Enhanced Piezoelectricity by Polarization Rotation through Thermal Strain Manipulation in PbZr_0.6Ti_0.4O₃ Thin Films

Lead based bulk piezoelectric materials, e.g., PbZr_xTi_1-xO₃ (PZT), are widely used in electromechanical applications, sensors, and transducers, for which optimally performing thin films are needed. The results of a multi-domain Landau–Ginzberg-Devonshire model applicable to clamped ferroelectric thin films are used to predict the lattice symmetry and properties of clamped PZT thin films on different substrates. Guided by the thermal strain phase diagrams that are produced by this model, experimentally structural transitions are observed. These can be related to changes of the piezoelectric properties in PZT(x = 0.6) thin films that are grown on CaF₂, SrTiO₃ (STO) and 70% PbMg_1/3Nb_2/3O₃-30% PbTiO₃ (PMN-PT) substrates by pulsed laser deposition. Through temperature en field dependent in situ X-ray reciprocal space mapping (RSMs) and piezoelectric force microscopy (PFM), the low symmetry monoclinic phase and polarization rotation are observed in the film on STO and can be linked to the measured enhanced properties. The study identifies a monoclinic -rhombohedral M_C-M_A-R crystal symmetry path as the polarization rotation mechanism. The films on CaF₂ and PMN-PT remain in the same symmetry phase up to the ferroelectric-paraelectric phase transition, as predicted. These results support the validity of the multi-domain model which provides the possibility to predict the behavior of clamped, piezoelectric PZT thin films, and design films with enhanced properties.