The Influence of Strain-Induced Ferroelectricity on the Fracture of Oxide Perovskites

Many materials can be cleaved to produce ideal atomically flat surfaces. However, for many perovskite oxides it was observed that a well-defined concentration of adatoms remains on one of the crack surfaces, mirrored by an equivalent amount of vacancies on the other side, even though this violates the charge neutrality of the created surfaces. In this work, we show for three prototypical oxide perovskites, SrTiO3, BaTiO3, and KTaO3, that these materials develop a large dielectric polarization by ferroelectric displacements of the atoms under the large strain at the crack tip. Upon fracture, the polarization creates a surface charge on the emerging surfaces, which is maintained by the transfer of ions between the crack surfaces and the formation of adatoms and vacancies. By quantifying the strain-induced ferroelectric atomic displacements at the point of fracture using density-functional theory calculations, we find a very good agreement between the surface charge from the evolving polarization and the experimentally observed concentration of adatoms, which are therefore a remnant of the cleaving process. When predicting results of fracture experiments, this strain-induced ferroelectric transition developing during fracture has to be taken into account even for oxide perovskites that are not intrinsically ferroelectric at room temperature.