Monoclinic and triclinic phases in higher-order Devonshire theory - art. no. 094108

Devonshire theory provides a successful phenomenological description of man y cubic perovskite ferroelectrics such as BaTiO3 via a sixth-order expansio n of the free energy in the polar order parameter. However, the recent disc overy of a novel monoclinic ferroelectric phase in the PZT system by Noheda et al. [Appl. Phys. Lett. 74, 2059 (1999)] poses a challenge to this theor y. Here, we confirm that the sixth-order Devonshire theory cannot support a monoclinic phase, and consider extensions of the theory to higher orders. We show that an eighth-order theory allows for three kinds of equilibrium p hases in which the polarization is confined not to a symmetry axis but to a symmetry plane. One of these phases provides a natural description of the newly observed monoclinic phase. Moreover, the theory mal;es testable predi ctions about the nature of the phase boundaries between monoclinic, tetrago nal, and rhombohedral phases. A ferroelectric phase of the lowest (triclini c) symmetry type, in which the polarization is not constrained by symmetry, does not emerge until the Devonshire theory is carried to twelfth order. A topological analysis of the critical points of the free-energy surface fac ilitates the discussion of the phase transition sequences.