从头开始评估从水铁矿到针铁矿的固态转化途径,ACS Earth and Space Chemistry

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从头开始评估从水铁矿到针铁矿的固态转化途径
ACS Earth and Space Chemistry ( IF 2.9 ) Pub Date : 2022-03-09 , DOI: 10.1021/acsearthspacechem.2c00026
Michel Sassi ₁ , Kevin M. Rosso ₁

Affiliation

铁（氧氢）氧化物矿物之间的相变可以通过固态离子的自扩散在局部发生，特别是在高温下。然而，在环境条件下，固态转化路径的重要性，特别是对于相关扩散路径长度较短的纳米粒子，仍然知之甚少。在这里，我们使用密度泛函理论，通过计算能量成本来评估从 Michel 等人的模型 Fh 结构获得 Gt 所需的最小铁阳离子集的能量成本，评估了水铁矿 (Fh) 转变为针铁矿 (Gt) 的前景。2007 年¹和 Manceau 等人。2014 年^。2虽然从 Manceau Fh 模型到 Gt 的转换很简单，并且涉及 1.04 eV/晶胞的中等活化能，但从 Michel 模型开始进行同样的操作更加复杂，并且可能涉及结构上类似于 Gt 的具有能量竞争性的中间正交相通过铁阳离子交换质子到 Gt。确定了产生正交中间体的两种过渡途径，其中最快的涉及具有2.09 eV /晶胞的大活化能的限速原子位移。作为伴随的结果，我们还计算了将 Fh 的 Michel 和 Manceau 模型相互连接的固态转换能量学，它们之间的活化能约为 1.82 eV/cell。我们讨论并比较了我们在 Fh 模型中计算的与温度相关的铁自扩散系数与从高温测量得出的赤铁矿和磁铁矿的自扩散系数。结果表明，尽管尚未明确处理 pH 影响，但即使对于 Fh 的纳米级粒径，在环境条件下也不太可能获得 Gt 的固态转化途径，这表明其他途径更为重要。

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Ab Initio Evaluation of Solid-State Transformation Pathways from Ferrihydrite to Goethite

Phase transformations between iron (oxyhydr)oxide minerals can occur topotactically through the self-diffusion of ions in the solid state, particularly at high temperatures. However, under ambient conditions, the extent to which solid-state transformation pathways could be important, especially for nanoparticles where the associated diffusion path lengths are short, remains poorly known. Using density functional theory, here, we evaluated this prospect for ferrihydrite (Fh) transformation to goethite (Gt) by computing the energetic cost to displace the minimum set of iron cations needed to obtain Gt starting from model Fh structures by Michel et al. in 2007¹ and Manceau et al. in 2014.² While transformation to Gt from the Manceau Fh model is straightforward and involves a moderate activation energy of 1.04 eV/unit cell, doing the same starting from the Michel model is more complex and could involve an energetically competitive intermediate orthorhombic phase structurally similar to Gt that converts to Gt through exchange of iron cations for protons. Two transition pathways yielding the orthorhombic intermediate were identified, with the fastest involving a rate-limiting atomic displacement having a large activation energy of 2.09 eV/unit cell. As a companion result, we also calculated the solid-state transformation energetics that interconnect the Michel and Manceau models of Fh, which are separated by an activation energy of approximately 1.82 eV/cell. We discuss and compare our calculated temperature-dependent iron self-diffusion coefficients in Fh models to those for hematite and magnetite derived from high-temperature measurements. The results show that although pH effects have not yet been treated explicitly, even for the nanoscale particle sizes of Fh it is unlikely that solid-state transformation pathways to Gt are accessible under ambient conditions, implicating other pathways as more important.

更新日期：2022-03-09

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