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 Pb_{1- x} Sr_x HfO₃ 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 Pb_{1- x} Sr_x HfO₃ (0 ≤ x < 1) system is explored. Strontium alloying increased the electric-breakdown strength (E_B ) and decreased hysteresis loss, thus enhancing the capacitive energy storage density (U_r ) and efficiency (η). The composition, Pb_0.5 Sr_0.5 HfO₃ produced the best combination of E_B  = 5.12 ± 0.5 MV cm^-1 , U_r  = 77 ± 5 J cm^-3 , and η = 97 ± 2%, well out-performing PbHfO₃ and other antiferroelectric oxides.

Thin-film materials with large electromechanical responses are fundamental enablers of next-generation micro-/nano-electromechanical applications. Conventional electromechanical materials (for example, ferroelectrics and relaxors), however, exhibit severely degraded responses when scaled down to submicrometre-thick films due to substrate constraints (clamping). This limitation is overcome, and substantial electromechanical responses in antiferroelectric thin films are achieved through an unconventional coupling of the field-induced antiferroelectric-to-ferroelectric phase transition and the substrate constraints. A detilting of the oxygen octahedra and lattice-volume expansion in all dimensions are observed commensurate with the phase transition using operando electron microscopy, such that the in-plane clamping further enhances the out-of-plane expansion, as rationalized using first-principles calculations. In turn, a non-traditional thickness scaling is realized wherein an electromechanical strain (1.7%) is produced from a model antiferroelectric PbZrO₃ film that is just 100 nm thick. The high performance and understanding of the mechanism provide a promising pathway to develop high-performance micro-/nano-electromechanical systems.