Effects of Strain and Film Thickness on the Stability of the Rhombohedral Phase of HfO2

The discovery of ferroelectric polarization in HfO2-based ultrathin films has\nspawned a lot of interest due to their potential applications in data storage.\nRecently, a new R3m rhombohedral phase was proposed to be responsible for the\nemergence of ferroelectricity in the [111]-oriented Hf0.5Zr0.5O2 thin films,\nbut the fundamental mechanism of ferroelectric polarization in such films\nremains poorly understood. In this paper, we employ density-functional-theory\ncalculations to investigate structural and polarization properties of the R3m\nHfO2 phase. We find that the film thickness and in-plane compressive strain\neffects play a key role in stabilizing the R3m phase leading to robust\nferroelectricity of [111]-oriented R3m HfO2.\n