Phonon lifetimes; unifying inelastic neutron scattering measurements with first-principle calculations

Solid-state materials, such as thermoelectrics can be used in thermal management, without the need for moving parts. These can harvest waste heat and convert energy to electricity or be used as spot-sized solid-state refrigerators. To discover new materials or optimize existing ones, it is important to fully understand how interactions between atomic vibrations and other degrees of freedom in the material can decrease thermal transport. In this project, scientists at the University of Stavanger, SINTEF, the Helmholtz-Zentrum Berlin in Germany, the Technical University of Denmark, and collaborators from various countries are working together to develop an experimental and computational methodology based on ab-initio calculations and high-resolution inelastic neutron scattering. This methodology aims at predicting the lattice thermal transport in thermoelectric materials, by accurately describing the atomic vibrations, their interactions and the extent to which these vibrations can travel through the material. Accuracy in these predictions will bring screening calculations closer to reality, facilitating the discovery of materials for innovative thermal management solutions. In this project, we started by investigating the atomic vibrations in a selected set of structurally simple but environmentally promising thermoelectric materials to move forward to researching more complex structures with more degrees of freedom. We have conducted multiple inelastic neutron scattering experiments using large-scale facilities in the UK, Switzerland and France. Calculations and analysis of these data are underway showing promising results.

We propose to develop a coherent strategy to compute, measure and analyse phonon lifetimes and thermal conductivities for advanced materials, which will make possible to predict new materials for thermal management and yield the possibility to predict new or better optimized thermoelectric materials containing non-toxic and earth-abundant elements. This coherent strategy is based on computational developments, implementing both anharmonic and incoherent effects, in addition to the temperature effects on the phonon and electrical properties. These will be based on density functional theory (DFT) and post-DFT methods like screened Green’s functions (GW) and time-dependent-DFT (TDFT) and developments therein. In combination, this will allow us to develop generic workflows for each computational task and execute a large-scale screening study after initial experimental verification using three selected thermoelectric materials, strontium titanate, a skutterudite compound and a clathrate compound. We will determine the phonon lifetimes in the selected benchmark systems with high precision. A novel combination of the highest energy and momentum resolution neutron scattering techniques and Raman scattering will be used. Instrumental resolution functions will be revised and resolution functions for the future spectrometer BIFROST at the European Spallation Source will be computed. The scope of this proposal demands a highly experienced project group with top international research partners from different fields, such as neutron instrumentation and atomic-scale modelling. The project will mainly be executed by two national partners: University of Stavanger (UiS) and SINTEF and two international partners: Helmholtz-Zentrum Berlin and the Technical University of Denmark, thus ensuring knowledge transfer and contributing to the development of national expertise within these techniques.