Optical absorption properties of NaxSi136 clathrate studied by diffuse reflection spectroscopy

• The optical absorption of NaxSi136 was clarified by diffuse reflection method.
• Fundamental absorption edge and strong free carrier absorption were observed.
• The inclusion of Na produces free carriers at room temperature.
• The inclusion of Na little influences the band structure of NaxSi136.

The optical absorption spectra of NaxSi136 clathrates (x = 0.6–14.7) were experimentally clarified with the diffuse reflection spectroscopy. Strong free carrier absorption which becomes intensive with increasing Na concentration in NaxSi136 was found in infrared region, in addition to the absorption due to the interband transition. The origin of the free carries was interpreted with the ionization of Na. The position of the interband absorption edge was almost independent of Na content, and was estimated to be 1.6–1.8 eV if NaxSi136 has the direct gap.