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 Pm3n 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 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.