Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userScale effects and the formation of polarization vortices in tetragonal ferroelectrics
107
0
0.0
(
0
)
Added by
Ananya Renuka Balakrishna
Publication date
2017
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
Vortices consisting of $90^circ$ quadrant domains are rarely observed in ferroelectrics. Although experiments show polarization flux closures with stripe domains, it is as yet unclear why pure single vortices are not commonly observed. Here we model and explore the energy of polarization patterns with vortex and stripe domains, formed on the square cross-section of a barium titanate nanowire. Using phase-field simulations, we calculate the associated energy of polarization patterns as a function of nanowire width. Further, we demonstrate the effects of surface energy and electrical boundary conditions on equilibrium polarization patterns. The minimum energy equilibrium polarization pattern for each combination of surface energy and nanowire width is mapped for both open-circuit and short-circuit boundary conditions. The results indicate a narrow range of conditions where single vortices are energetically favorable: nanowire widths less than about 30nm, open-circuit boundary condition, and surface energy of less than 4N/m. Short-circuit boundary conditions tend to favor the formation of a monodomain, while surface energy greater than 4N/m can lead to the formation of complex domain patterns or loss of ferroelectricity. The length scale at which a polarization vortex is energetically favorable is smaller than the typical size of nanoparticle in recent experimental studies. The present work provides insight into the effects of scaling, surface energy and electrical boundary conditions on the formation of polarization patterns.
rate research
Read More
Using density-functional calculations we study the structure and polarization response of tetragonal PbTiO3, BaTiO3 and SrTiO3 in a strain regime that is previously overlooked. Different from common expectations, we find that the polarizations in all three substances saturate at large strains, demonstrating a universal phenomenon. The saturation is shown to originate from an unusual and strong electron-ion correlation that leads to cancellation between electronic and ionic polarizations. Our results shed new insight on the polarization properties, and reveal the existence of a fundamental limit to the strain-induced polarization enhancement.
Ferroelectrics form domain patterns that minimize their energy subject to imposed boundary conditions. In a linear, constrained theory, that neglects domain wall energy, periodic domain patterns in the form of multi-rank laminates can be identified as minimum-energy states. However, when these laminates (formed in a macroscopic crystal) comprise domains that are a few nanometers in size, the domain-wall energy becomes significant, and the behaviour of laminate patterns at this scale is not known. Here, a phase-field model, which accounts for gradient energy and strain energy contributions, is employed to explore the stability and evolution of the nanoscale multi-rank laminates. The stress, electric field, and domain wall energies in the laminates are computed. The effect of scaling is also discussed. In the absence of external loading, stripe domain patterns are found to be lower energy states than the more complex, multi-rank laminates, which mostly collapse into simpler patterns. However, complex laminates can be stabilized by imposing external loads such as electric field, average strain and polarization. The study provides insight into the domain patterns that may form on a macroscopic single crystal but comprising of nanoscale periodic patterns, and on the effect of external loads on these patterns.
Consecutive stochastic 90{deg} polarization switching events, clearly resolved in recent experiments, are described by a new nucleation and growth multi-step model. It extends the classical Kolmogorov-Avrami-Ishibashi approach and includes possible consecutive 90{deg}- and parallel 180{deg}-switching events. The model predicts the results of simultaneous time-resolved macroscopic measurements of polarization and strain, performed on a tetragonal Pb(Zr,Ti)O3 ceramic in a wide range of electric fields over a time domain of five orders of the magnitude. It allows the determination of the fractions of individual switching processes, their characteristic switching times, activation fields, and respective Avrami indices.
The mechanocaloric effect is the temperature change of a material upon application or removal of an external stress. Beyond its fundamental interest, this caloric response represents a promising and ecofriendly alternative to current cooling technologies. To obtain large mechanocaloric effects, we need materials whose elastic properties (e.g., strain, elastic compliance) are strongly temperature dependent. This is the case of ferroelectric perovskite oxides, where the development of the spontaneous electric polarization is accompanied by significant strains and lattice softening. Thus, in this work we study the mechanocaloric properties of model ferroelectric PbTiO$_{3}$, by means of predictive atomistic (second-principles) simulations and a perturbative formalism here introduced. Our calculations reveal relatively large effects (up to $-$4 K for relatively small applied compressions of $-$0.1 GPa) and several striking features. In particular, we find that the mechanocaloric response is highly anisotropic in the ferroelectric phase, as it can be either conventional (temperature increases upon compression) or inverse (temperature decreases) depending on the direction of the applied stress. We discuss and explain these surprising results, which compare well with existing experimental information. Our analysis suggests that the coexistence of conventional and inverse mechanocaloric responses is probably common among ferroelectrics and materials displaying a negative thermal expansion.
Doping is a widely used method to tune physical properties of ferroelectric perovskites. Since doping can induce charges due to the substitution of certain elements, charge effects shall be considered in doped samples. To understand how charges can affect the system, we incorporate the dipole-charge interaction into our simulations, where the pinched hysteresis loops can well be reproduced. Two charge compensation models are proposed and numerically investigated to understand how lanthanum doping affect BaTiO$_{3}$s ferroelectric phase transition temperature and hysteresis loop. The consequences of the two charge compensation models are compared and discussed.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
