Tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: The structure of PbZr0.52Ti0.48O3

The perovskitelike ferroelectric system PbZr1-xTixO3 (PZT) has a nearly ver tical morphotropic phase boundary (MPB) around x = 0.45-0.50. Recent synchr otron x-ray powder diffraction measurements by Noheda et al. [Appl. Phys. L ett. 74, 2059 (1999)] have revealed a monoclinic phase between the previous ly established tetragonal and rhombohedral regions. In the present work we describe a Rietveld analysis of the detailed structure of the tetragonal ac id monoclinic PZT phases on a sample with x = 0.48 for which the lattice pa rameters are, respectively, a(1) = 4.044 Angstrom, c(t) = 4.138 Angstrom, a t 325 K, and a(m) = 5.721 Angstrom, b(m) = 5.708 Angstrom, c(m) = 4.138 Ang strom, beta = 90.496 degrees, at 20 K. In the tetragonal phase the shifts o f the atoms along the polar [001] direction are similar to those in PbTiO3 but the refinement indicates that there are, in addition, local disordered shifts of the Pb atoms of similar to 0.2 Angstrom perpendicular to the pola r axis. The monoclinic structure can be viewed as a condensation along one of the (110) directions of the local displacements present in the tetragona l phase. It equally well corresponds to a freezing-out of the local displac ements along one of the (100) directions recently reported by Corker et al. [J. Phys.: Condens. Matter 10, 6251 (1998)] for rhombohedral PZT. The monoc linic structure therefore provides a microscopic picture of the MPB region in which one of the ''locally'' monoclinic phases in the "average" rhombohe dral or tetragonal structures freezes out, and thus represents a bridge bet ween these two phases.