Max-Planck-Institut für Chemische Physik fester Stoffe , 01187 Dresden, Germany.
Ivan-Franko National University , Lviv 79000, Ukraine.
Acc Chem Res. 2018 Feb 20;51(2):214-222. doi: 10.1021/acs.accounts.7b00561. Epub 2018 Jan 9.
Materials with the crystal structure of γ-brass type (CuZn type) are typical representatives of intermetallic compounds. From the electronic point of view, they are often interpreted using the valence electron concentration approach of Hume-Rothery, developed previously for transition metals. The γ-brass-type phases of the main-group elements are rather rare. The intermetallic compound BePt, a new member of this family, was synthesized, and its crystal structure, chemical bonding, and physical properties were characterized. BePt crystallizes in the cubic space group F4̅3m with lattice parameter a = 15.90417(3) Å and 416 atoms per unit cell. From the crystallographic point of view, the binary substance represents a special family of intermetallic compounds called complex metallic alloys (CMA). The crystal structure was solved by a combination of synchrotron and neutron powder diffraction data. Besides the large difference in the scattering power of the components, the structure solution was hampered by the systematic presence of very weak reflections mimicking wrong symmetry. The structural motif of BePt is described as a 2 × 2 × 2 superstructure of the γ-brass structure (CuZn type) or 6 × 6 × 6 superstructure of the simple bcc structural pattern with distinct distribution of defects. The main building elements of the crystal structure are four types of nested polyhedral units (clusters) with the compositions BePt and BePt. Each cluster contains four shells (4 + 4 + 6 + 12 atoms). Clusters with different compositions reveal various occupation of the shells by platinum and beryllium. Polyhedral nested units with the same composition differ by the distance of the shell atoms to the cluster center. Analysis of chemical bonding was made applying the electron localizability approach, a quantum chemical technique operating in real space that is proven to be especially efficient for intermetallic compounds. Evaluations of the calculated electron density and electron localizability indicator (ELI-D) revealed multicenter bonding, being in accordance with the low valence electron count per atom in BePt. A new type of atomic interactions in intermetallic compounds, cluster bonds involving 8 or even 14 atoms, is found in the clusters with shorter distances between the shell atoms and the cluster centers. In the remaining clusters, four- and five-center bonds characterize the atomic interactions. Multicluster interactions within the polyhedral nested units and three-center polar intercluster bonds result in a three-dimensional framework resembling the structural pattern of NaCl. BePt is a diamagnetic metal and one of rather rare CMA compounds revealing superconductivity (T = 2.06 K).
具有 γ-黄铜型（CuZn 型）晶体结构的材料是金属间化合物的典型代表。从电子的角度来看，它们通常使用 Hume-Rothery 之前为过渡金属开发的价电子浓度方法来解释。主族元素的 γ-黄铜型相相当罕见。作为这个家族的新成员，合成了新的 BePt 金属间化合物，并对其晶体结构、化学成键和物理性质进行了表征。BePt 在立方空间群 F4̅3m 中结晶，晶格参数 a = 15.90417(3) Å，每个晶胞有 416 个原子。从晶体学的角度来看，二元物质代表了一类特殊的金属间化合物，称为复杂金属合金（CMA）。晶体结构是通过同步加速器和中子粉末衍射数据的组合来解决的。除了成分散射能力的巨大差异外，结构解决方案还受到非常弱的反射的系统存在的阻碍，这些反射模拟了错误的对称性。BePt 的结构基元被描述为 γ-黄铜结构（CuZn 型）的 2×2×2 超结构或简单 bcc 结构模式的 6×6×6 超结构，具有明显的缺陷分布。晶体结构的主要建筑元素是四种嵌套多面体单元（簇），组成分别为 BePt 和 BePt。每个簇包含四个壳层（4+4+6+12 个原子）。具有不同组成的簇揭示了铂和铍在壳层中的不同占据。具有相同组成的多面体嵌套单元通过壳层原子与簇中心的距离来区分。化学成键的分析是应用电子局域化方法进行的，这是一种在实空间中运行的量子化学技术，被证明对金属间化合物特别有效。对计算得到的电子密度和电子局域化指标（ELI-D）的评估表明存在多中心键合，这与 BePt 中每个原子的低价电子计数一致。在具有较短壳层原子与簇中心之间距离的簇中发现了金属间化合物中一种新的原子相互作用类型，即涉及 8 甚至 14 个原子的簇键。在其余的簇中，四中心和五中心键合特征化了原子相互作用。多面体嵌套单元内的多簇相互作用和三中心极性间簇键导致了类似于 NaCl 结构模式的三维框架。BePt 是一种抗磁性金属，是为数不多的具有超导性（T = 2.06 K）的 CMA 化合物之一。
