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A systematic study of (1− x )Pb(Fe 0.5 Nb 0.5 )O 3 – x BiFeO 3 ( x = 0–0.5) was performed by combining dielectric and electromechanical measurements with structural and microstructural characterization in order to investigate the strengthening of the relaxor properties when adding BiFeO 3 into Pb(Fe 0.5 Nb 0.5 )O 3 and forming a solid solution. Pb(Fe 0.5 Nb 0.5 )O 3 crystalizes in monoclinic symmetry exhibiting ferroelectric-like polarization versus electric field ( P–E ) hysteresis loop and sub-micron-sized ferroelectric domains. Adding BiFeO 3 to Pb(Fe 0.5 Nb 0.5 )O 3 favors a pseudocubic phase and a gradual strengthening of the relaxor behavior of the prepared ceramics. This is indicated by a broadening of the peak in temperature-dependent permittivity, narrowing of P–E hysteresis loops and decreasing size of ferroelectric domains resulting in polar nanodomains for x = 0.20 composition. The relaxor behavior was additionally confirmed by Vogel–Fulcher analysis. For the x ≥ 0.30 compositions, broad high-temperature anomalies are observed in dielectric permittivity versus temperature measurements in addition to the frequency-dispersive peak located close to room temperature. These samples also exhibit pinched P–E hysteresis loops. The observed pinching is most probably related to the reorganization of polar nanoregions under the electric field as shown by synchrotron X-ray diffraction measurements as well as by piezo-response force microscopy analysis, while in part affected by the presence of charged point defects and anti-ferroelectric order, as indicated from rapid cooling experiments and high-resolution transmission electron microscopy, respectively. 

Lead‐based relaxor ferroelectrics are characterized by outstanding piezoelectric and dielectric properties, making them useful in a wide range of applications. Despite the numerous models proposed to describe the relation between their nanoscale polar structure and the large properties, the multiple contributions to these properties are not yet revealed. Here, by combining atomistic and mesoscopic‐scale structural analyses with macroscopic piezoelectric and dielectric measurements across the (100–x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃(PMN–xPT) phase diagram, a direct link is established between the multiscale structure and the large nonlinear macroscopic response observed in the monoclinic PMN‐xPT compositions. The approach reveals a previously unrecognized softening effect, which is common to Pb‐based relaxor ferroelectrics and arises from the displacements of low‐angle nanodomain walls, facilitated by the nanoscale polar character and lattice strain disorder. This comprehensive comparative study points to the multiple, distinct mechanisms that are responsible for the large piezoelectric response in relaxor ferroelectrics.