Formation and atomic structure of the (PbTi)O phase for epitaxial super-tetragonal PbTiO₃

Lattice strain has been used to modify the properties of ferroelectric oxide films. Introducing strain in thin film is usually realized based on the lattice mismatch of the heterostructural materials, which form multilayered systems or composite structure. For the composite structure of PbTiO₃/α-PbO-type (PTO/PO-T) structure phase, the PO-T structure phase plays the key role in introducing a high strain in the PTO phase and resulting in a giant polarization of the PTO phase. Here, we perform a detailed study on the PO-T structure phase on atomic scale by means of high-resolution (scanning) transmission electron microscopy. The atomic details, including cations and oxygen of the PO-T structure phase, are revealed, which show the α-PbO structure with parts of the Pb atoms being substituted by Ti atoms. Local inhomogeneity in Ti atom distribution is found in nanometer scale. Experimental investigations show that the PO-T structure phase is formed on a layer of the PTO phase with a thickness of one or two unit cells, which grows covering the underneath La_0.3Sr_0.7MnO₃ (LSMO) electrode layer. The formation mechanism of the PO-T structure phase is discussed on the basis of the atomic details of the PO-T/PTO/LSMO interfaces and the growth condition of the film.