Unique switching mode of HfO2 among fluorite-type ferroelectric candidates

Abstract

While demonstrating technical significance in recent years, the physical understanding into ferroelectric hafnia and related materials is still less satisfactory. This is partly due to the limited number of sample materials in the fluorite-type ferroelectrics. In this work, five more fluorite-type ferroelectric candidate materials are further considered (SrI₂, SrBr₂, CaCl₂, YbCl₂, YbBr₂), where their common features as well as the uniqueness of hafnia within this class are extracted. It turns out that other materials typically prefer to establish a new bond after breaking an old bond during polarization switching, but hafnia shows a strong tendency to first forming the new bond. The most favorable switching path of hafnia is thus very special and is not preferred by other materials. The reason is discussed within the context of the seven-coordination theory, and the indispensable covalent feature of the Hf-O bonds is identified as a key. While the simultaneous ionic feature of Hf-O bonds permits HfO₂ to be a member of the fluorite-type ferroelectric, the covalent feature helps to restrict the switching path preference in HfO₂, explaining its unique characteristics in this class. In addition, the special feature of fluorite-type ferroelectrics, compared with perovskite and wurtzite ferroelectrics, is also discussed and revealed from the structural point of view.