Electric-Field-Induced Ferroelectricity in 5%Y-doped Hf_0.5Zr_0.5O₂: Transformation from the Paraelectric Tetragonal Phase to the Ferroelectric Orthorhombic Phase

The ferroelectric phase transformation from the tetragonal phase to the orthorhombic phase, induced by an electric field, is demonstrated in a 5%YO_1.5-doped Hf_0.5Zr_0.5O₂ epitaxial film which is grown on Sn-doped In₂O₃-covered (111) yttria-stabilized zirconia by the pulsed laser deposition method at room temperature and subsequent heat treatment. Although X-ray diffraction shows the film to consist of a paraelectric tetragonal phase after the heat treatment, polarization–electric field (P–E) measurements reveal a hysteresis loop attributed to the ferroelectricity. To clarify the discrepancy between the crystal structure and electric characteristics, the crystal structure after electric field loading is determined by scanning transmission electron microscopy and synchrotron X-ray diffraction measurements. Both structural characterizations clearly reveal that the application of an electric field promotes the phase transition from the paraelectric tetragonal phase to the ferroelectric orthorhombic phase. This ferroelectric transition is irreversible, as the ferroelectric phase remains after the removal of the electric field. These results facilitate the elucidation of the mechanism by which ferroelectricity is displayed in HfO₂-based fluorite ferroelectric materials and imply unimportance of the orthorhombic phase for as-prepared films.