Multiple strains and polar states in PbZr0.52Ti0.48O3/PbTiO3 superlattices revealed by aberration-corrected HAADF-STEM imaging.

Tuning multiple strain and polar states of ferroelectrics by using strain engineering is an essential approach for designing multifunctional electric devices such as multiple state memories. However, integrating multiple strain states is still a challenge, and in addition, revealing such strains and the resultant polar behaviors on the atomic level remains difficult. In this work we prepare PbZr0.52Ti0.48O3/PbTiO3 (PZT/PTO) superlattices on SrRuO3-buffered SrTiO3(001) substrates. Aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) reveals that the superlattice is coherent in both c (out-of-plane polar direction) and a (in-plane polar direction) domains. We find that the strain states of both PZT and PTO in c and a domains are variant, leading to four special strain states. For example, the tetragonality for PTO in c and a domains is 1.061 and 1.045, respectively. In contrast, PZT in c domains displays a tetragonality as giant as 1.107, which corresponds to 110 µC cm-2 spontaneous polarization, much larger than the bulk PZT; while PZT in a domains exhibits 1.010 tetragonality with about 70 µC cm-2 polarization. This study reveals a practical way to integrate multiple strain states and enhanced polarizations in ferroelectric films, which could be used as multifunctional electric elements.