  Hafnia-based ferroelectrics have become a valuable class of electronic
functional materials at the nanoscale, showing great potential for
next-generation memory and logic devices. However, more robust ferroelectric
properties and better understanding of the polarization mechanisms are
currently needed both in technology and science. Herein, we report the
properties of oxygen-deficient Hf0.5Zr0.5O2 films with ultralarge remanent
polarization (Pr) of 387 uC cm-2 at room temperature (1 kHz). Structure
characterizations identify a new ferroelectric monoclinic Pc phase in these
Hf0.5Zr0.5O2 films. The in-situ STEM measurements evidence polar displacements
of the oxygen atoms, which move up and down in the Pc structure under applied
DC bias fields, showing a huge displacement (1.6 A). DFT calculations optimized
the Pc structure and also predicted a large polarization. The coexistence of
the ferroelectric monoclinic (Pc) phases and orthorhombic (Pca21) is
responsible for this superior ferroelectric properties. These findings are
promising for hafnia-based ferroelectric applications in integrated
ferroelectric devices, energy harvesting and actuators, etc.
