Electric-field-induced structural changes for cubic system of lead-free and lead-based perovskite-type oxides

Changes in crystal structures of lead-free BaTiO₃ (BT) and lead-based 0.70Pb(Mg_1/3Nb_2/3)-0.30PbTiO₃ (PMN-PT) single crystals with a perovskite-type structure have been investigated under applied electric field by analyzing the high-energy synchrotron radiation X-ray diffraction data. The experiment is performed within the cubic phase in the phase diagram with the range of temperatures and electric fields where no electric field-induced phase transition is caused. The crystal structure analysis demonstrates that the crystal system is change to a tetragonal system by electrostriction when the electric field is applied along the crystal axis. The atomic arrangement in BT corresponds to the displacement pattern attributed to the Slater mode where Ti and O ions are mutually shifted in the opposite directions along the electric field, while that in PMN-PT is of the Last mode where Pb ions and (Mg, Nb)O₆ oxygen octahedra are mutually shifted. The electron charge density study demonstrates that the Ba-O bonding in BT is ionic, whereas the Pb-O bonding in PMN-PT is highly covalent in the cubic structure. PMN-PT is revealed to have a characteristic Pb-O covalent bond which induces the atomic displacement attributed to the Last mode under applied electric field within the cubic phase despite no phase transition. (author)