Lead diffusion in CaTiO3: A combined study using Rutherford backscattering and TOF-SIMS for depth profiling to reveal the role of lattice strain in diffusion processes
Lead diffusion in CaTiO3: A combined study using Rutherford backscattering and TOF-SIMS for depth profiling to reveal the role of lattice strain in diffusion processes
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DOI：
10.2138/am-2019-6730
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发表时间：
2019-03
影响因子：
3.1
通讯作者：
C. Beyer;R. Dohmen;D. Rogalla;H. Becker;K. Marquardt;C. Vollmer;U. Hagemann;N. Hartmann;S. Chakraborty
中科院分区：
文献类型：
--
作者：
C. Beyer;R. Dohmen;D. Rogalla;H. Becker;K. Marquardt;C. Vollmer;U. Hagemann;N. Hartmann;S. Chakraborty
摘要我们给出了CaTiO 3钙钛矿中Pb扩散系数的实验数据，CaTiO 3钙钛矿通常用于金伯利岩和碳酸岩的定年。定向合成和天然CaTiO 3单晶进行实验。Pb源是激光沉积的（Ca0.83Pb0.07）Ti1.05O3薄膜或（Ca0.9Pb0.1）TiO 3粉末储存器。将晶体在736至1135 °C的高温炉中退火2至283小时。的扩散配置文件进行了测量卢瑟福背散射和飞行时间二次离子质谱的深度剖析模式。用两种分析方法对相同样品测得的浓度曲线一致。测得的浓度分布图显示了两个区域-在扩散界面处的陡峭梯度，其急剧过渡（在距离表面约50至150 nm处）到渗透到晶体更深处的低浓度尾部。该扩散行为可以使用作为Pb浓度的函数的扩散系数来最佳地建模，其中分别针对分布的高浓度区域和低浓度区域具有不同的扩散系数集合。从薄膜和粉末源实验中提取的扩散系数在不确定性范围内是相似的。Pb在8.5和0.6重量%之间的浓度下扩散较慢，在低于约0.5重量% Pb的浓度下扩散较快1.6至2.6对数单位。文中讨论了每个区域的温度依赖性，与天然样品最相关的快速扩散状态的Arrhenius关系为DPbfast=2.5×10−13×exp（−158（24）kJ/mol/RT）m2/s。开始{array}{} D_{\text{Pb}}^{\text{fast}}=2.5 \times 10^{-13}\times \exp \Big（-158\big（24\big）\text{kJ/mol}/RT\Big）m^2/s。\end{array} $$我们发现了一个明显的变化，在结构中的CaTiO 3在单晶的表面区域，这是一致的扩散率的变化。该初始区域以平面缺陷为主。我们建议，铅被困在平面缺陷，已形成由于高应变引入钙钛矿结构的离子半径之间的不匹配所造成的钙钛矿2+和铅2+。如果发生CaTiO 3中Pb的扩散重置，则对于一系列不同的冷却方案，此处获得的激活能产生CaTiO 3中Pb的闭合温度在300 °C至400 °C之间。在上升金伯利岩的典型冷却速率为数小时至数天时，晶体生长的年龄被保存下来，闭合温度与岩浆温度相似。
Abstract We present experimental data on the diffusivity of Pb in CaTiO3 perovskite, which is commonly used for dating kimberlites and carbonatites. Experiments were performed on oriented synthetic and natural CaTiO3 single crystals. The Pb-source was either a laser deposited (Ca0.83Pb0.07)Ti1.05O3 thin film or a (Ca0.9Pb0.1)TiO3 powder reservoir. The crystals were annealed in a high-temperature furnace between 736 and 1135 °C and for durations from 2 to 283 h. The diffusion profiles were measured with Rutherford backscattering and time-of-flight secondary ion mass spectrometry in the depth-profiling mode. The concentration profiles measured on the same samples with the two analytical methods are in agreement. The measured concentration profiles show two regions—a steep gradient at the diffusion interface that transitions sharply (at ~50 to 150 nm from the surface) to a low concentration tail that penetrates deeper into the crystal. This diffusion behavior could be modeled best using diffusion coefficients that are a function of the Pb concentration, with a different set of diffusion coefficient for the high and the low concentration region of the profile, respectively. The diffusion coefficients extracted from the thin film and powder source experiments are similar within uncertainties. Pb diffuses slower at concentrations between 8.5 and 0.6 wt% and 1.6 to 2.6 log units faster below ~0.5 wt% Pb. Temperature dependency for each region is discussed in the text, and the Arrhenius relation for the fast diffusion regime that is most relevant for natural samples is DPbfast=2.5×10−13×exp(−158(24)kJ/mol/RT)m2/s.$$\begin{array}{} D_{\text{Pb}}^{\text{fast}}=2.5 \times 10^{-13}\times \exp \Big(-158\big(24\big)\text{kJ/mol}/RT\Big)m^2/s. \end{array} $$ We found a distinct change in the structure of CaTiO3 in the surface region of the single crystal that is coincidental with the change in diffusivity. This initial region is dominated by planar defects. We propose that Pb is trapped in planar defects that have formed due to the high strain introduced into the perovskite structure caused by the mismatch in the ionic radius between Ca2+ and Pb2+. The activation energies obtained here yield closure temperature for Pb in CaTiO3 between 300 and 400 °C for a range of different cooling scenarios if diffusive resetting of Pb in CaTiO3 occurs at all. At typical cooling rates of hours to days for ascending kimberlite, the age of crystal growth is preserved, with closure temperatures similar to the magma temperature.