Abstract
The optical properties of ternary type-I Ge clathrate Ba8Ga16Ge30 are investigated by optical reflectance and transmittance measurements. The refractive index and extinction coefficient are calculated from the reflectance spectrum via the modified Kramers–Kronig analysis method between 0.5 and 3.2 eV. The photon energy dependence of the optical absorption coefficient reveals that Ba8Ga16Ge30 is an indirect band gap semiconductor with a gap energy Eg of 0.69 eV at 5.7 K and 0.66 eV at 285 K. The temperature dependence of Eg can be satisfactorily described by an equation based on the Bose–Einstein statistics model. When compared with those of common elemental, III–V, and II–VI semiconductors, the small temperature coefficient dEg/dT of the Ba8Ga16Ge30 can be considered to represent the weak electron–phonon interaction in the Ba8Ga16Ge30 clathrate.
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