Collaborative Research: A New Mechanism for Metal Isotope Fractionation Induced by Natural Solid-State Ion Conduction

自然生成的金属银有时呈线状、线状或绳状形态，通常被称为“线银”。直到最近，人们对线银的结构特征知之甚少，更不知道它是如何形成的。令人惊讶的是，这种形态可以在实验室中在与自然中发现的非常不同的简单条件下创造出来，但它们的结构特征与自然样品非常相似。从对天然和合成样品的研究中发现，金属丝银的形成是通过一种称为离子传导的固态过程进行的，在这种情况下，金属离子可以直接通过晶体结构移动，在这种情况下，银(I)离子通过矿物银辉石移动。更令人惊讶的是，金属丝生长过程中的离子传输导致金属丝中重的稳定同位素109Ag的浓缩。这与已知在自然界中发生的同位素浓缩机制所预测的相反。因此，对丝银生长的进一步研究可能会阐明自然界中一种以前不被认识的同位素分离机制。了解这种机制和可能的同位素富集度，对于我们对物理和化学现象的基本理解具有重要意义。这些反过来又直接应用于研究地球化学和地质过程，如金属迁移和在矿床中发现的同位素特征。更广泛的影响包括可能的技术应用，在这些应用中，材料的同位素化学可以影响其物理性质。这项研究涉及来自多个学术部门和不同学科的科学家，并将包括从本科生到博士生的多个层次的学生培训和参与。该项目将实验矿物学、分析地球化学和计算方法结合在一起，研究高度受控条件下线银的形成，以阐明所观察到的同位素效应的基本机制。同位素分馏被认为是在快离子传导过程中预解离过程的结果，当109Ag+离子通过硫化银结构时优先发生。这项研究还将测试这种同位素效应是否延伸到铜、锌和镍等其他过渡金属。了解同位素分馏的机制、大小和方向对于在地质学研究中使用和准确解释同位素数据至关重要，并使“调整”的同位素化学物质的工程化开发成为可能。最后，这种未被利用的物理现象可能被用来改进利用快速离子传导的技术设备的特征。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Naturally occurring metallic silver is sometimes found with a wire-, string- or rope-like morphology, commonly called “wire silver”. Until recently, very little was known about the structural characteristics of wire silver and even less about how it forms. Surprisingly, this morphology can be created in the laboratory under simple conditions very different from those found in nature, yet their structural characteristics are very similar to natural samples. From the study of natural and synthetic specimens, it has been found that wire silver formation occurs by a solid-state process called “ion conduction”, where metal ions can move directly through a crystal structure, and in this case, silver(I) ions move through the mineral argentite. Even more surprising, this ion transport during wire growth causes an enrichment of the heavy stable isotope 109Ag in the wire. This is opposite of that predicted by mechanisms of isotope enrichment known to occur in nature. Thus, further study of wire silver growth may elucidate a previously unrecognized mechanism for isotope separation in nature. Understanding this mechanism and the degree of possible isotope enrichment is significant for our basic understanding of physical and chemical phenomena. These in turn have direct application to the study of geochemical and geological processes such as metal transport and the isotopic signatures found in ore deposits. Broader impacts include possible technological applications where the isotope chemistry of materials can affect their physical properties. This study involves scientists from multiple academic departments and different disciplines, and will include student training and participation at multiple levels, from undergraduates through doctoral candidates. This project integrates experimental mineralogy, analytical geochemistry and computational methods in the study of wire silver formation under highly controlled conditions as a means to elucidate the fundamental mechanism of the observed isotope effect. Isotope fractionation is hypothesized to result from the process of predissociation during superionic conduction, occurring preferentially for 109Ag+ ions as they move through the silver sulfide structure. The study will also test whether this isotope effect extends to other transition metals such as copper, zinc and nickel. Understanding the mechanisms, magnitude and direction of isotope fractionation is essential for the use and accurate interpretation of isotope data in geological studies, and allows for the engineered development of materials with “tuned” isotope chemistries. Finally, this unexploited physical phenomenon might be leveraged to improve the characteristics of technological devices that utilize fast ion conduction.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.