Open Access

Rachid Belrhiti‐Nejjar, Manuel Zahn, Patrice Limelette, Max Haas, Lucile Féger, Isabelle Monot‐Laffez, Nicolas Horny, Dennis Meier, Fabien Giovannelli, Jan Schultheiß, Guillaume F. Nataf

• A common strategy for reducing the thermal conductivity of polycrystalline systems is to increase the number of grain boundaries. Indeed, grain boundaries enhance the probability of phonon scattering events, which has been applied to control the thermal transport in a wide range of materials, including hard metals, diamond, oxides, and two-dimensional (2D) systems such as graphene. Here, the opposite behavior in improper ferroelectric ErMnO3 polycrystals is reported, where the thermal conductivity decreases with increasing grain size. This unusual relationship between heat transport and microstructure is attributed to phonon scattering at ferroelectric domain walls. The domain walls are more densely packed in larger grains, leading to an inversion of the classical grain-boundary-dominated transport behavior. The findings open additional avenues for microstructural engineering of materials for thermoelectric and thermal management applications, enabling simultaneous control overA common strategy for reducing the thermal conductivity of polycrystalline systems is to increase the number of grain boundaries. Indeed, grain boundaries enhance the probability of phonon scattering events, which has been applied to control the thermal transport in a wide range of materials, including hard metals, diamond, oxides, and two-dimensional (2D) systems such as graphene. Here, the opposite behavior in improper ferroelectric ErMnO3 polycrystals is reported, where the thermal conductivity decreases with increasing grain size. This unusual relationship between heat transport and microstructure is attributed to phonon scattering at ferroelectric domain walls. The domain walls are more densely packed in larger grains, leading to an inversion of the classical grain-boundary-dominated transport behavior. The findings open additional avenues for microstructural engineering of materials for thermoelectric and thermal management applications, enabling simultaneous control over mechanical, electronic, and thermal properties.…