Metastable Layered Cobalt Chalcogenides from Topochemical Deintercalation

We present a general strategy to synthesize metastable layered materials via topochemical deintercalation of thermodynamically stable phases. Through kinetic control of the deintercalation reaction, we have prepared two hypothesized metastable compounds, CoSe and CoS, with the anti-PbO type structure from the starting compounds KCo2Se2 and KCo2S2, respectively. Thermal stability, crystal structure from X-ray and neutron diffraction, magnetic susceptibility, magnetization, and electrical resistivity are studied for these new layered chalcogenides; both CoSe and CoS are found to be weak itinerant ferromagnets with Curie temperatures close to 10 K. Due to the weak van der Waals forces between the layers, CoSe is found to be a suitable host for further intercalation of guest species such as Li-ethylenediamine. From first-principles calculations, we explain why the Co chalcogenides are ferromagnets instead of superconductors as in their iron analogues. Bonding analysis of the calculated electronic density of states both explains their phase stability and predicts the limits of our deintercalation technique. Our results have broad implications for the rational design of new two-dimensional building blocks for functional materials.

本研究提出一种通过对热力学稳定相进行拓扑化学脱插（topochemical deintercalation）来合成亚稳态层状材料（metastable layered materials）的通用策略。通过对脱插反应的动力学控制，我们分别从前驱体KCo₂Se₂与KCo₂S₂中制备出两种此前被假说存在的、具有反PbO型（anti-PbO）结构的亚稳态化合物CoSe与CoS。我们对这两种新型层状硫族化合物（layered chalcogenides）的热稳定性、基于X射线与中子衍射的晶体结构、磁化率、磁化强度以及电阻率展开了研究；结果发现CoSe与CoS均为居里温度（Curie temperature）接近10 K的弱巡游铁磁体。由于层间存在较弱的范德瓦尔斯力（van der Waals force），CoSe可作为合适的宿主材料，用于后续插入Li-乙二胺等客体物种。通过第一性原理（first-principles）计算，我们阐释了为何钴基硫族化合物会成为铁磁体，而非与其铁基类似物（iron analogues）一样表现出超导性。对计算得到的电子态密度（electronic density of states）进行键合分析，既解释了它们的相稳定性，也预测了本研究脱插技术的应用极限。本研究结果可为功能材料新型二维结构基元（two-dimensional building blocks）的合理设计提供广泛的借鉴意义。