  Whereas ferroelectricity may vanish in ultra-thin ferroelectric films, it is
expected to emerge in ultra-thin anti-ferroelectric films, sparking people's
interest in using antiferroelectric materials as an alternative to
ferroelectric ones for high-density data storage applications. Lead Zirconate
(PbZrO3) is considered the prototype material for antiferroelectricity, and
indeed previous studies indicated that nanoscale PbZrO3 films exhibit
ferroelectricity. The understanding of such phenomena from the microstructure
aspect is crucial but still lacking. In this study, we fabricated a PbZrO3 film
with thicknesses varying from 5 nm to 80 nm. Using Piezoresponse Force
Microscopy, we discovered the film displayed a transition from
antiferroelectric behaviour in the thicker areas to ferroelectric behaviour in
the thinner ones, with a critical thickness between 10 and 15 nm. In this
critical thickness range, a 12 nm PZO thin film was chosen for further study
using aberration-corrected scanning transmission electron microscopy. The
investigation showed that the film comprises both ferroelectric and
ferrielectric phases. The ferroelectric phase is characterized by polarisation
along the pseudocubic [011] projection direction. The positions of Pb, Zr, and
O were determined using the integrated differential phase contrast method. This
allowed us to ascertain that the ferroelectric PbZrO3 unit cell is half the
size of that in the antiferroelectric phase on the ab plane. The observed unit
cell is different from the electric field-induced ferroelectric rhombohedral
phases. Additionally, we identified a ferrielectric phase with a unique
up-up-zero-zero dipole configuration. The finding is crucial for understanding
the performance of ultrathin antiferroelectric thin films and the subsequent
design and development of antiferroelectric devices.
