Mr. Asaf Hershkovitz, PhD. Candidate
Ferroelectrics are functional materials that demonstrate switchable macroscopic polarization and strong electro-mechanical coupling. Therefore, these materials are used in everyday technologies including medical ultrasound imaging, low-energy data storage, and cell-phones. In these materials, neighboring unit cells arrange in mesoscale polarization domains that mediate between the atomic-scale structure and macroscopic functionality. The emergence of ferroelectricity in perovskites is typically accompanied by a crystallographic phase transition. Domain imaging at the mesoscale is usually much slower than the typical domain dynamic time scale. Thus, although both the atomic-scale and macroscopic-scale mechanisms of this phase transition are understood, the mesoscopic domain dynamics and stabilization mechanism around the transition have remained elusive. In this work, we used piezoresponse force microscopy (PFM)–an electromechanical AFM—based imaging method–to directly observe the domain organization at the ferroic transition. Assisted with a unique temperature-control capability (heating/cooling rate as low as 0.015 °C/min) we successfully provided a complete description of the orthorhombic-tetragonal transition in BaTiO3, presenting its mixed diffusive-displacive nature. Likewise, by investigating the tetragonal-cubic transition in these materials we redefined the ferroic order parameter and found an unpredicted mediating phase between these two crystal structures.
Advisor: Asst. Prof. Yachin Ivry