Boron: Enabling Exciting Metal-Rich Structures and Magnetic Properties.
Boron: Enabling Exciting Metal-Rich Structures and Magnetic Properties.
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DOI：
10.1021/acs.accounts.7b00268
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发表时间：
2017-08
影响因子：
18.3
通讯作者：
J. P. Scheifers;Yuemei Zhang;B. Fokwa
中科院分区：
文献类型：
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作者：
J. P. Scheifers;Yuemei Zhang;B. Fokwa
硼独特的化学性质及其与金属的反应产生了一大类金属硼化物，其成分范围从硼含量最高的 YB66（用作同步辐射的单色仪）到金属含量最高的 Nd2Fe14B（迄今为止最好的永磁体）。后一种化合物优异的磁性能源于其独特的晶体结构，而硼的存在对于该结构至关重要。一般来说，了解任何给定扩展固体的晶体结构是了解其物理性质并最终预测具有所需性质的新合成目标的先决条件。硼与自身以及金属元素形成牢固化学键的能力使我们能够构建具有令人兴奋特性的新结构。近年来，我们发现了新的硼化物结构，其中包含一些前所未有的硼碎片（三角平面B4单元、平面B6环）和磁活性元素的低维子结构（梯子、支架、三角形链）。新的硼化物结构催生了新的超导材料（例如 NbRuB）和新的流动磁性材料（例如 Nb6Fe1-xIr6+xB8）。对含有磁活性元素链（反铁磁性 Sc2FeRu5B2 中的 Fe 链）、梯子（铁磁性 Ti9Fe2Rh18B8 中的 Fe 梯子）和三角形链（亚铁磁性和受挫 TiCrIr2B2 中的 Cr3 链）的硼化物的研究使我们能够深入了解这些因素（使用密度泛函） 理论计算）可以影响此类低维磁性单元的磁排序。我们发现，包含这些磁性亚基的相的磁性可以通过金属非磁性网络内的化学取代来大幅调整。例如，通过逐渐用Ru取代Rh，Ti2FeRh5B2的小磁滞（磁能存储的量度）可以连续增加至24倍，这一结果甚至超过了Ru/Ir替代的结果（高达初始值的54倍）。此外，正如实验和理论方法所证明的那样，长程磁相互作用的类型可以通过金属非磁性网络中的适当替代来彻底调整。事实证明，采用 Ti3Co5B2 或 Th7Fe3 结构类型的富 Ru 和贫价电子金属硼化物具有主要的反铁磁相互作用，而在富 Rh（或富 Ir）和富价电子相中，铁磁相互作用占主导地位，例如在 Sc2FeRu5-xRhxB2 和 FeRh6-xRuxB3 系列中发现的那样。令人着迷的是，硼簇（例如 B6 环）在某些情况下甚至直接与磁性亚基相互作用，这种相互作用被发现有利于铁磁性 Nb6Fe1-xIr6+xB8 中的 Fe-Fe 磁交换相互作用。使用较便宜的过渡金属，我们最近预测了新的流动磁体，其实验证明仍在等待中。此外，还发现了新的结构，所有这些结构都正在进行实验和计算研究，目的是寻找新的超导体、磁体和机械硬质材料。我们的团队正在追求一个新的方向，因为二元和三元过渡金属硼化物作为微米和纳米尺度的高效水分解电催化剂显示出巨大的前景。
Boron's unique chemical properties and its reactions with metals have yielded the large class of metal borides with compositions ranging from the most boron-rich YB66 (used as monochromator for synchrotron radiation) up to the most metal-rich Nd2Fe14B (the best permanent magnet to date). The excellent magnetic properties of the latter compound originate from its unique crystal structure to which the presence of boron is essential. In general, knowing the crystal structure of any given extended solid is the prerequisite to understanding its physical properties and eventually predicting new synthetic targets with desirable properties. The ability of boron to form strong chemical bonds with itself and with metallic elements has enabled us to construct new structures with exciting properties. In recent years, we have discovered new boride structures containing some unprecedented boron fragments (trigonal planar B4 units, planar B6 rings) and low-dimensional substructures of magnetically active elements (ladders, scaffolds, chains of triangles). The new boride structures have led to new superconducting materials (e.g., NbRuB) and to new itinerant magnetic materials (e.g., Nb6Fe1-xIr6+xB8). The study of boride compounds containing chains (Fe-chains in antiferromagnetic Sc2FeRu5B2), ladders (Fe-ladders in ferromagnetic Ti9Fe2Rh18B8), and chains of triangles (Cr3 chains in ferrimagnetic and frustrated TiCrIr2B2) of magnetically active elements allowed us to gain a deep understanding of the factors (using density functional theory calculations) that can affect magnetic ordering of such low-dimensional magnetic units. We discovered that the magnetic properties of phases containing these magnetic subunits can be drastically tuned by chemical substitution within the metallic nonmagnetic network. For example, the small hysteresis (measure of magnetic energy storage) of Ti2FeRh5B2 can be successively increased up to 24-times by gradually substituting Ru for Rh, a result that was even surpassed (up to 54-times the initial value) for Ru/Ir substitutions. Also, the type of long-range magnetic interactions could be drastically tuned by appropriate substitutions in the metallic nonmagnetic network as demonstrated using both experimental and theoretical methods. It turned out that Ru-rich and valence electron poor metal borides adopting the Ti3Co5B2 or the Th7Fe3 structure types have dominating antiferromagnetic interactions, while in Rh-rich (or Ir-rich) and valence electron rich phases ferromagnetic interactions prevail, as found, for example, in the Sc2FeRu5-xRhxB2 and FeRh6-xRuxB3 series. Fascinatingly, boron clusters (e.g., B6 rings) even directly interact in some cases with the magnetic subunits, an interaction which was found to favor the Fe-Fe magnetic exchange interactions in the ferromagnetic Nb6Fe1-xIr6+xB8. Using less expensive transition metals, we have recently predicted new itinerant magnets, the experimental proof of which is still pending. Furthermore, new structures have been discovered, all of which are being studied experimentally and computationally with the aim of finding new superconductors, magnets, and mechanically hard materials. A new direction is being pursued in our group, as binary and ternary transition metal borides show great promise as efficient water splitting electrocatalysts at the micro- and nanoscale.