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Effect of the oxygen vacancy on the ferroelectricity of 90° domain wall structure in PbTiO3: A density functional theory study

The effect of oxygen vacancy on the ferroelectricity of the 90° domain wall structure in PbTiO3 has been investigated by ab initio (first-principles) density functional theory calculations. Based on the position of oxygen vacancies with respect to the 90° domain wall, the oxygen vacancies are classified into two types, i.e., vacancy on the PbO plane (O1-vac) and vacancy on the TiO plane (O2- and O3-vac). Oxygen vacancies in the region adjacent to the 90° domain wall give the minimum formation energies, which indicates that the oxygen vacancies tend to accumulate in the area near the 90° domain wall. The minimum vacancy energies for all oxygen vacancies occur in the region adjacent to the 90° domain wall. The pinning energies of the oxygen vacancies to the 90° domain wall are determined to be 0.330 eV, 0.130 eV, and 0.228 eV for the O1-, O2-, and O3-vacancies, respectively. Such a pinning effect of oxygen vacancies results in a shift of the central position of the domain wall in the crystal lattice, and also, the width of the domain wall is enlarged. This result is found to be the most pronounced for O1-vac on the PbO plane. Finally, the suppression of ferroelectricity caused by the domain wall is strengthened by the effect of the pinned oxygen vacancies.