Structure, thermal, and transport properties of the clathrates Sr8Zn8Ge38, Sr8Ga16Ge30, and Ba8Ga16Si30

Abstract

The structural parameters, thermal properties, and transport properties of three type I clathrates, namely Sr8Zn8Ge38, Sr8Ga16Ge30, and Ba8Ga16Si30, 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 cm(-1). The heat capacities of these three clathrate materials increase smoothly with increasing temperatures and approach the Dulong-Petit value around room temperature. The Gruneisen parameter of these materials is constant between 100 and 300 K but increases below 100 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.