Defect-mediated lattice relaxation and domain stability in ferroelectric oxides

The mechanical, optical, thermal, and magnetic properties of materials are strongly affected and, sometimes, dominated by point defects and their complexes. In ferroic materials, changes of the ferroelectric, dielectric, and piezoelectric properties, which either occur spontaneously or accumulate in the course of numerous ferroelectric axis switching events, have been attributed to the presence of defects.

Here we consider oxygen vacancies in tetragonal ferroelectric perovskites (general formula ABO₃, see Figure to the right). The oxygen axial sites located in the B-O-B chains, oriented along the tetragonal (polar) axis c, and the oxygen equatorial sites located in the BO₂ planes perpendicular to this axis, are not equivalent. Consequently, the oxygen vacancies at these sites, referred to as axial (V_ax) and equatorial (V_eq ) vacancies, are not isoenergetic and they induce strong site-specific lattice relaxation.

The mechanisms of the transformation between the vacancy's stable (V_ax) and metastable (V_eq) configurations are examined using the density functional theory. Upon switching the polar axis by 90 degrees the axial vacancy becomes equatorial and vice versa.

Our results suggest that two equatorial vacancy stabilization scenarios are possible in this case: (i)diffusion of the vacancy from the metastable to the stable site and (ii) rotation of the polar axis near the vacancy. The interplay between these scenarios is determined by the vacancy diffusion rate and the energy of the 90 degree domain walls. Optimizing these properties by judicious materials design can help to produce better ferroelectrics and exploit defects in order to maximize piezoelectric response.