Difference in ferroelectric aging between A-site and B-site acceptor doped BaTiO$_{3}$ crystals.

Aging, the time-dependent changing of material properties, has been widely found in acceptor-doped ABO$_{3}$ ferroelectrics. The origin is usually ascribed to gradual domain stabilization by acceptor-dopant-generated oxygen vacancies. As in ABO$_{3}$ systems both A-site and B-site acceptor doping can induce oxygen vacancies, they are expected to cause similar aging effect. However, here we report that there exists a significant difference in aging effect between A-site (K-doped) and B-site (Mn-doped) acceptor-doped BaTiO$_{3}$ crystals. The B-site acceptor doping has much stronger aging effect. This new phenomenon can be fully explained by a semi-quantitative model based on the defect symmetry principle$^{1-4}$. According to this model, the ``strength'' of aging is determined by a symmetry-conforming force of the defect symmetry to crystal symmetry. This model may also have potential applications in predicting and understanding the strength of the aging effect in other systems. [1] X. Ren, Nat. Mater., 3:91, 2004; [2] L.X. Zhang, W. Chen and X. Ren, Appl. Phys. Lett., 85:5658, 2004; [3-4] L.X. Zhang and X. Ren, Phys. Rev. B, 71:174108, 2005; Phys. Rev. B 73:094121, 2006.