Structure, thermal, and transport properties of the clathratesSr8Zn8Ge38,Sr8Ga16Ge30, andBa8Ga16Si30

Abstract

The structural parameters, thermal properties, and transport properties of three type I clathrates, namely ${\mathrm{Sr}}_{8}{\mathrm{Zn}}_{8}{\mathrm{Ge}}_{38}$, ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$, and ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Si}}_{30}$, have been determined at or below room temperature. The structural parameters of these clathrates were determined by powder neutron diffraction. Their lattice thermal expansion is two to four times greater than that of the diamond phases of silicon and germanium, consistent with more anharmonic lattice vibrations. From the temperature dependence of the isotropic atomic displacement parameters, the estimated rattling frequencies of guests in the large cages of these clathrates are in the range $50--60\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The heat capacities of these three clathrate materials increase smoothly with increasing temperatures and approach the Dulong--Petit value around room temperature. The Gr\"uneisen parameter of these materials is constant between $100$ and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ but increases below $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, due to the dominance of the low-frequency guest-rattling modes. The room-temperature electrical resistivity and the Seebeck coefficient show that these materials are metallic. The temperature profile of the thermal conductivities and calculated phonon mean free paths of these materials show glasslike behavior, although they are crystalline materials, indicating strong resonant scattering of heat-carrying acoustic phonons via the rattling of the guests in the clathrate cages.