Semiconductive Flexoelectric Piezoelectric Metamaterials with Strong Electromechanical Response
Conventional piezoelectric materials, exemplified by Sm-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals, exhibit a maximum piezoelectric coefficient d33 of approximately 4000 pC N−1, yet their operational temperature is fundamentally constrained by the Curie temperature (TC), above which the non-centrosymmetric ferroelectric phase transitions to a non-piezoelectric paraelectric state. This study demonstrates that reduction-sintered BaTiO3 ceramics overcome both the magnitude and temperature limitations through a flexoelectric piezoelectric metamaterial (FPM) architecture. Reduction sintering yields an effective flexoelectric coefficient μeff exceeding 50 mC m−1, a 25-fold enhancement over the highest previously reported value. The resulting BaTiO3−δ FPMs exhibit a giant effective d33 above 20000 pC N−1 with no depoling observed above TC. The mechanism is attributed to spontaneously polarized surface layers, formed by inhomogeneous oxygen vacancy distributions, coupled with a negative capacitance amplification effect that boosts the dielectric and piezoelectric responses of these surface layers. This work establishes a generic strategy for designing high-performance piezoelectric materials with extended working temperatures, and identifies a new route for engineering negative capacitance materials for low-energy memory applications.