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Lecture

Visualization of polar nanoregions in bismuth-alkali-based relaxor ferroelectrics revealed by high-resolution PFM and quantification of the relaxation behavior via high-speed PFM

Wednesday (28.09.2016)
10:15 - 10:30
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Bismuth-alkali- and alkali-niobium-based ceramics are potential candidates to replace lead-containing materials in sensor and actuator applications because they exhibit a large strain at relatively low electric field and their dielectric and electromechanical properties can be conveniently tailored by various dopants. Changes in degree of ergodicity, vacancies and chemical pressure are crucial in determining relaxor features, such as their macroscopic piezoelectric properties that have been widely studied. The nanoscopic dynamic relaxor properties given by the electromechanical behavior and distribution of polar nanoregions (PNRs) in the material, however, are rarely explored. We developed a high-resolution piezoresponse force microscopy (PFM) technique based on dual ac resonance tracking (DART) to visualize polar nanoregions and speeded up the imaging rate to ascertain the dynamic relaxor behavior of the Lanthanum-doped ferroelectrics Bi1/2Na1/2TiO3 (BNT) - Bi1/2K1/2TiO3 (BKT) that are characterized by different ergodicity through selective A-site doping causing distinct dielectric and electromechanical properties on the macroscale. Thus, we locally poled areas on the surface with a biased tip of an atomic force microscope and subsequently observed the relaxation process of the specimen by piezoresponse force microscopy with tip velocities up to 60 µm/s. Three fundamental mechanisms during relaxation of the long-range ordered domain were identified, characterized by a slow movement of the domain wall, the sudden collapse of the main area to PNR clusters, followed by a slow relaxation with pinning points that stabilize the residual domain at the end of the process. Interestingly, the relaxation times we determined on the ergodic and non-ergodic samples suggest that the degree of the macroscopically determined ergodicity is a result of the coexistence and interactions between ergodic and non-ergodic PNRs on the nanoscale.

Speaker:
Dr. Christian Dietz
Technische Universität Darmstadt