下地幔压力下CaCO $_{3}$的后文石相
Post-aragonite phases of CaCO$_{3}$ at lower mantle pressures
摘要：
下地幔条件下碳酸盐矿物的稳定性、结构和性质对我们理解全球碳循环和地球内部组成有重要影响。近年来，人们对下地幔条件下碳酸盐岩的行为，特别是它们的碳杂化产生了极大的兴趣，这与深部地幔中碳的储存有关。使用高压同步辐射X射线衍射在金刚石压砧单元耦合直接激光加热的CaCO $_{3}$使用CO $_{2}$激光器，我们确定的结晶相的材料高于40 GPa $-$对应的下地幔深度约1,000公里$-$已首先预测的\textit{ab initio}结构预测。在40 GPa下观察到的$sp^{2}$碳杂化物质是具有$P2_{1}/c$对称性的单斜晶系，并且在高达50 GPa下是稳定的，在50 GPa以上，它转变成不能由现有已知相索引的结构。结合\textit{ab initio}随机结构搜索（AIRSS）和准谐波近似（QHA）计算，重新探索CaCO富相图的相对相稳定性$_{3}$。采用弹性带（NEB）方法研究了$_{3}$相关晶相之间的反应机理，并推测该矿物能够发生$sp^{2}$杂化变化，其杂化变化完全发生在$P2_{1}/c$结构$-$中，而放弃了已被接受的后文石$Pmmn$结构。
英文摘要：
The stability, structure and properties of carbonate minerals at lower mantle conditions has significant impact on our understanding of the global carbon cycle and the composition of the interior of the Earth. In recent years, there has been significant interest in the behavior of carbonates at lower mantle conditions, specifically in their carbon hybridization, which has relevance for the storage of carbon within the deep mantle. Using high-pressure synchrotron X-ray diffraction in a diamond anvil cell coupled with direct laser heating of CaCO$_{3}$ using a CO$_{2}$ laser, we identify a crystalline phase of the material above 40 GPa $-$ corresponding to a lower mantle depth of around 1,000 km $-$ which has first been predicted by \textit{ab initio} structure predictions. The observed $sp^{2}$ carbon hybridized species at 40 GPa is monoclinic with $P2_{1}/c$ symmetry and is stable up to 50 GPa, above which it transforms into a structure which cannot be indexed by existing known phases. A combination of \textit{ab initio} random structure search (AIRSS) and quasi-harmonic approximation (QHA) calculations are used to re-explore the relative phase stabilities of the rich phase diagram of CaCO$_{3}$. Nudged elastic band (NEB) calculations are used to investigate the reaction mechanisms between relevant crystal phases of CaCO$_{3}$ and we postulate that the mineral is capable of undergoing $sp^{2}$-$sp^{3}$ hybridization change purely in the $P2_{1}/c$ structure $-$ forgoing the accepted post-aragonite $Pmmn$ structure.
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