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Electric‐field‐induced phase transitions in Bi0.5Na0.5TiO3‐based relaxor ferroelectrics are essential to the control of their electrical properties and consequently in revolutionizing their dielectric and piezoelectric applications. However, fundamental understanding of these transitions is a long‐standing challenge due to their complex crystal structures. Given the structural inhomogeneity at the nanoscale or sub‐nanoscale in these materials, dielectric response characterization based on terahertz (THz) electromagnetic‐probe beam fields is intrinsically coordinated to lattice dynamics during DC‐biased poling cycles. The complex permittivity reveals the field‐induced phase transitions to be irreversible. This profoundly counters the claim of reversibility, the conventional support for which is based upon the peak that is manifest in each of four quadrants of the current–field curves. The mechanism of this irreversibility is solely attributed to polar clusters in the transformed lattices. These represent an extrinsic factor, which is quiescent in the THz spectral domain. [ABSTRACT FROM AUTHOR]

In article number 1901373, J Wu, B Yang, D Zhang, H Yan and co‐workers demonstrate the importance of terahertz (THz) technology in decoding the intrinsic structural dynamics of Bi0.5Na0.5TiO3‐based ferroelectrics. During DC‐field poling cycles, the variated dielectric response probed by reflected‐THz beam is coordinated to the permanent change of polar clusters. The evolution of perovskite unit cells symbolizes the irreversible field‐induced structural changes at the lattice scale. [ABSTRACT FROM AUTHOR]