Exploring the Pb1−xSrxHfO3 System and Potential for High Capacitive Energy Storage Density and Efficiency

The hafnate perovskites PbHfO3 (antiferroelectric) and SrHfO3 (“potential” ferroelectric) are studied as epitaxial thin films on SrTiO3 (001) substrates with the added opportunity of observing a morphotropic phase boundary (MPB) in the Pb1−xSrxHfO3 system. The resulting (240)-oriented PbHfO3 (Pba2) films exhibited antiferroelectric switching with a saturation polarization ≈53 µC cm−2 at 1.6 MV cm−1, weak-field dielectric constant ≈186 at 298 K, and an antiferroelectric-to-paraelectric phase transition at ≈518 K. (002)-oriented SrHfO3 films exhibited neither ferroelectric behavior nor evidence of a polar P4mm phase . Instead, the SrHfO3 films exhibited a weak-field dielectric constant ≈25 at 298 K and no signs of a structural transition to a polar phase as a function of temperature (77–623 K) and electric field (–3 to 3 MV cm−1). While the lack of ferroelectric order in SrHfO3 removes the potential for MPB, structural and property evolution of the Pb1−xSrxHfO3 (0 ≤ x < 1) system is explored. Strontium alloying increased the electric-breakdown strength (EB) and decreased hysteresis loss, thus enhancing the capacitive energy storage density (Ur) and efficiency (η). The composition, Pb0.5Sr0.5HfO3 produced the best combination of EB = 5.12 ± 0.5 MV cm−1, Ur = 77 ± 5 J cm−3, and η = 97 ± 2%, well out-performing PbHfO3 and other antiferroelectric oxides.