Ferroelectric switching at edge dislocations in BaTiO$_3$ modelled at the atomic scale

TL;DR

The study addresses the lack of an atomistic picture of how line defects, specifically <100> edge dislocations, influence ferroelectric switching in BaTiO3. It employs an isotropic core-shell potential within atomistic simulations to model a pair of dislocations ( Burgers vectors $\vec{b}=\pm[100]$) embedded in a tetragonal BaTiO3 matrix and applies electric fields along the three Cartesian directions, tracking switching via local dipoles $u_k$, macroscopic polarization $\mathbf{P}$, and strain $\epsilon_{ii}$. The results show that dislocation cores can nucleate switching for all field directions, with the strongest coupling when the field is parallel to the Burgers vector; coercive fields shift to $E_c \approx 7$, 6, and 8 MV/cm for $E_x$, $E_y$, and $E_z$ respectively, and remnant polarization $P_r$ remains around $0.39$–$0.43$ C/m$^2$. Additionally, 180° walls can be pinned by the cores due to compressive strain, reducing the overall switchable polarization. These atomistic insights illuminate defect-mediated tuning of ferroelectric switching and offer design guidance for engineering dislocation structures at interfaces or under high-temperature deformation to tailor device performance.

Abstract

Ferroelectric switching governs the functional properties of ferroelectric perovskites. It is widely accepted that this switching depends on domain nucleation and pinning and that these processes can be controlled by the defect structure. However, an atomistic picture of the influence of one important class of defects - dislocations on ferroelectric switching is missing. This is an important gap in knowledge as dislocations cannot be avoided at interfaces and can also be engineered by plastic deformation at high temperatures. Using atomistic simulations, we show how the cores of $\langle100\rangle$ edge dislocations in BaTiO$_3$ can either act as nucleation centers for ferroelectric switching or pin walls depending on the direction of the applied field. The coupling between electric field and polarization is strongest when the field is applied parallel to the Burgers vector of the dislocation.

Figures (9)

• Figure 1: Simulation setup: (a) Starting from a $40\times 80 \times 1$ supercell of cubic BaTiO$_3$, a BaO-TiO$_2$ double layer along $[010]$, i.e. along y, with a length of $d= 87$ Å has been removed. (b) After relaxation, a pair of TiO2-type and BaO-type dislocation lines along [001], i.e. along $z$, with Burger's vectors $b=\pm [100]$ form. (c)-(d) give these cores with atomic resolution.
• Figure 2: Change of the coercive field ($E_c$) of pristine BaTiO$_3$ for bi-axial strain in the x-y plane computed using the core-shell model. $\epsilon=0\%$ and $\epsilon=0.2\%$ correspond to the in-plane lattice parameters of the tetragonal phase and of the cubic phase, respectively. Blue and red curves show the coercive fields for the out-of-plane and in-plane directions. Lines are guides for the eye only.
• Figure 3: (a)--(b) Strain map around the pair of edge dislocations shown in Fig. \ref{['dislocation_setup']} in the tetragonal phase polarized along $x$. The strain along (a) $x$ ($\epsilon_{xx}$) and (b) $y$ ($\epsilon_{yy}$) directions are given relative to the pristine material with tetragonal axis along $x$-direction. Ba atoms are given as spheres and red or blue colour indicate tensile or compressive strains, respectively.
• Figure 4: Field hysteresis for an electric field applied along $z$-direction in the presence of a pair of dislocations with $\vec{b}$ and $\vec{l}$ along $x$ and $z$ directions, respectively. Colors mark the three polarization components and the hysteresis of the pristine material is given in black. (a)--(f) Underlying microscopic processes color encoded by the magnitude of the local polarization with red and blue indicating $P_z$ and $-P_z$, respectively.
• Figure 5: Field hystereses for an electric field applied along $y$ in the presence of a pair of dislocation with $\vec{b}$ and $\vec{l}$ along x and z. Colors mark the three polarization components and the hysteresis of the pristine material is given in black. (a)--(f) Underlying microscopic processes color encoded by the magnitude of the local polarization with red and blue indicating $P_y$ and $-P_y$.