Formation of the orthorhombic phase in CeO₂-HfO₂ solid solution epitaxial thin films and their ferroelectric properties

The formation of the metastable orthorhombic phase (Pca2₁) in CeO₂-HfO₂ solid solution epitaxial thin films has been demonstrated. The films were deposited at room temperature on (001)yttria stabilized zirconia substrates by an Ar ion-beam sputtering method and subsequent annealing, where the Ce content of the films was controlled by changing the composition, x = [CeO₂]/([HfO₂]+[CeO₂]), of the sputtering target. The chemical states of cations in xCeO₂ − (1 − x)HfO₂ (x = 0–0.5) thin films have been investigated by X-ray photoelectron spectroscopy, which confirmed the coexistence of Ce⁴⁺ and Ce³⁺. The crystal structure has been investigated by using X-ray diffraction and transmission electron microscopy. These analyses revealed that the metastable orthorhombic phase was formed in the films with x = 0.03–0.1, and the lattice constants of that phase increased with the Ce content. Microstructural analysis has been performed by using scanning transmission electron microscopy, which revealed a multidomain structure consisting of the orthorhombic phase. The polarization-electric field loop for the film with x = 0.1 indicated ferroelectricity, demonstrating that CeO₂-HfO₂ solid solution thin films are candidates for fluorite-type ferroelectrics.