Impact of Strain Engineering on Antiferroelectricity in NaNbO₃ Thin Films

Thin films of NaNbO₃ were grown on various substrates to investigate the effect of epitaxial strain on their structural and electrical properties. Reciprocal space maps confirmed the presence of epitaxial strain from +0.8% to -1.2%. A bulk-like antipolar ground state was detected via structural characterization for NaNbO₃ thin films grown with strains ranging from a compressive strain of 0.8% to small tensile strains, up to -0.2%. For larger tensile strains on the other hand, no indication of antipolar displacements can be detected, even beyond the relaxation of the film at larger thicknesses. Electrical characterization revealed a ferroelectric hysteresis loop for thin films under a strain of +0.8% to -0.2%, while the films under larger tensile strain showed no out-of-plane polarization component. However, the films with a compressive strain of 0.8% present a saturation polarization of up to 55 μC·cm^-2, more than twice as large for films grown under conditions with small strain, which is also larger than the highest values reported for bulk materials. Our results indicate the high potential for strain engineering in antiferroelectric materials, as the antipolar ground state could be retained with compressive strain. The observed enhancement of the saturation polarization by strain allows for substantial increase of energy density of the capacitors with antiferroelectric materials.