Electron Paramagnetic Resonance Study of Type-Ii Silicon Clathrate with Low Sodium Guest Concentration

An electron paramagnetic resonance (EPR) study of type-II silicon (Si) clathrate with low sodium (Na) guest concentration provides insights into the properties of this cagelike Si allotrope and into Na as an n-type dopant. An EPR line arising from unterminated Si bonds in a disordered Si phase is identified in the system. The presence of this amorphouslike component helps explain the thermally activated, hopping-related conductivity that has long been observed in the system but which has been poorly understood. This study also identifies "superhyperfine" lines surrounding each Na hyperfine line due to interactions of the donor electron with a Si-29 nucleus on the cage and confirms the identification of hyperfine lines due to interactions of two neighboring Na nuclei. Analysis of the strength of these interactions shows that the electron wave function associated with the Na donor extends onto surrounding cages consistent with identification of Na as a shallow donor, and provides quantitative evidence of Na clustering.