Role Of Point Defects On The Aging Of Ferroelectrics And Its Related Phenomena

Ferroelectric materials have been widely applied in industrial fields as capacitors, sensors, and transducers, etc. due to their functional properties. Despite their usefulness, there have been a number of long-standing unsolved problems to be solved. Most of these problems are related to the point defects, which are inevitably introduced either intentionally as dopants or unintentionally as impurities. Among these problems, the ferroelectric aging effect (the gradual change of properties with time) is a well observed but a puzzling phenomenon in ferroelectrics. Aging strongly affects the stability and reliability of ferroelectric devices; however, it can not be understood by conventional theory. To explore the origin of aging, historically, a number of models had been proposed. Unfortunately, a unified microscopic explanation remains unclear. Thus the study of ferroelectric aging is an important issue both for applications and for a fundamental understanding.Recently, a general symmetry-conforming principle for point defects (defect symmetry principle for short) has been proposed in ferroelectrics. Research work based on this principle has generated exiting discoveries (large recoverable electrostrain) in aged ferroelectrics; and more importantly, it shows potential to microscopically understand the ferroelectric aging. Therefore, in the present work, we utilize such a principle to systematically investigate the aging in doped BaTiO₃ ferroelectrics, aiming to give a unified microscopic explanation on ferroelectric aging.Firstly, based on defect symmetry principle, large recoverable electrostrain had been achieved in aged ferroelectric single crystals, which indicating aging is not always a detrimental effect; it may be even"useful"for applications. However, in practical viewpoint, poly-crystals/ceramics are suitable for mass production. So we investigated aging-induced strain effect in ceramic samples. It is found that, after aging 1.0mol% Mn-doped (Ba_0.95Sr_0.05)TiO₃ ceramics can generate a large recoverable electrostrain of about 0.12-0.15% , which is comparable to good piezoelectric Pb(Zr,Ti)O₃ (PZT) ceramics. This result demonstrates the potential of our approach in achieving large recoverable electrostrain in environmental-friendly (Pb-free) ceramics and thus is a crucial step towards the real application of this novel strain effect.Secondly, the aging-induced recoverable strain effect comes from reversible domain switching mechanism based on defect symmetry principle. Experimentally the macroscopic strain effect has been obtained in aged ferroelectric single and poly crystals. However, the reversible domain-switching process itself is yet to be directly verified. So we performed in-situ domain observation for an aged Mn-doped BaTiO₃ single crystal and simultaneously measured its polarization (P)-field (E) hysteresis loop. Besides, electrostrain behavior of the sample was also characterized at the same time. It was found that the aged sample shows a remarkable reversible domain switching during electric field cycling; it corresponds well to the giant recoverable electrostrain effect. This provides direct mesoscopic evidence for defect-mediated reversible domain-switching mechanism.Thirdly, aging and its related multi-scale phenomena had been well explained by our aging model based on defect symmetry principle . However, historically there exist several other aging models without consensus. The main controversy comes from whether aging is due to domain wall pinning effect or due to a volume effect. To solve this problem, we made a single-domain (domain-wall-free) Mn-doped BaTiO₃ single crystal and found that aging effect exists even in the single-domain sample: the single domain state was stabilized during electric field cycling. This direct evidence strongly suggests that volume effect is the governing mechanism for the ferroelectric aging. Furthermore, the microscopic origin of volume effect is provided by defect symmetry principle. Such an aging model explains both the aging in hysteresis loop (large signal properties) and the aging in dielectric and piezoelectric constants (small signal properties), thus proving a unified microscopic explanation for all ferroelectric aging.Finally, the unified microscopic aging model based on defect symmetry principle can clearly explain the drastic change in aging effect dependent on doping site and doping type. Our experimental results showed that A-site K acceptor doped BaTiO₃ crystal has a weaker aging compared with B-site Mn acceptor doped one. Such a phenomenon is yet to be reported so far. Besides, it is well known that aging exists only in acceptor doped BaTiO₃ crystal, while La or Nb donor doped BaTiO₃ crystal has no aging effect. A microscopic explanation on the non-existence of aging in donor-doped case has been missing so far, but our work provides a simple explanation. According to our semi-quantitative aging model, the weaker aging in A-site acceptor doped sample is due to the small symmetry-conforming force of A-site related defects, while donor doped sample has no aging effect is because symmetry-conforming force of donor related defects is too small to overcome the large migration barrier of defects. This indicates the necessary conditions of aging, which can be a guideline for how to reduce or enhance the aging effect. This guideline may be important in the application of ferroelectric materials.In conclusion, based on the defect symmetry principle, large recoverable electrostrain has been obtained in environment-friendly ferroelectrics, especially in economic ceramics, thus being important to applications; furthermore, ferroelectric aging is well explained by a unified microscopic model, which have been verified with a number of critical experiments. The microscopic understanding of the aging may provide insights into other defect-related problems in ferroelectrics.