绝热线上，MgSiO3 的后钙钛矿相在 1.04 TPa 下破裂，并产生 Fe2P 型 SiO2 和 CsCl（B2）型 MgO。另一方面，CaSiO3 钙钛矿直接分解为 SiO2 和金属 CaO，跳过了后钙钛矿多型体。预测的 SiO2、MgSiO3 和 CaSiO3 的超高压和温度相图表明，Fe2P 型 SiO2 可能是类地系外行星深部地幔和类木行星核心的主要成分之一。
