Temperature dependent structural and transport properties of the type II clathrates A8Na16E136 (A=Cs or Rb and E=Ge or Si)

Variable temperature single-crystal structure analyses for Cs8Na16Si136, Rb8Na16Si136, Cs8Na16Ge136, and Rb8Na16Ge136 are reported along with electrical and thermal transport measurements on two polycrystalline specimens. The strong temperature dependence of the atomic displacement parameters for the alkali-metal atoms is indicative of significant disorder associated with the “rattling” alkali-metal atoms inside the two different polyhedra (sixteen dodecahedra and eight hexakaidecahedra per cubic unit cell) that makeup the type II clathrate hydrate framework. This disorder can lead to low lattice thermal conductivities. Transport measurements show these compounds to be metallic. The potential of type II clathrates for thermoelectric applications is discussed.