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

天然存在的金属银有时被发现具有线状、线状或绳状形态，通常被称为“线银”。直到最近，人们对银线的结构特征知之甚少，对它是如何形成的更是知之甚少。令人惊讶的是，这种形态可以在实验室中在与自然界中发现的条件非常不同的简单条件下创建，但它们的结构特征与天然样品非常相似。从对天然和合成标本的研究中发现，线银的形成是通过一种被称为“离子传导”的固态过程发生的，其中金属离子可以直接穿过晶体结构移动，在这种情况下，银（I）离子穿过矿物辉银矿移动。更令人惊讶的是，在金属丝生长过程中的这种离子传输导致金属丝中重稳定同位素109 Ag的富集。这与自然界中已知的同位素富集机制所预测的相反。因此，进一步研究线银生长可能阐明以前未认识到的同位素分离的自然机制。了解这种机制和可能的同位素富集程度对于我们基本理解物理和化学现象具有重要意义。这些又直接应用于地球化学和地质过程的研究，如金属迁移和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，其中材料的同位素化学可能影响其物理特性。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候选人的多个层次的学生培训和参与。 该项目综合了实验矿物学、分析地球化学和计算方法，在高度控制的条件下研究线银的形成，作为阐明所观察到的同位素效应的基本机制的一种手段。同位素分馏被假设为导致超离子传导过程中的预解离，发生优先为109 Ag+离子，因为他们通过银硫化物结构移动。该研究还将测试这种同位素效应是否延伸到其他过渡金属，如铜，锌和镍。了解同位素分馏的机制、幅度和方向对于在地质研究中使用和准确解释同位素数据至关重要，并允许工程开发具有“调谐”同位素化学性质的材料。最后，这一未被利用的物理现象可能被用来改善利用快速离子传导的技术设备的特性。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值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.