Mesoscale mechanisms of the diffuse dielectric behaviour and retention of the polar nano-regions in the polycrystalline ferroelectric BaTiO3

Barium titanate is a classical ferroelectric material that exhibits a jump-like behavior in the order parameter, spontaneous polarization, near the temperature of its transition to the paraelectric phase. This serves as a textbook example of a first-order phase transition, marked by the coexistence of polar and non-polar phase regions. Despite compelling evidence of the gradual phase transformation across Curie temperature ( T c ) and partial retention of ferroelectric properties above T c , the microscopic mechanisms of the phase retention remain unclear. Current study explains temperature anomalies in the macroscopic characteristics of polycrystalline barium titanate by employing complementary macroscopic and local techniques. Our findings reveal that retention of the polar phase regions is driven by the charged defects, which act as the origin of the spatially non-uniform internal electric fields. The insights from this research offer a deeper understanding of the fundamental mechanisms governing ferroelectric behavior and open new possibilities for tailoring materials with phase coexistence for a wide range of technological applications. • The retention of polar nano-regions in polycrystalline BTO above the Curie temperature was confirmed using complementary macroscopic and local methods, which also revealed their role in the diffuse behavior of heat capacity and dielectric permittivity. • Several distinct temperature ranges and critical temperatures, respectively, were identified in the macroscopic functional responses. High-resolution piezoresponse force microscopy revealed the evolution of the polar phase and detected the formation of a complex state involving ordered PNRs and their long-range interactions. • The retention of polar nano-regions well above the Curie temperature was found to be driven by internal electric fields originating from charged defects. These internal electric fields typically persist up to around 170-200°C before relaxing due to the dispersal of the charged defects. • The high-temperature residual second harmonic generation response can be attributed to disordered polar nano-regions pinned by immobile charged defects.