In situ observation of ferroelectric domain evolution of (K,Na)NbO₃ single crystals during heating

Since the spontaneous polarization and domain switching is closely related with the physical properties of ferroelectric materials, significant research efforts have been carried out to understand domain switching under external fields. However, investigations on the evolution of ferroelectric domains under thermal fields have comparatively been limited. To elucidate the thermal stability of strain and high pyroelectric coefficient observed in potassium sodium niobate (KNN) single crystals, we conducted in situ TEM studies with heating. Our findings reveal that the strain thermal stability originates from the thermal stability of ferroelectric domains. The nucleation of domain configurations of the tetragonal phase from the orthorhombic phase enhances the pyroelectric effect during the phase transition. Furthermore, near the Curie temperature, we identified an intermediate state exhibiting a labyrinth domain configuration between the cubic and tetragonal phases. Our investigation elucidates the underlying mechanisms behind the thermal properties of the material and advances the understanding of the thermal response of ferroelectric domains.