Flexoelectricity enables piezoelectric single crystals to be self-poled

Piezoelectric materials used in transducer applications suffer from thermal depolarization, resulting in performance degradation or complete loss of their piezoelectricity. Self-poling is a promising phenomenon, as it enables ferroelectric materials to spontaneously develop net polarization and exhibit piezoelectric response without a conventional poling process. However, the mechanism of self-poling in bulk ferroelectric materials remains controversial and unclear. Here, we demonstrate that inhomogeneous physical properties in the bulk ferroelectric single crystals can induce flexoelectricity-driven unidirectional self-poling. We confirmed the presence of inhomogeneous lattice parameters, thermal expansion coefficients, and phase transition temperatures along the [001] direction in the Mn-doped 0.71Pb(Mg1/3Nb2/3)O3−0.29PbTiO3 single crystal through in-situ high-energy synchrotron radiation X-ray diffraction analysis. These inhomogeneities allow the single crystal to preserve its non-centrosymmetric state along the [001] direction well above TC and enable thermal activation of the self-poling even below TC. These self-poling ferroelectrics are expected to resolve critical reliability issues in future electronic applications.