Influence of vacancy damage on He diffusion in apatite, investigated at atomic to mineralogical scales
Influence of vacancy damage on He diffusion in apatite, investigated at atomic to mineralogical scales
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DOI：
10.1016/j.gca.2016.10.018
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发表时间：
2017-01
影响因子：
5
通讯作者：
C. Gerin;C. Gautheron;E. Oliviero;C. Bachelet;Duval Mbongo Djimbi;A. Seydoux‐Guillaume;L. Tassan-got;P. Sarda;J. Roques;F. Garrido
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作者：
C. Gerin;C. Gautheron;E. Oliviero;C. Bachelet;Duval Mbongo Djimbi;A. Seydoux‐Guillaume;L. Tassan-got;P. Sarda;J. Roques;F. Garrido
氦在富U-Th矿物，特别是磷灰石中的扩散被认为是受损伤的强烈影响，即使在低U-Th含量时也是如此。为了获得直接证据并更好地了解损伤对He扩散的影响，我们使用不同的方法对磷灰石中的空位损伤进行了纳米到原子尺度的研究。首先，在He注入量为2×10~(15)~1×10~(17)He/cm~2的范围内，对磷灰石晶体造成了损伤，对应于表面下前200 nm处原子位移的12~100%。用透射电子显微镜对损伤结构进行了成像，以获得最低的He注量。透射电子显微镜图像在纳米尺度上没有可见的损伤区，这意味着所产生的损伤很好地对应于Frenkel缺陷(空位和间隙)。其次，通过使用弹性反冲检测分析(ERDA)绘制He浓度随深度的分布图，对这些样品进行扩散实验。在测量了注入的氦分布和氦浓度后，在145℃到250℃的温度下，在15-45小时内对样品进行加热，以扩散注入的分布。获得的氦与深度加热的分布和氦浓度揭示了损伤对氦扩散率的影响。这一结果只能用一个扩散依赖于损伤剂量的模型来解释，该模型考虑了在较高损伤剂量下的空位俘获和损伤互连。第三，用密度泛函理论(DFT)对F-磷灰石晶体中的空位进行了模拟。空位使结构略有变形，He原子在空位中的插入能低于通常的插入位。相应地，He原子跳出空位的额外能量为ΔEa≈为30-40千焦耳/摩尔，与已发表的估计很好地一致。因此，这一计算表明，由于空位的存在而对结构进行的微小修改有效地捕获了He原子，从而降低了扩散系数。最后，对于具有空位型损伤的磷灰石晶体，我们提出了一个能够很好地再现辐照样品上的He扩散数据的He扩散模型。我们预计，对于天然磷灰石，与空位聚集相对应的反冲损伤将具有更高的捕获能力，ΔEa>；约为50kJ/m ol。
Helium diffusion in U–Th-rich minerals, especially apatite, is considered as strongly impacted by damage, even at low U–Th content. To get direct evidence and better understand the impact of damage on He diffusion, we conducted a study on vacancy damage in apatite, at nanometric to atomic scales, using different methodologies. Firstly, damage was created on apatite crystals by He implantation at different He fluences ranging from 2 × 1015to 1 × 1017He/cm2, corresponding to atomic displacement ranging from 12 to more than 100% of the total structure in the first 200 nm below the surface. Transmission Electron Microscopy (TEM) was used to image the damage structure, for the lowest He fluence. TEM images present no visible damage zone at nano-scale, implying that the created damage corresponds well to Frenkel defects (vacancies and interstitials). Secondly, diffusion experiments were performed on those samples by mapping He concentration vs. depth profiles using Elastic Recoil Detection Analysis (ERDA). After measurement of implanted-He profiles and He concentrations, the samples were heated in order to diffuse the implanted profile during 15–45 h at temperatures from 145 to 250 °C. The obtained He vs. depth heated profiles and He concentrations reveal the impact of damage on He diffusivity. The results can only be explained by a model where diffusion depends on damage dose, taking into account He trapping in vacancies and damage interconnectivity at higher damage dose. Thirdly, Density Functional Theory (DFT) calculations were performed to simulate a vacancy in a F-apatite crystal. The structure becomes slightly deformed by the vacancy and the insertion energy of a He atom in the vacancy is lower than for an usual insertion site. Accordingly, the additional energy for a He atom to jump out of the vacancy is ΔEa≈ 30–40 kJ/mol, in good agreement with published estimates. This calculation thus shows that small modifications of the structure due to the presence of vacancies efficiently trap He atoms, thus reducing diffusivity. Finally, for apatite crystal having vacancy-type damage, we propose a He diffusion model able to reproduce well He diffusion data obtained on irradiated samples. We anticipate that, for natural apatite, the recoil-damage that corresponds to vacancy clustering, would have a higher trapping power with ΔEa> 50 kJ/mol.