Potential of Chevrel phases for thermoelectric applications

A low lattice thermal conductivity is one of the requirements to achieve high thermoelectric figures of merit. Several low thermal conductivity materials were identified and developed over the past few years at the Jet Propulsion Laboratory (JPL), including filled skutterudites and Zn4Sb3-based materials. A. study of the mechanisms responsible for the high phonon scattering rates in these compounds has demonstrated that materials with structures that can accommodate additional atoms in their lattice are likely to possess low lattice thermal conductivity values. Chevrel phases (Mo6Se8-type) are just such materials and are currently being investigated at JPL for thermoelectric applications. The crystal structures of the Chevrel phases present cavities which can greatly vary in size and can contain a large variety of atoms ranging from large ones such as Ph to small ones such as Cu. In these materials, small inserted atoms usually show large thermal parameters which indicate that they move around and can significantly scatter the phonons. The electronic and thermal properties of these materials can potentially be controlled by a careful selection of the filling element(s). We have synthesized (Cu, Cu/Fe, Ti)(x)Mo6Se8 samples and report in this paper on their thermoelectric properties. Approaches to optimize the properties of these materials for thermoelectric applications are discussed.