First principles study on oxygen deficiency induced new stable low-temperature rhombohedral HfO_2-x

Hafnium dioxide (HfO₂) has been extensively studied due to its promising properties, such as a high dielectric constant, wide band gap, and good thermodynamic stability. A number of polymorphs of HfO₂ such as monoclinic, cubic, tetragonal, and orthorhombic were experimentally observed depending on temperature and pressure. Interestingly, a recent report showed the cubic phase of HfO₂, normally crystalized above 2600 K, can be realized at low temperatures with a tiny rhombohedral distortion by adopting oxygen deficiency. [1] Here we address this new rhombohedral HfO_2-x phase by means of density functional theory-based ab initio calculations which concur with the observations. Our calculations reveal that oxygen deficiency can lead to a stable rhombohedral HfO_2-x phase; in addition, our analysis explains observed electronic properties in rhombohedral HfO_2-x. Details in DFT as well as observations will be discussed.