Switchable Auxetic Piezoelectricity in Rhombohedral Relaxor Ferroelectrics

Piezoelectric materials typically exhibit opposite longitudinal and transverse mechanical responses under an electric field, reflecting the electromechanical analogue of a positive Poisson’s ratio. Auxetic piezoelectricity, in which longitudinal and transverse coefficients share the same sign, has recently emerged as an unconventional electromechanical response; yet its intrinsic realization and electrical tunability in relaxor ferroelectrics with complex domain structures remain elusive. Here, we demonstrate intrinsic and electrically switchable auxetic piezoelectricity in rhombohedral relaxor ferroelectric Pb(Mg₁/₃Nb₂/₃)O₃-PbTiO₃ by combining first-principles calculations and quantitative piezoelectric measurements. Density functional perturbation theory reveals that a positive transverse piezoelectric response is symmetry-allowed in the rhombohedral phase and primarily originates from internal-strain contributions associated with cooperative Pb–O displacements. Experiments on single-domain (111)_pc-oriented crystals confirm the existence of an intrinsic auxetic state. Beyond the single-domain limit, in situ bias-dependent measurements reveal continuous and reversible electric-field-driven switching between auxetic and conventional piezoelectric states within the same crystal. A tensor-rotation-based multi-domain model quantitatively reproduces the bias-dependent response and identifies domain redistribution, enabled by the large shear piezoelectric coefficient, as the microscopic origin of the switching behavior. These findings establish auxetic piezoelectricity as an intrinsic and electrically programmable response in rhombohedral relaxor ferroelectrics, providing a general route toward reconfigurable electromechanical functionalities.