层状金属间化合物中二维Kagomé片的扭转
Twisting of 2D kagomé sheets in layered intermetallics
摘要：
二维层状材料和范德华固体中的化学键是理解和利用其独特的电子，磁性，光学，热学和超导特性的核心。本文报道了在T = 100（2）K时，通过X射线衍射测量发现金属kagomé MgCo 6 Ge 6中自发的双向双层扭曲（扭曲角~ 4.5°），这是通过激光布里奇曼法制备单晶实现的。尽管从T ~ 300 K到100 K冷却时出现静态扭曲，但在物理性质测量中没有发现相变的证据。结合存在的爱因斯坦声子模式贡献的比热，这意味着扭曲存在于所有温度下，但在室温下是热波动。晶体轨道Hamilton布居分析表明，当耦合到连接相邻层的强键合和刚性（Ge 2）二聚体时，层间的合作扭曲稳定了Co-kagomé网络。在晶体结构中的位移无序的进一步建模显示存在第二，镁缺乏，堆叠序列。这种替代的堆叠顺序也表现出层间扭曲，但具有不同的图案，与由于Mg的去除而导致的电子计数的变化一致。磁化强度，电阻率和低温比热测量都与泡利顺磁性，强相关金属一致。我们的研究结果提供了至关重要的洞察化学概念如何导致有趣的电子结构和层状材料的行为。
英文摘要：
Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal and superconducting properties. Here we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ~ 4.5°) in the metallic kagomé MgCo6Ge6 at T = 100(2) K via X-ray diffraction measure-ments, enabled by the preparation of single crystals by the Laser Bridgman method. Despite the appearance of static twisting on cooling from T ~ 300 K to 100 K, no evidence for a phase transition was found in physical properties measurements. Combined with the presence of an Einstein phonon mode contribution in the specific heat, this implies that the twisting exists at all temperatures but is thermally fluctuating at room temperature. Crystal Orbital Hamilton Population analysis demonstrates that the cooperative twisting between layers stabilizes the Co-kagomé network when coupled to strongly bonded and rigid (Ge2) dimers that connect adjacent layers. Further modelling of the displacive disorder in the crystal structure shows the presence of second, Mg-deficient, stacking sequence. This alternative stacking sequence also exhibits inter-layer twisting, but with a different pattern, consistent with the change in electron count due to removal of Mg. Magnetization, resistivity, and low-temperature specific heat measurements are all consistent with a Pauli paramagnetic, strongly correlated metal. Our results provide crucial insight into how chemical concepts lead to interesting electronic structures and behaviors in layered materials.
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