In fabrication of ferroelectric thin films, the films are generally grown at high temperatures above the Curie temperature (T_C) holding for the bulk ferroelectrics. The strain relaxation in the films normally takes places during cooling. After the phase transition from the paraelectric to ferroelectric phase occurred, the symmetry is broken and differences in lattice parameters are introduced. That is, after cooling through T_C, the symmetry is lowered due to the transition from cubic to tetragonal phases. This symmetry lowering can lead to a preferential formation of domains that is able to relieve the accumulated stress in the film.

In general, the stress in ferroelectric thin films is relieved by forming misfit dislocations and by splitting into different ferroelastic domains [1, 2]. However, the term “domain” can not only refer to crystallographic domains but also to the domains where only the polarization reverses.

[1] S. Stemmer, S. K. Streiffer, F. Ernst , M. Ruhle. Philos. Mag. A 71,713,.(1995).
[2] M. W Chu, I. Szafraniak, R. Scholz, C. Harnegea, D. Hesse, et al. Nat. Mater. 3, 87, (2004).