Exploring the Morphotropic Phase Boundary in Epitaxial PbHf1-XTiXO3 Thin Films

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 (similar to 87 mu C/cm2) behavior, respectively. PbHf0.75Ti0.25O3 and PbHf0.25Ti0.75O3 solid solutions were both found to be ferroelectric with rhombohedral (R3c, similar to 22 mu C/cm2) and tetragonal (P4mm, similar to 46 mu C/ 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 mu C/cm2) and saturation (18.5-36 mu C/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.