Peterson Gordon G C, Geisler Emmett E, Fredrickson Daniel C
Department of Chemistry, University of Wisconsin (UW)-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States.
Inorg Chem. 2020 Apr 6;59(7):5018-5029. doi: 10.1021/acs.inorgchem.0c00246. Epub 2020 Mar 9.
The structural chemistry of intermetallic phases is generally viewed in terms of what crystal structure will be most stable for a given combination of metallic atoms. Yet, individual atoms do not always make the best reference points. As the number of elements involved in compounds increases, their structures can often appear to be assembled from structural motifs derived from simpler compounds nearby in the phase diagram rather than fundamentally new arrangements of atoms. In this Article, we explore the notion that complex multinary phases can be viewed productively in terms of motif-preserving reactions between binary compounds, as opposed to direct reactions of the component elements. We present the targeted synthesis and structure solution of CaCuAl, an intermetallic phase whose placement in the phase diagram is suggestive of a reaction between CaAl and CuAl. Single-crystal X-ray diffraction analysis reveals that this compound crystallizes in the YTaNiAl (or stuffed BaHg) structure type and is constructed from three modules: Ca@(Cu/Al) polyhedra derived from the BaAl type, Cu@Al cubes, and Al cuboctahedra. To help understand this arrangement, we identify forces driving the reactivity of one of the supposed starting materials, CaAl, through visualization of its atomic charge distribution and chemical pressure (CP) scheme, which suggest that the Al sites closest to the Ca atoms should show a high affinity for substitution by Cu atoms. Such a process on its own, however, would lead to overly long Ca-Cu distances and electron deficiency. When Cu is made available to CaAl in the Ca-Cu-Al ternary system, its incorporation in the Ca coordination environments instead nucleates domains of a fluorite-like CuAl phase, which act as nodes in the primitive cubic framework of CaAl- and fluorite-like units. The cubic holes created by this framework are occupied by Al face-centered-cubic fragments that donate electrons while also resolving negative CPs in the Ca environments. This structural chemistry illustrates how new elements added to a binary compound at sites with conflicting electronic and atomic size preferences can serve as anchor points for the growth of domains of different bonding types, a notion that can be applied as a more general design strategy for new intermetallic intergrowth structures.
金属间相的结构化学通常是从对于给定金属原子组合而言哪种晶体结构最稳定的角度来考虑的。然而，单个原子并不总是最佳的参考点。随着参与化合物的元素数量增加，它们的结构常常看起来是由相图中附近较简单化合物衍生出的结构基序组装而成，而不是原子的全新排列。在本文中，我们探讨了这样一种观点，即复杂的多元相可以有效地从二元化合物之间的基序保留反应的角度来审视，而不是从组成元素的直接反应的角度。我们展示了CaCuAl的定向合成和结构解析，CaCuAl是一种金属间相，其在相图中的位置表明它是CaAl和CuAl之间反应的产物。单晶X射线衍射分析表明，该化合物以YTaNiAl（或堆积型BaHg）结构类型结晶，并且由三个模块构成：源自BaAl型的Ca@(Cu/Al)多面体、Cu@Al立方体和Al立方八面体。为了帮助理解这种排列方式，我们通过可视化其原子电荷分布和化学压力（CP）方案来确定假定的起始原料之一CaAl的反应驱动力，这表明最靠近Ca原子的Al位点对被Cu原子取代应表现出高亲和力。然而，仅这样一个过程会导致Ca - Cu距离过长且电子不足。当在Ca - Cu - Al三元体系中向CaAl提供Cu时，Cu掺入Ca的配位环境中反而会形成萤石型CuAl相的畴，这些畴在CaAl和萤石型单元的原始立方框架中充当节点。由这个框架产生的立方孔洞被Al面心立方片段占据，这些片段提供电子，同时也消除了Ca环境中的负CP。这种结构化学说明了添加到二元化合物中具有相互冲突的电子和原子尺寸偏好的位点上的新元素如何能够作为不同键合类型畴生长的锚点，这一概念可以作为新型金属间共生结构更通用的设计策略来应用。
