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A mechanism for aragonite to post-aragonite transition in MCO3 (M = Ca, Sr and Ba) carbonates: evidence of a hidden metastable polymorph
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2022-11-24 , DOI: 10.1039/d2cp04209f Miguel A Salvadó 1 , Pilar Pertierra 1 , J Manuel Recio 1
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2022-11-24 , DOI: 10.1039/d2cp04209f Miguel A Salvadó 1 , Pilar Pertierra 1 , J Manuel Recio 1
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To advance in the understanding of the Earth's carbon cycle, it is necessary to determine thermodynamic boundaries and kinetic barriers associated with the pressure-induced polymorphic sequence of alkaline-earth carbonates. Following a symmetry-based strategy within the martensitic approximation, we propose a two-step mechanism mediated by a hexagonal P63/mmc structure for the aragonite to post-aragonite transformation in the MCO3 (M = Ca, Sr, Ba) crystal family. The calculated transition pressures and activation energies, from ∼7 to 42 GPa and ∼0.3 to 0.6 eV, respectively, are low enough to allow this transformation to occur under mantle conditions. Our analysis reveals that the intermediate hexagonal structure is the early one proposed by Holl et al., Phys. Chem. Miner., 2000, 27, 467–473 for high pressure BaCO3, and later considered as metastable. Phonon calculations inform that this P63/mmc structure is in fact unstable at zero pressure. Remarkably, our molecular dynamics calculations showed that this instability smoothly leads to a dynamically stable P63mc structure, which we confirm is actually the phase observed by Holl et al. This finding allows us to reconcile previous controversial data and contributes to clarifying the role of carbonates in the Earth's interior.
中文翻译：
MCO3（M = Ca、Sr 和 Ba）碳酸盐中文石向后文石转变的机制：隐藏的亚稳态多晶型物的证据
为了推进对地球碳循环的理解，有必要确定与压力诱导的碱土碳酸盐多晶型序列相关的热力学边界和动力学障碍。遵循马氏体近似内基于对称的策略，我们提出了一种由六角P 6 3 / mmc结构介导的两步机制，用于 MCO 3中文石到后文石的转变(M = Ca, Sr, Ba) 晶体族。计算出的转变压力和活化能分别为 ~7 到 42 GPa 和 ~0.3 到 0.6 eV，足够低以允许这种转变在地幔条件下发生。我们的分析表明，中间六角结构是 Holl等人提出的早期结构。,物理。化学。矿工。, 2000, 27 , 467–473 用于高压 BaCO 3，后来被认为是亚稳态的。声子计算表明，这种P 6 3 / mmc结构实际上在零压力下不稳定。值得注意的是，我们的分子动力学计算表明，这种不稳定性平稳地导致动态稳定的P6 3 mc结构，我们确认这实际上是 Holl等人观察到的相位。这一发现使我们能够调和以前有争议的数据，并有助于阐明碳酸盐在地球内部的作用。
更新日期：2022-11-29
中文翻译：
MCO3（M = Ca、Sr 和 Ba）碳酸盐中文石向后文石转变的机制：隐藏的亚稳态多晶型物的证据
为了推进对地球碳循环的理解，有必要确定与压力诱导的碱土碳酸盐多晶型序列相关的热力学边界和动力学障碍。遵循马氏体近似内基于对称的策略，我们提出了一种由六角P 6 3 / mmc结构介导的两步机制，用于 MCO 3中文石到后文石的转变(M = Ca, Sr, Ba) 晶体族。计算出的转变压力和活化能分别为 ~7 到 42 GPa 和 ~0.3 到 0.6 eV，足够低以允许这种转变在地幔条件下发生。我们的分析表明，中间六角结构是 Holl等人提出的早期结构。,物理。化学。矿工。, 2000, 27 , 467–473 用于高压 BaCO 3，后来被认为是亚稳态的。声子计算表明，这种P 6 3 / mmc结构实际上在零压力下不稳定。值得注意的是，我们的分子动力学计算表明，这种不稳定性平稳地导致动态稳定的P6 3 mc结构，我们确认这实际上是 Holl等人观察到的相位。这一发现使我们能够调和以前有争议的数据，并有助于阐明碳酸盐在地球内部的作用。
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