Structural characterization and thermal conductivity of type-I tin clathrates

Three tin compounds, Cs8Sn44, Cs8Ga8Sn38, and Cs8Zn4Sn42, representative of the type-I clathrate hydrate crystal structure are structurally characterized by temperature-dependent neutron powder diffraction, 87 K Sn-119 Mossbauer spectroscopy, and room-temperature single-crystal X-ray diffraction. These compounds form in cubic space group Pm (3) over bar n with alkalimetal 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 Cs8Ga8Sn38 and Cs8Zn4Sn42; however, in the defect Cs8Sn44 compound this is not the case. This is due to two Sn vacancies in Cs8Sn44 which affect the local symmetry and Sn-Sn bonding. Temperature-dependent ADPs for the defect Cs8Sn44 compound are compared to those for Cs8Zn8Sn42. These data help elucidate the cause of the different lattice thermal conductivities of these two compounds.