Multiscale simulations of defect dipole–enhanced electromechanical coupling at dilute defect concentrations

Abstract

The role of defects in solids of mixed ionic-covalent bonds such as ferroelectric oxides is complex. Current understanding of defects on ferroelectric properties at the single-defect level remains mostly at the empirical level, and the detailed atomistic mechanisms for many defect-mediated polarization-switching processes have not been convincingly revealed quantum mechanically. We simulate the polarization-electric field (P-E) and strainelectric field (ε-E) hysteresis loops for BaTiO 3 in the presence of generic defect dipoles with large-scale molecular dynamics and provide a detailed atomistic picture of the defect dipole-enhanced electromechanical coupling. We develop a general first-principles-based atomistic model, enabling a quantitative understanding of the relationship between macroscopic ferroelectric properties and dipolar impurities of different orientations, concentrations, and dipole moments. We find that the collective orientation of dipolar defects relative to the external field is the key microscopic structure feature that strongly affects materials hardening/softening and electromechanical coupling. We show that a small concentration (≈0.1 at.%) of defect dipoles dramatically improves electromechanical response. This offers the opportunity to improve the performance of inexpensive polycrystalline ferroelectric ceramics through defect dipole engineering for a range of applications including piezoelectric sensors, actuators, and transducers.The presence of defects in ferroelectrics can be the unintended consequence of sample impurities or the result of designed materials engineering.1 A dipolar defect can be intentionally introduced by substituting a cation with a dopant of different valence. The charge mismatch between the acceptor dopant and the replaced ion is often compensated by nearby oxygen Unlike defects in metals and semiconductors where the electrostatic interactions are strongly screened, the dipolar impurities in ferroelectrics couple strongly with the bulk ferroelectrics through the dipolar electric field, resulting in long-range, nonlocal elastic fields. 25 These longrange interactions make it challenging to model and predict the effects of defects on mesoscale ferroelectric properties at finite temperatures. The goal of this work is to develop a quantitative Page 3 of 10 theoretical framework that links the atomic structure of defects and macroscopic ferroelectric properties under realistic doping conditions.The P-E and ε-E hysteresis loops contain information on both intrinsic bulk and extrinsic defect-related ferroelectric properties, 26 and decoding intrinsic and extrinsic effects is critical for a complete understanding of defect-mediated polarization switching. We simulated the hysteresis loops in tetragonal BaTiO 3 in the presence of dipolar defects using large-scale molecular dynamics simulations (see Methods) under different defect conditions and electric field orientations. A defect dipole is introduced by placing a pair of particles of opposite-charge (q) equidist...

Effects of quenching on phase transformations and ferroelectric properties of 0.35BCZT-0.65KBT ceramics

“…The occurrence of such pinched, or slim, P-E loops is a result of restriction of ferroelectric domain switching, which might be attributed to several causes [46]. For instance, hindering of domain wall movement can occur due to domain wall pinning by oxygen vacancy-cation defect dipoles [47,48], or to existence of antiferroelectric phases [14,49]. In relaxor ferroelectrics, it could arise as a result of ergodicity phenomena, in which the transformation between PNRs and ordered ferroelectric domains exhibits complete or partial reversibility [12,13].…”

Understanding doped perovskite ferroelectrics with defective dipole model

“…While there is a large amount of literature on exploiting doping effects experimentally, the nature and cause of the observed effects are not fully understood, and several possible factors are proposed to explain experimental results. The effects of doping induced oxygen vacancies and ferroelectric domains, among others, are often believed to play important roles [5][6][7][8][9][10][11][12][13][14] .…”

Charge effects in donor doped perovskite ferroelectrics

“…Having shown the models and the effective Hamiltonian, we proceed to use MC simulations to obtain basic properties of La donor doping BaTiO 3 , including hysteresis loops, polarization, as well as phase transition temperatures, with different charge concentration and configurations. Before showing such results, we firstly certify our numerical scheme, including the Ewald interaction matrix and the newly developed MC program with E chg-dpl ({u} , q), against the defect dipole model [38][39][40] , which is known to gives rise to the pinched hysteresis loop.…”