Large imprint in epitaxial 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ thin films for piezoelectric energy harvesting applications

Tuning and stabilizing a large imprint in epitaxial relaxor ferroelectric thin films is one of the key factors for designing micro-electromechanical devices with an enhanced figure of merit (FOM). In this work, epitaxial 500 nm-thick 0.67Pb(Mg_1/3Nb_2/3)O₃–0.33PbTiO₃ (PMN–33PT) films, free from secondary phases and with extremely low rocking curves (FWHM < 0.05°), are grown on ScSmO₃ (SSO) and DyScO₃ (DSO) substrates buffered with SrRuO₃ (SRO). The PMN–33PT is observed to grow coherently on SSO substrates (lattice mismatch of −0.7%), which is c-axis oriented and exhibits large tetragonality compared to bulk PMN–33PT, while on DSO substrates (lattice mismatch of −1.9%), the PMN–33PT film is almost completely relaxed and shows reduced tetragonality. Due to the compressive epitaxial strain, the fully strained PMN–33PT film displays typical ferroelectric P–E hysteresis loops, while the relaxed sample shows relaxor-like P–E loops. Samples present large negative imprints of about −88.50 and −49.25 kV/cm for PMN–33PT/SRO/SSO and PMN–33PT/SRO/DSO, respectively, which is more than threefold higher than the coercive field. The imprint is induced by the alignment of defect dipoles with the polarization and is tuned by the epitaxial strain. It permits the stabilization of a robust positive polarization state (P_r ∼ 20 μC/cm²) and low dielectric permittivity (<700). In addition, the relaxed PMN–33PT film shows improved piezoelectric properties, with a 33% enhancement in d_33,eff relative to the fully strained sample. The obtained low dielectric permittivity and the high piezoelectric coefficients at zero electric field in the studied PMN–33PT films hold great promise to maximize the FOM toward applications in piezoelectric devices.