Ferroelastic Domain Switching in (K,Na)NbO₃-Based Polymorphic Perovskites Under Combined Stress and Temperature Stimuli

Stress, other than temperature, serves as a thermodynamic driving force that alters the structure of ceramics at atomic and mesoscale levels. In oxides, stress-induced structural evolution is often linked to materials functionalities such as ferroelasticity and shape-memory effect. Understanding these stress-induced effects on functional oxides, particularly from the perspective of atomic-scale structural evolution, is crucial for optimizing materials and exploring advanced properties. (K,Na)NbO₃ (KNN)–based perovskites are among the most promising lead-free ferroelectric materials, with wide applications as sensors and transducers across a broad temperature range. Their rich structural transitions and the various domain-switching modes make domain behavior highly responsive to the combined stimuli of temperature and stress. However, the temperature-dependent ferroelasticity in polymorphic KNN-based ceramics remains insufficiently explored. In this study, bulk ceramic samples are investigated using time-of-flight neutron diffraction with load frame coupled with heater. In-situ neutron diffraction reveals the anisotropic domain switching behaviors under compression with individual grains responding differently to stress depending on their domain orientation relative to the loading direction. Notably, the (111) plane behaves like a habit plane, showing the least tendency to switch domains. Irreversible switching contributes significantly to the large macro strain (~0.95%) observed during first loading at 300MPa at the room temperature, in contrast to reversible switching during unloading. In addition to the individual switching, comprehensive analysis suggests an increases in domain size under low stress. The study further examines domain behaviors at elevated temperatures, across the orthorhombic-tetragonal and tetragonal-cubic transformations. The dynamic switching and recovery under a combination of stress, pre-strain, temperature and phase transformation are unraveled.