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 characteri zed by temperature-dependent neutron powder diffraction, 87 K Sn-119 Mossba uer spectroscopy, and room-temperature single-crystal X-ray diffraction. Th ese compounds form in cubic space group Pm (3) over bar n with alkalimetal atoms residing in the polyhedral cavities formed by the tetrahedrally bonde d network of Sn atoms. Of particular interest are the atomic displacement p arameters (ADPs) exhibited by the alkali-metal atom inside the polyhedral " cages" formed by the framework Sn atoms. The "guest" Cs atoms inside the la rger tetrakaidecahedra show a relatively large room-temperature ADP for Cs8 Ga8Sn38 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 loca l symmetry and Sn-Sn bonding. Temperature-dependent ADPs for the defect Cs8 Sn44 compound are compared to those for Cs8Zn8Sn42. These data help elucida te the cause of the different lattice thermal conductivities of these two c ompounds.