Semiconducting Ge clathrates: Promising candidates for thermoelectric applications

Abstract

Transport properties of polycrystalline Ge clathrates with general composition ${\mathrm{Sr}}_{\mathrm{8}}{\mathrm{Ga}}_{\mathrm{16}}{\mathrm{Ge}}_{\mathrm{30}}$ are reported in the temperature range $\text{5\hspace{0.5em}}\mathrm{K}\text{⩽T⩽300\hspace{0.5em}}\mathrm{K.}$ These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of ${10}^{17}{\text{–10}}^{18}\hspace{0.5em}{\mathrm{cm}}^{\mathrm{-3}}.$ The thermal conductivity is more than an order of magnitude smaller than that of crystalline germanium and has a glasslike temperature dependence. The resulting thermoelectric figure of merit, $\mathrm{ZT,}$ at room temperature for the present samples is $\frac{1}{4}$ that of ${\mathrm{Bi}}_{\mathrm{2}}{\mathrm{Te}}_{\mathrm{3}}$ alloys currently used in devices for thermoelectric cooling. Extrapolating our measurements to above room temperature, we estimate that $\text{ZT>1}$ at $\text{T>700\hspace{0.5em}}\mathrm{K,}$ thus exceeding that of most known materials.