Temperature dependent structural and transport properties of the type II clathrates A ₈Na ₁₆E ₁₃₆ (A=Cs or Rb and E=Ge or Si)

Variable temperature single-crystal structure analyses for Cs ₈Na ₁₆Si ₁₃₆, Rb ₈Na ₁₆Si ₁₃₆, Cs ₈Na ₁₆Ge ₁₃₆, and Rb ₈Na ₁₆Ge ₁₃₆ 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.