Unique switching mode of HfO2 among fluorite-type ferroelectric candidates

As a technically significant dielectric, the physical understanding of ferroelectric hafnia is still not 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, and YbBr2), in which their common features and the uniqueness of hafnia within this class are extracted. It was revealed that while other materials typically prefer to establish a new bond only after breaking an old bond during polarization switching, hafnia shows a strong tendency of forming a new bond first. The most favorable switching path of hafnia is thus very special and is not preferred by other materials. The reason for this 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 element. While the simultaneous ionic feature of Hf-O bonds permits HfO2 to be a member of the fluorite-type ferroelectric class of materials, its 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 a structural point of view.