A previously unreported ferroelectric phase has been discovered in a highly homogeneous sample of ${\mathrm{PbZr}}_{\mathrm{0.52}}{\mathrm{Ti}}_{\mathrm{0.48}}{\mathrm{O}}_{\mathrm{3}}$ by high-resolution synchrotron x-ray powder diffraction measurements. At ambient temperature the sample has tetragonal symmetry ${\mathrm{(a}}_t\text{=4.037\hspace{0.05em}Å,}$ $c_t\text{=4.138\hspace{0.05em}Å),}$ and transforms below ∼250 K into a phase which, unexpectedly, has monoclinic symmetry ${\mathrm{(a}}_m\text{=5.717\hspace{0.05em}Å,}$ $b_m\text{=5.703\hspace{0.05em}Å,}$ $c_m\text{=4.143\hspace{0.05em}Å,}$ $\text{β=90.53°,}$ at 20 K). The intensity data strongly indicate that the polar axis lies in the monoclinic ac plane close to the pseudocubic [111] direction, which would be an example of the species $\text{m3m(12)A2Fm}$ predicted on symmetry grounds by Shuvalov.

High resolution x-ray powder diffraction measurements on poled ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ (PZT) ceramic samples close to the rhombohedral-tetragonal phase boundary (the so-called morphotropic phase boundary) have shown that for both rhombohedral and tetragonal compositions the piezoelectric elongation of the unit cell does not occur along the polar directions but along those directions associated with the monoclinic distortion. This work provides the first direct evidence for the origin of the very high piezoelectricity in PZT.

The perovskitelike ferroelectric system ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ (PZT) has a nearly vertical morphotropic phase boundary (MPB) around $x=0.45\mathrm{}–\mathrm{0.50.}$ 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 $x=0.48\mathrm{}$ for which the lattice parameters are, respectively, $a_t=4.044\mathrm{}$ Å, $c_t=4.138\mathrm{}$ Å, at 325 K, and $a_m=5.721\mathrm{}$ Å, $b_m=5.708\mathrm{}$ Å, $c_m=4.138\mathrm{}$ Å, $\beta =90.496\mathrm{}°,$ 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 $\sim 0.2$ Å 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.

Synchrotron x-ray powder diffraction measurements have been performed on unpoled ceramic samples of $\left(1\mathrm{}-x\right)\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3-x{\mathrm{PbTiO}}_3$ (PMN-xPT) with $30\%<~x<~39\%$ as a function of temperature around the morphotropic phase boundary, which is the line separating the rhombohedral and tetragonal phases in the phase diagram. The experiments have revealed very interesting features previously unknown in this or related systems. The sharp and well-defined diffraction profiles observed at high and intermediate temperatures in the cubic and tetragonal phases, respectively, are in contrast to the broad features encountered at low temperatures. These peculiar characteristics, which are associated with the monoclinic phase of ${\mathrm{M}}_C$-type previously reported by Kiat et al. [Phys. Rev. B 65, 064106 (2000)] and Singh and Pandey [J. Phys. Condens Matter 13, L931 (2001)], can only be interpreted as multiple coexisting structures with ${\mathrm{M}}_C$ as the major component. An analysis of the diffraction profiles has allowed us to properly characterize the PMN-xPT phase diagram and to determine the stability region of the monoclinic phase, which extends from $x=31\%$ to $x=37\%$ at 20 K. The complex lansdcape of observed phases points to an energy balance between the different PMN-xPT phases which is intrinsically much more delicate than that of related systems such as ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ or $\left(1\mathrm{}-x\right)\mathrm{Pb}\left({\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{1/3}\right){\mathrm{O}}_3-x{\mathrm{PbTiO}}_3.$ These observations are in good accord with an optical study of $x=33\%$ by Xu et al. [Phys. Rev. B 64, 020102 (2001)], who observed monoclinic domains with several different polar directions coexisting with rhombohedral domains, in the same single crystal.

The origin of ultrahigh piezoelectricity in the relaxor ferroelectric ${\mathrm{PbZn}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3-{\mathrm{PbTiO}}_3$ was studied with an electric field applied along the [001] direction. The zero-field rhombohedral $R$ phase starts to follow the direct polarization path to tetragonal symmetry via an intermediate monoclinic $M$ phase, but then jumps irreversibly to an alternate path involving a different type of monoclinic distortion. Details of the structure and domain configuration of this novel phase are described. This result suggests that there is a nearby $R-M$ phase boundary as found in the $\mathrm{Pb}(\mathrm{Ti},\mathrm{Zr}){\mathrm{O}}_3$ system.