Department of Chemical and Biological Engineering, Colorado School of Mines, Golden, Colorado80401, United States.
Department of Physics, Colorado School of Mines, Golden, Colorado80401, United States.
Inorg Chem. 2023 May 8;62(18):6882-6892. doi: 10.1021/acs.inorgchem.2c03703. Epub 2023 Jan 30.
At low guest atom concentrations, Si clathrates can be viewed as semiconductors, with the guest atoms acting as dopants, potentially creating alternatives to diamond Si with exciting optoelectronic and spin properties. Studying Si clathrates with different guest atoms would not only provide insights into the electronic structure of the Si clathrates but also give insights into the unique properties that each guest can bring to the Si clathrate structure. However, the synthesis of Si clathrates with guests other than Na is challenging. In this study, we have developed an alternative approach, using thermal diffusion into type II Si clathrate with an extremely low Na concentration, to create Si clathrate with Li guests. Using time-of-flight secondary-ion mass spectroscopy, X-ray diffraction, and Raman scattering, thermal diffusion of Li into the nearly empty Si clathrate framework is detected and characterized as a function of the diffusion temperature and time. Interestingly, the Si clathrate exhibits reduced structural stability in the presence of Li, converting to polycrystalline or disordered phases for anneals at temperatures where the starting Na guest Si clathrate is quite stable. The Li atoms inserted into the Si clathrate lattice contribute free carriers, which can be detected in Raman scattering through their effect on the strength of Si-Si bonds in the framework. These carriers can also be observed in electron paramagnetic resonance (EPR). EPR shows, however, that Li guests are not simple analogues of Na guests. In particular, our results suggest that Li atoms, with their smaller size, tend to doubly occupy cages, forming "molecular-like" pairs with other Li or Na atoms. Results of this work provide a deeper insight into Li guest atoms in Si clathrate. These findings are also relevant to understanding how Li moves through and interacts with Si clathrate anodes in Li-ion batteries. Additionally, techniques presented in this work demonstrate a new method for filling the Si clathrate cages, enabling studies of a broad range of other guests in Si clathrates.
在低客体原子浓度下，硅笼合物可以被视为半导体，客体原子作为掺杂剂，有可能创造出具有令人兴奋的光电和自旋性质的钻石硅替代品。研究具有不同客体原子的硅笼合物不仅可以深入了解硅笼合物的电子结构，还可以深入了解每个客体原子可以为硅笼合物结构带来的独特性质。然而，合成具有除钠以外的客体原子的硅笼合物具有挑战性。在这项研究中，我们开发了一种替代方法，使用热扩散将极低浓度的钠引入 II 型硅笼合物中，从而创造出具有锂客体原子的硅笼合物。使用飞行时间二次离子质谱、X 射线衍射和拉曼散射，检测到并表征了锂在几乎空的硅笼合物框架中的热扩散，其扩散温度和时间均为函数。有趣的是，在存在锂的情况下，硅笼合物的结构稳定性降低，在开始的钠客体硅笼合物非常稳定的退火温度下，转化为多晶或无序相。插入硅笼合物晶格中的锂原子提供了自由载流子，它们可以通过对框架中硅-硅键强度的影响在拉曼散射中检测到。这些载流子也可以在电子顺磁共振（EPR）中观察到。然而，EPR 表明，锂客体原子并不是钠客体原子的简单类似物。特别是，我们的结果表明，由于锂原子较小的尺寸，它们倾向于双重占据笼，与其他锂或钠原子形成“分子样”对。这项工作的结果提供了对硅笼合物中锂客体原子的更深入了解。这些发现也与理解锂离子电池中锂如何穿过和与硅笼合物阳极相互作用有关。此外，本工作中提出的技术展示了填充硅笼合物笼的新方法，使对硅笼合物中其他各种客体原子的研究成为可能。
