In a collaboration between the University of Reading, ISIS Neutron and Muon Source and STFC Scientific Computing Department, researchers used neutron spectroscopy and molecular dynamic simulations to investigate the behaviour of tetrahedrite, a copper antimony sulphide, as a potential component of thermoelectric devices.
Thermoelectricity has the potential to make a valuable contribution to net zero targets. The process of converting waste heat back into useful forms of electricity could help reduce reliance on fossil fuels and therefore decrease greenhouse gas emissions. Unfortunately, it is not a widely used technology, due to the high materials costs and low efficiency.
One method for boosting the efficiency of thermoelectric devices is the use of superionic materials which exhibit “liquid-like” behaviour. Superionic conductors have mobile ions, creating a changing crystal structure which disrupts heat propagation and therefore reduces thermal conductivity. However, under the conditions that a thermoelectric device operates, this leads to degradation of the materials, as mobile ions diffuse out of the crystal structure and the device fails.
The study of tetrahedrite at ISIS led to the identification of a behaviour termed incipient ionic conduction. Incipient ionic conduction enables ultralow lattice thermal conductivities without degradation in thermoelectric performance. Through a combination of neutron spectroscopy and molecular dynamics simulations— carried out using the Automated Potential Development (APD) workflow —researchers demonstrated that tetrahedrite has mobile copper ions, but they are trapped by the crystal structure of the material. This enables tetrahedrite to behave like an incipient ionic conductor.
Two tetrahedrite samples were compared: a copper-rich sample with long-range diffusion and stochiometric sample without long-range diffusion. Using the LET spectrometer, copper diffusion through quasielastic neutron scattering (QENS) and lattice dynamics through inelastic neutron scattering (INS) were examined simultaneously. QENS revealed that both samples exhibit copper ion diffusion, but further analysis found that, at temperatures below the onset of long-range diffusion, the mobile copper ions are contained inside the cages found in the tetrahedrite crystal structure.
The suppression of thermal conductivity in “liquid-like” superionic conductors is attributed to strong anharmonicity. The team’s INS measurements of the tetrahedrite samples revealed strong anharmonicity in the low-energy optical mode of the copper ions but require single-crystal INS data as a function of temperature to increase understanding of the relationship between lattice structure and the migration of copper ions.
The team hopes that discovery of this incipient ionic conduction in tetrahedrite will open new avenues in the hunt for materials with ultralow thermal conductivity. They envisage that crystallographic databases could be exploited to find other materials with incipient ionic conductivity- materials that have cages in which ion motion is confined. In the long term, this will hopefully allow for increased use of thermoelectricity as a sustainable energy resource.
