Atomic structure and ferroelectricity of Pb(Zr(1-x)Ti(x))O(3)

Zr-rich lead zirconium titanate (PZT) presents an excellent system for a study on the nature of ferroelectricity. It exists in a variety of phases all of which represent distortions of the perfect perovskite structure with large atomic displacements. The structural changes of PZT with temperature and composition have been discussed so far primarily through the crystallographic studies of the lattice structure, while the real structure may be different from the average lattice structure. The atomic structure of PZT has been studied using both the Rietveld refinement and the atomic pair distribution function (PDF) analysis of pulsed neutron diffraction data. While the Rietveld refinement of the diffraction data describes the average structure, a real-space refinement of the PDF provides information on the short-range, local structure. The present study demonstrates that the PDF analysis can be more accurate than the Rietveld refinement in determining the atomic positions and the thermal factors even for a perfectly periodic crystalline structure, and is a powerful method of structural study for crystals at finite temperatures since differences can develop between the local structure and the long range structure. Two main observations were made. It is the long range structure that changes most significantly with temperature and composition, rather than the local structure. Second, the changes in the long range structure are explained by the variation in the arrangement of the same local structure. The Pb favors a particular local atomic arrangement in which it accommodates the lone-pair 6s-p electrons on one side and form stronger covalent bonding with a small number of oxygen ions on the other. The same local atomic assemblies arrange themselves in various patterns depending on the temperature and composition forming different long-range structures. In PZT Pb atoms are locally displaced even at temperatures well in the paraelectric phase, and the correlation among Pb displacements determines the long-range structure. These considerations may be of special relevance for the Pb-based relaxor ferroelectrics, such as La-doped (PLZT) and $\rm Pb(Mg\sb{1/3}Nb\sb{2/3})O\sb3$ (PMN). Indeed, the relaxor ferroelectrics are characterized with very diffuse phase transitions and a local polarization that exists even at much higher temperatures