Point-defect-driven flattened polar phonon bands in fluorite ferroelectrics

The scale-free ferroelectric polarization of fluorite M O 2 ( M = Hf, Zr) due to flat polar phonon bands are promising for nonvolatile memories. Defects are also widely introduced to improve the emergent ferroelectricity. However, their roles are still not fully understood at the atomic-level. Here, we report a significant effect of point-defect-driven flattening of polar phonon bands with more polar modes and polarization contribution in doped M O 2 . The polar phonon bands in La-doped M O 2 ( M = Hf, Zr) can be significantly flattened, compared with pure ones. However, the lower energy barrier with larger polarization of V O -only doped M O 2 compared with La-doped cases suggest that V O and local lattice distortion should be balanced for high-performance fluorite ferroelectricity. The work is believed to bridge the relation between point defects and the generally enhanced induced ferroelectricity in fluorite ferroelectrics at the atomic-level and inspire their further property optimization via defect-engineering.