The aim of the study is the investigation of the oxygen vacancy (VO) role for the stabilization of orthorhombic noncentrosymmetric (Pbc21) phase in Hf_0.5Zr_0.5O₂ films. The simulations carried out in terms of the hybrid DFT using Quantum-ESPRESSO code for the four low temperatures Hf_0.5Zr_0.5O₂ crystalline phases: orthorhombic ferroelectric (of-), monoclinic (P21c), orthorhombic oI (Pbca) and orthorhombic oII (Pnma).

It is shown that the VO concentration increasing leads to the fact that the pressure range under which the of-Hf_0.5Zr_0.5O₂ structure can exist is narrowed. It is concluded that high VO concentration has a negative impact on the ferroelectric phase stabilization even for thin Hf_0.5Zr_0.5O₂ films. It is established that of-Hf_0.5Zr_0.5O₂ with O vacancies in any concentration is not stable for bulk crystal at any value of external pressure in which the structure is able to exist. However, it is shown that VO have a positive impact on the stabilization of the centrosymmetric oII phase of Hf_0.5Zr_0.5O₂. Thus, the presence of oxygen vacancies and polyvacancies not only does not contribute to the Hf_0.5Zr_0.5O₂ ferroelectric phase stabilization, but also leads to the destruction of the ferroelectric structure. This is consistent with the hypothesis that the degradation ferroelectric response of FeRAM elements during repeated reprogramming, is caused by the oxygen vacancies generation.

The work is supported by the Russian Science Foundation grant #14-19-00192.