Enhanced understanding of silicon clathrates and guests through magnetic spectroscopies

ABSTRACT Low-density silicon allotropes, including types I and II Si clathrates, represent a class of metastable, cage or channel like materials with unique structural and electronic properties. These allotropes exhibit expanded frameworks compared to conventional diamond cubic Si, with potential applications in thermoelectrics, photovoltaics, optoelectronics, and quantum information science. However, the synthesis and characterization of these microcrystalline materials remain challenging due to their metastability and sensitivity to synthesis conditions. In this study, magnetic spectroscopic techniques are used to characterize type I and type II Si clathrates synthesized as powders, films, and single crystals. Additional structural characterization techniques like scanning electron microscopy and X-ray diffraction provide further understanding of these materials. Our XRD and 29 Si NMR results clearly indicate the presence of type II clathrates in samples prepared to maximize its phase. In contrast, 23 Na NMR spectra of these samples are often consistent with type I structure, providing an additional means to detect trace phases not identified by other methods. This observation aids in clearing up conflicting reports in the Si clathrate NMR literature. EPR spectra of iodine-treated clathrate films showed significant reduction in Na-related features and Si dangling bond defects. This treatment effectively reduces Na signal in powders where iodine was thought to remove Na. Our new EPR results of iodine-treated films suggest an alternative explanation: Na and defects are passivated by iodine. This work also highlights the advantage of a multi-technique approach.