Perovskite oxides (of formula unit ABO3) are ionic materials with high dielectric constants. Some of them are ferroelectric, i.e. capable of storing a permanent electric polarization and of switching between two polarization states. These properties make them a promising class of materials for electronic applications, as dielectrics in transistors or to store data in ferroelectric memories. As often, defects in the crystalline structure have a major impact on functional properties, as well as on the reliability and life time of components. Moreover, the most commonly used ferroelectric perovskites are based on lead titanate PbTiO3, like lead zirconate titanate Pb(Zr,Ti)O3 or PZT. Although such materials have remarkable properties, the major drawback is that they contain lead, which is toxic for living beings and the environment.
We chose to study potassium niobate KNbO3, a lead-free ferroelectric perovskite material and a good candidate for the replacement of PZT. In particular, we investigated the properties of dislocations in this material, for two reasons: first, to gain knowledge of their atomic structure and their role in the plastic deformation of the material; this point is discussed in the page dedicated to the plastic deformation of perovskites. And second, to understand the interactions between dislocations and the ferroelectric domains.
Potassium niobate KNbO3 or KNO, adopts various crystal symmetries depending on temperature, as summarized in Fig. 1. Above its Curie temperature TC=708 K it looses its ferroelectric properties and becomes cubic. Below, it adopts tetragonal, orthorhombic, and then rhombohedral lattice with decreasing temperature. The direction of the ferroelectric polarization changes from <001> to <011>, and then to <111> respectively.
Fig. 1 - Ferroelectric perovskites usually overcome several phase transformations as a function of temperature. The blue arrow indicates one of the possible directions of the electric polarization P. The ion displacements and lattice distortions are exagerated. The temperatures given here correspond to phase transitions in potassium niobate KNbO3.
Ferroelectrics have several metastable polarizations. For instance in the tetragonal phase the polarization can point in six directions: ±[100], ±[010], and ±[001]. As a result the misorientation between two domains with different polarizations can be 90° or 180°.
[1] P. Hirel et al., Phys. Rev. B 92 (2016) 214101. doi: 10.1103/PhysRevB.92.214101
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