Exploring the Morphotropic Phase Boundary in Epitaxial PbHf1–xTixO3Thin Films

Epitaxial PbHf1–xTixO3/SrTiO3(001) thin-film heterostructures are studied for a potential morphotropic phase boundary (MPB) akin to that in the PbZr1–xTixO3system. End members, PbHfO3and PbTiO3, were found to possess orthorhombic (Pbam) and tetragonal (P4mm) crystal structures and antiferroelectric and ferroelectric (∼87 μC/cm2) behavior, respectively. PbHf0.75Ti0.25O3and PbHf0.25Ti0.75O3solid solutions were both found to be ferroelectric with rhombohedral (R3c, ∼22 μC/cm2) and tetragonal (P4mm, ∼46 μC/cm2) structures, respectively. For intermediate PbHf1–xTixO3compositions (e.g., x= 0.4, 0.45, 0.5, and 0.55), a structural transition was observed from rhombohedral (hafnium-rich) to tetragonal (titanium-rich) phases. These intermediate compositions also exhibited mixed-phase structures including R3c, monoclinic (Cm), and P4mmsymmetries and, in all cases, were ferroelectric with remanent (5–22 μC/cm2) and saturation (18.5–36 μC/cm2) polarization and coercive field (24–34.5 kV/cm) values increasing with x. While the dielectric constant was the largest for PbHf0.6Ti0.4O3, the MPB is thought to be near x= 0.5 after separation of the intrinsic and extrinsic contributions to the dielectric response. Furthermore, piezoelectric displacement–voltage hysteresis loops were obtained for all chemistries revealing displacement values as good as PbZr0.52Ti0.48O3films in the same geometry. Thereby, the PbHf1–xTixO3system is a viable alternative to the PbZr1–xTixO3system offering comparable performance.