Exploring the Morphotropic Phase Boundary in Epitaxial PbHf_1–<i>x</i>Ti_<i>x</i>O₃ Thin Films

Abstract

Epitaxial PbHf1–xTixO3/SrTiO3(001) thin-film heterostructures are studied for a potential morphotropic phase boundary (MPB) akin to that in the PbZr1–xTixO3 system. End members, PbHfO3 and PbTiO3, were found to possess orthorhombic (Pbam) and tetragonal (P4mm) crystal structures and antiferroelectric and ferroelectric (∼87 μC/cm2) behavior, respectively. PbHf0.75Ti0.25O3 and PbHf0.25Ti0.75O3 solid solutions were both found to be ferroelectric with rhombohedral (R3c, ∼22 μC/cm2) and tetragonal (P4mm, ∼46 μC/cm2) structures, respectively. For intermediate PbHf1–xTixO3 compositions (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 P4mm symmetries 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 PbZr0.52Ti0.48O3 films in the same geometry. Thereby, the PbHf1–xTixO3 system is a viable alternative to the PbZr1–xTixO3 system offering comparable performance.