Persistence of strong and switchable ferroelectricity despite vacancies

Abstract

Vacancies play a pivotal role in affecting ferroelectric polarization and switching properties, and there is a possibility that ferroelectricity may be utterly eliminated when defects render the system metallic. However, sufficient quantitative understandings of the subject have been lacking for decades due to the fact that vacancies in ferroelectrics are often charged and polarization in charged systems is not translationally invariant. Here we perform first-principles studies to investigate the influence of vacancies on ferroelectric polarization and polarization switching in prototypical BaTiO3 of tetragonal symmetry. We demonstrate using the modern theory of polarization that, in contrast to common wisdom, defective BaTiO3 with a large concentration of vacancies (or , or ) possesses a strong nonzero electric polarization. Breaking of Ti-O bonds is found to have little effect on the magnitude of polarization, which is striking. Furthermore, a previously unrecognized microscopic mechanism, which is particularly important when vacancies are present, is proposed for polarization switching. The mechanism immediately reveals that (i) the switching barrier in the presence of is small with ΔE = 8.3 meV per bulk formula cell, and the polarization is thus switchable even when vacancies exist; (ii) The local environment of vacancy is surprisingly insignificant in polarization switching. These results provide profound new knowledge and will stimulate more theoretical and experimental interest on defect physics in FEs.

Oxygen Deficiency and Migration-Mediated Electric Polarization in Magnetic Fe,Co-Substituted SrTiO3−δ

“…However, as the Bloch functions used to evaluate the Berry connections are not unique [65][66][67][68], two different sets of periodic functions follow the relation…”

“…This leaves us with a property that can be found from a lattice of polarization, the different components of which along the lattice vectors are expanded by P Q , such that P = P ion + P elec + P Q . ∆P is only determined within such a quantum uncertainty; hence, the process of resolving the polarization requires evaluation at intermediate steps connecting the end structures of interest; this will result in branches of polarization separated by quanta as displayed in Figure 2 [65][66][67][68]. Some of the processes that can be used to generate intermediate linking structures are ionic displacements, defect migration, applying strain, and/or inducing structural changes with pressure or external fields [65,66].…”

Lead-Free Hybrid Material with an Exceptional Dielectric Phase Transition Induced by a Chair-to-Boat Conformation Change of the Organic Cation

“…According to what we know, this might be the first discovery of a solid-state structural phase transition triggered by concrete conformation changes. The invertible switching of dielectric constants between two distinct states discloses its potential as a switchable dielectric material. − Because of the chainlike inorganic framework, 1 exhibits notable semiconducting properties that are confirmed by the positive temperature-dependent conductivity and theoretical analysis of the electronic structure. This study opens up new possibilities of developing electric-ordered materials accompanied by excellent semiconducting behavior. − …”

Oxygen deficiency and migration mediated electric polarization in Fe,Co-substituted SrTiO$_{3-δ}$

“…V is defined as the quantum-of-polarization, R being a lattice vector and V the corresponding volume of the cell. P Q defines a series of branches for the polarization, which result in a lattice-valued quantity as displayed in Figure 2 [44][45][46][47] . ∆P is only determined within such a P Q uncertainty but can be resolved by calculating the polarization at intermediate steps connecting the end-structures e.g., resulting from ionic displacements, defect migration or strain and pressure induced structural changes 44,45 .…”