Size effects on ferroelectricity of ultrafine particles of PbTiO3

High resolution transmission electron microscopy is used to image the nanostructure of ultrafine ferroelectric lead titanate particles ranging from 20 to 2000 nm in diameter. The crystal structure, surface morphology, domain–wall structure, as well as surface reconstruction under a 400 KeV electron beam are studied. High resolution images and selected area diffraction patterns showed that all the particles had tetragonal structure; the $\mathrm{c/a}$ ratio and domain size both decreased with decreasing particle size and the particles became monodomain when their diameter was less than 20 nm. A domain wall width of 14 Å was deduced from strain contrast shown by 90° domain walls. There is no evidence of amorphous surface layers; however surface reconstruction of lead titanate particles under the electron beam irradiation may produce small particles identified as face-centred PbO. The relationship between the physical properties and the observed nanostructures is discussed.