Tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: The structure ofPbZr0.52Ti0.48O3
The perovskitelike ferroelectric system ${\mathrm{PbZr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{3}$ (PZT) has a nearly vertical morphotropic phase boundary (MPB) around $x=0.45--0.50.$ Recent synchrotron x-ray powder diffraction measurements by Noheda et al. [Appl. Phys. Lett. 74, 2059 (1999)] have revealed a monoclinic phase between the previously established tetragonal and rhombohedral regions. In the present work we describe a Rietveld analysis of the detailed structure of the tetragonal and monoclinic PZT phases on a sample with $x=0.48$ for which the lattice parameters are, respectively, ${a}_{t}=4.044$ \AA{}, ${c}_{t}=4.138$ \AA{}, at 325 K, and ${a}_{m}=5.721$ \AA{}, ${b}_{m}=5.708$ \AA{}, ${c}_{m}=4.138$ \AA{}, $\ensuremath{\beta}=90.496\ifmmode^\circ\else\textdegree\fi{},$ at 20 K. In the tetragonal phase the shifts of the atoms along the polar [001] direction are similar to those in ${\mathrm{PbTiO}}_{3}$ but the refinement indicates that there are, in addition, local disordered shifts of the Pb atoms of $\ensuremath{\sim}0.2$ \AA{} perpendicular to the polar axis. The monoclinic structure can be viewed as a condensation along one of the $〈110〉$ directions of the local displacements present in the tetragonal phase. It equally well corresponds to a freezing-out of the local displacements along one of the $〈100〉$ directions recently reported by Corker et al. [J. Phys.: Condens. Matter 10, 6251 (1998)] for rhombohedral PZT. The monoclinic structure therefore provides a microscopic picture of the MPB region in which one of the ``locally'' monoclinic phases in the ``average'' rhombohedral or tetragonal structures freezes out, and thus represents a bridge between these two phases.