Ferroelectric hafnia as an ionic conductor

Abstract

Abstract The intensively concerned hafnia-based ferroelectric (FE) material has been controversial over whether the origin of its observed ferroelectricity being structural or electrochemical. We revisit the rigorous application of modern theory of polarization on displacive FE-HfO2, and make clear the microscopic mechanism of ionic conductance intertwined with continuous nucleation-and-growth FE switching in HfO2 from first principles. Independent from the involvement of vacancies, active oxygen ions in FE-HfO2 can be collectively conducted along continuous FE uniaxial-connected-paths (UCPs) in a typical nucleation-and-growth manner. The ionic conductance should have a nonlinear electric-field dependence from the Merz’s law, which is consistent with the strongly correlated ionic conductance. Based on our established physical picture, some abnormal experimental observations of HfO2 may be explained beyond the pristine understanding of FE switching within double-well potentials.