The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures. Rattling of ions in cages results in low thermal conductivity, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells.

We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na_0.8CoO₂, which has a large-period superstructure. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO₂. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.

—Voneshen et al.

Led by scientists at Royal Holloway, University of London, the team conducted a series of experiments on crystals of sodium cobaltate grown in the University’s Department of Physics. X-ray and neutron scattering experiments were carried out at the European Synchrotron Radiation Facility and in the Institut Laue-Langevin in Grenoble, using the UK’s national supercomputer facility HECToR to make their calculations.

They believe their approach can easily be applied to other substances, since they only require tiny crystals and will, therefore, guide the design of the next generation of thermoelectric materials.

If we can design better thermoelectric materials, we will be able to reduce the energy consumption of cars by converting waste heat in exhausts into electrical power, as well as cooling hot spots on computer chips using solid state refrigerators.

The development of thermoelectric oxides offers an environmentally clean alternative to current materials that contain elements that are harmful, such as lead, bismuth or antimony, or those that are in limited supply, such as tellurium.

—Professor Jon Goff from the Department of Physics at Royal Holloway

Resources

• D. J. Voneshen, K. Refson, E. Borissenko, M. Krisch, A. Bosak, A. Piovano, E. Cemal, M. Enderle, M. J. Gutmann, M. Hoesch, M. Roger, L. Gannon, A. T. Boothroyd, S. Uthayakumar, D. G. Porter and J. P. Goff (2013) Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate. Nature Materials, doi: 10.1038/nmat3739