Prediction of a hexagonal SiO2 phase affecting stabilities of MgSiO3 and CaSiO3 at multimegabar pressures
Prediction of a hexagonal SiO2 phase affecting stabilities of MgSiO3 and CaSiO3 at multimegabar pressures
复制标题
DOI：
10.1073/pnas.1013594108
复制
发表时间：
2011-01
期刊：
影响因子：
--
通讯作者：
T. Tsuchiya;J. Tsuchiya
中科院分区：
文献类型：
--
作者：
T. Tsuchiya;J. Tsuchiya
SiO_2-SiO_2在几兆巴压力条件下的超高压相关系尚不清楚。在这里，我们报告了一个理论上的预测，以前未表征的稳定结构的二氧化硅与一个意想不到的六方Fe 2 P型形式。这一阶段，更稳定的cotunnite型结构，以前假设的postpyrite相，被发现稳定在640 GPa通过仔细的结构搜索通过从头算密度泛函计算在各种结构模型。这是所有二氧化物化合物中压力诱导相变到Fe 2 P型结构的第一个证据结果。晶体结构由紧密堆积的，相当规则的SiO 9三帽三角棱镜与一个显着紧凑的晶格。进一步的研究进一步阐明了这种SiO2相变对MgSiO 3和CaSiO 3在多兆巴压力下的稳定性的巨大影响。MgSiO 3的后钙钛矿相在1.04 TPa下沿着假定的超地球层破裂，并产生Fe 2 P型SiO2和CsCl（B2）型MgO。另一方面，CaSiO 3钙钛矿直接解离成SiO2和金属CaO，跳过后钙钛矿多晶型物。预测的SiO2、MgSiO 3和CaSiO 3的超高压和温度相图表明，Fe 2 P型SiO2可能是类地系外行星深部地幔和气态巨星核心的主要成分之一。
Ultrahigh-pressure phase relationship of SiO2 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 Fe2P-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 Fe2P-type structure among all dioxide compounds. The crystal structure consists of closely packed, fairly regular SiO9 tricapped trigonal prisms with a significantly compact lattice. Additional investigation further elucidates large effects of this phase change in SiO2 on the stability of MgSiO3 and CaSiO3 at multimegabar pressures. A postperovskite phase of MgSiO3 breaks down at 1.04 TPa along an assumed adiabat of super-Earths and yields Fe2P-type SiO2 and CsCl (B2)-type MgO. CaSiO3 perovskite, on the other hand, directly dissociates into SiO2 and metallic CaO, skipping a postperovskite polymorph. Predicted ultrahigh-pressure and temperature phase diagrams of SiO2, MgSiO3, and CaSiO3 indicate that the Fe2P-type SiO2 could be one of the dominant components in the deep mantles of terrestrial exoplanets and the cores of gas giants.