Ferroelectrics are attractive materials for various low-power applications at the nanoscale thanks to the reversible spontaneous polarization they exhibit. The spontaneous polarization originates from collective interactions of ion that are displaced off-center. In the non-centrosymmetric tetragonal structure of ferroelectrics, bound charges accumulate at the material surface to form depolarization field opposite to the bulk polarization direction. The ion organization at the surface is also responsible to reduce mechanical strain, giving rise to a unique electro-mechanical coupling. In the absence of surface-charge compensation, the ferroelectric state is unstable. To-date, full realization of the surface structure has remained elusive, hindering the theoretical and even conceptual understanding of ferroelectricity as well as the technological potential of ferroelectric materials.
Here, we report on direct observation of an unpredicted formation of a thin cubic TiO shell that helps stabilize the ferroelectric polarization of tetragonal BaTiO3 nanoparticles. Moreover, we showed and quantified the dynamic evolution of this surface structure growth during exposure time to the electron beam, along with Ba escape. Our structural and chemical observations were confirmed using HRTEM/STEM equipped with HAADF as well as iDPC (integrated differential phase contrast) and atomic-resolution EDX detectors that allowed us to image the 20-pm ion displacement and to record the growth of 3-6 atomic layers of TiO shell on the BaTiO3 surface. Finally, the effect of temperature was also examined, allowing us to demonstrate directly how the surface affects the ferroelectric phase transformation.
