An ESR study on the thermal electron excitation of a sodium atom incorporated in a silicon clathrate compound

An ESR study was carried out for sodium-containing silicon clathrate compounds, NaxSi136 with 3 ⩽ × ⩽10. For Na3Si136 a quartet with an isotropic hf splitting of 13.3 mT was observed together with the weak septet of 6.7 mT at 4 K; the former and the latter being attributed to the Na atom and the dimer cation Na2+, respectively. The temperature-dependent ESR spectral intensity of the Na atom was observed over a wide temperature range from 4 to 200 K. A deviation from the paramagnetic Curie law was observed in the higher temperature region and is discussed in terms of a thermal electron excitation from the Na impurity level to the conduction band of the silicon clathrate compound.