Structural Characterization and Thermal Conductivity of Type-I Tin Clathrates

Three tin compounds, Cs 8 Sn 44, Cs 8 Ga 8 Sn 38, and Cs 8 Zn 4 Sn 42, representative of the type-I clathrate hydrate crystal structure are structurally characterized by temperature-dependent neutron powder diffraction, 87 K Sn-119 Mössbauer spectroscopy, and room-temperature single-crystal X-ray diffraction. These compounds form in cubic space group Pm 3̄ n with alkali-metal atoms residing in the polyhedral cavities formed by the tetrahedrally bonded network of Sn atoms. Of particular interest are the atomic displacement parameters (ADPs) exhibited by the alkali-metal atom inside the polyhedral “cages” formed by the framework Sn atoms. The “guest” Cs atoms inside the larger tetrakaidecahedra show a relatively large room-temperature ADP for Cs 8 Ga 8 Sn 38 and Cs 8 Zn 4 Sn 42; however, in the defect Cs 8 Sn 44 compound this is not the case. This is due to two Sn vacancies in Cs 8 Sn 44 which affect the local symmetry and Sn−Sn bonding. Temperature-dependent ADPs for the defect Cs 8 Sn 44 compound are compared to those for Cs 8 Zn 8 Sn 42 . These data help elucidate the cause of the different lattice thermal conductivities of these two compounds.