  A single crystal of lead-zirconate-titanate (PZT), composition
Pb(Zr0.80Ti0.20)O3, was studied by polarized-Raman scattering as a function of
temperature. Raman spectra reveal that the local structure deviates from the
average structure in both ferroelectric and paraelectric phases. We show that
the crystal possesses several, inequivalent complex domain boundaries which
show no sign of instability even 200 K above the ferroelectric-to-paraelectric
phase transition temperature TC. Two types of boundaries are addressed. The
first boundary was formed between ferroelectric domains below TC. This boundary
remained stable up to the highest measurement temperatures, and stabilized the
domains so that they had the same orientation after repeated heating and
cooling cycles. These domains transformed normally to the cubic paraelectric
phase. Another type of boundary was formed at 673 K and exhibited no signs of
instability up to 923 K. The boundary formation was reversible: it formed and
vanished between 573 and 673 K during heating and cooling, respectively. A
model in which the crystal is divided into thin slices with different Zr/Ti
ratios is proposed. The physical mechanism behind the thermal-stress-induced
structural changes is related to the different thermal expansion of the slices,
which forces the domain to grow similarly after each heating and cooling cycle.
The results are interesting for non-volatile memory development, as it implies
that the original ferroelectric state can be restored after the material has
been transformed to the paraelectric phase. It also suggests that a
low-symmetry structure, stable up to high-temperatures, can be prepared through
controlled deposition of layers with desired compositions.
