By Josh Perry, Editor
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Researchers from the Vienna (Austria) University of Technology (TU Wien) demonstrated a new effect in clathrates, crystals with a specific lattice structure that isolates individual atoms, in which the material is both a thermal insulator and a conductor of electricity.

The atom in the cage can oscillate in two different directions. (TU Wien)

According to a report from the university, this understanding of the properties in clathrates could be a breakthrough in the creation of thermoelectric materials for converting waste heat into electrical energy.

Clathrate cages create weak bonds between neighboring atoms, which limits the ability for heat to spread. Each of the atoms is independent, so the vibration of one atom only weakly impacts the next atom and keeps the heat from spreading quickly.

Researchers tested and measured a series of crystals with different properties and detailed a new Kondo-like phono scattering effect in clathrates. In these structures, an atom vibrates in two specific directions. The direction can be altered when a heat wave hits the atom in its cage, which causes the thermal conductivity to decrease.

“This is exactly the combination of material properties which is required in order to use the thermoelectric effect on an industrial scale,” the article said. “Something hot is connected to something cold using the right material, and the energy flow in between can be directly converted into electricity. On the one hand, the material must conduct electrical current, but on the one hand, it should not equilibrate the temperatures by conducting the heat too quickly, otherwise the effect can no longer be used.”

The research was recently published in Nature Communications. The abstract read:

“Crystalline solids are generally known as excellent heat conductors, amorphous materials or glasses as thermal insulators. It has thus come as a surprise that certain crystal structures defy this paradigm. A prominent example are type-I clathrates and other materials with guest-host structures.

“They sustain low-energy Einstein-like modes in their phonon spectra, but are also prone to various types of disorder and phonon-electron scattering and thus the mechanism responsible for their ultralow thermal conductivities has remained elusive.

“Our thermodynamic and transport measurements on various clathrate single crystal series and their comparison with ab initio simulations reveal an all phononic Kondo effect as origin.

“This insight devises design strategies to further suppress the thermal conductivity of clathrates and other related materials classes, with relevance for thermoelectric waste heat recovery and, more generally, phononic applications. It may also trigger theoretical work on strong correlation effects in phonon systems.”