Understanding the influence of defects and surface chemistry on ferroelectric switching: a ReaxFF investigation of BaTiO3

Ferroelectric Phase Transition in Barium Titanate Revisited with Ab Initio Molecular Dynamics

The ferroelectric phase transition of the perovskite barium titanate as well as its technical importance regarding the switching of respective polar properties is well known and has been thoroughly studied, both experimentally and on theoretical grounds. While details about the phase diagram as well as transition temperatures are experimentally well known, the theoretical approaches still face difficulties in contributing a detailed description of these phase transitions. Within this work, a new methodological approach is introduced to revisit the ferroelectric phase transition with first-principles methods. With the chosen ab initio molecular dynamics (AIMD) method in combination with the applied NpT ensemble, we are able to join the accuracy of density functional theory (DFT) with ambient conditions, realized using a thermostat and barostat in an MD simulation. The derived phase diagram confirms recent corrections in the theoretical models and reproduces the phase boundary pressure dependence of TC. In conclusion of the statistical atomistic dynamics, the nature of the transition can be described in a more detailed way. In addition, this work paves the way towards locally patterned piezoelectrica by means of acoustic standing waves as well as piezoelectrically induced acoustic resonators.

Interface-Tuning of Ferroelectricity and Quadruple-Well State in CuInP2S6 via Ferroelectric Oxide

Ferroelectric van der Waals CuInP2S6 possesses intriguing quadruple-well states and negative piezoelectricity. Its technological implementation has been impeded by the relatively low Curie temperature (bulk TC ∼ 42 °C) and the lack of precise domain control. Here we show that CuInP2S6 can be immune to the finite size effect and exhibits enhanced ferroelectricity, piezoelectricity, and polar alignment in the ultrathin limit when it is interfaced with ferroelectric oxide PbZr0.2Ti0.8O3 films. Piezoresponse force microscopy studies reveal that the polar domains in thin CuInP2S6 fully conform to those of the underlying PbZr0.2Ti0.8O3, where the piezoelectric coefficient changes sign and increases sharply with reducing thickness. High temperature in situ domain imaging points to a significantly enhanced TC of >200 °C for 13 nm CuInP2S6 on PbZr0.2Ti0.8O3. Density functional theory modeling and Monte Carlo simulations show that the enhanced polar alignment and TC can be attributed to interface-mediated structure distortion in CuInP2S6. Our study provides an effective material strategy to engineer the polar properties of CuInP2S6 for flexible nanoelectronic, optoelectronic, and mechanical applications.

In silico characterization of nanoparticles

Nanoparticles (NPs) make for intriguing heterogeneous catalysts due to their large active surface area and excellent and often size-dependent catalytic properties that emerge from a multitude of chemically different surface reaction sites. NP catalysts are, in principle, also highly tunable: even small changes to the NP size or surface facet composition, doping with heteroatoms, or changes of the supporting material can significantly alter their physicochemical properties. Because synthesis of size- and shape-controlled NP catalysts is challenging, the ability to computationally predict the most favorable NP structures for a catalytic reaction of interest is an in-demand skill that can help accelerate and streamline the material optimization process. Fundamentally, simulations of NP model systems present unique challenges to computational scientists. Not only must considerable methodological hurdles be overcome in performing calculations with hundreds to thousands of atoms while retaining appropriate accuracy to be able to probe the desired properties. Also, the data generated by simulations of NPs are typically more complex than data from simulations of, for example, single crystal surface models, and therefore often require different data analysis strategies. To this end, the present work aims to review analytical methods and data analysis strategies that have proven useful in extracting thermodynamic trends from NP simulations.

Critical Roles of Impurities and Imperfections in Various Phases of Materials

In many materials, impurities and imperfections play a critical role on the physical and chemical properties. In the present review, some examples of such materials are discussed. A bulk nanobubble (an ultrafine bubble) is stabilized against dissolution by hydrophobic impurities attached to the bubble surface. An acoustic cavitation threshold in various liquids decreases significantly by the presence of impurities such as solid particles, etc. The strength of brittle ceramics is determined by the size and number of pre-existing microcracks (imperfections) in the specimen. The size effect of a BaTiO3 nanocrystal is influenced by the amount and species of adsorbates (impurities) on its surface as adsorbate-induced charge-screening changes the free energy. The dielectric constant of an assembly of BaTiO3 nanocubes is influenced by a small tilt angle (imperfection) between two attached nanocubes, which induces strain inside a nanocube, and is also influenced by the spatial strain-relaxation due to defects and dislocations (imperfections), resulting in flexoelectric polarization.

Oxygen Vacancy Injection as a Pathway to Enhancing Electromechanical Response in Ferroelectrics

Since their discovery in late 1940s, perovskite ferroelectric materials have become one of the central objects of condensed matter physics and materials science due to the broad spectrum of functional behaviors they exhibit, including electro-optical phenomena and strong electromechanical coupling. In such disordered materials, the static properties of defects such as oxygen vacancies are well explored but the dynamic effects are less understood. In this work, the first observation of enhanced electromechanical response in BaTiO₃ thin films is reported driven via dynamic local oxygen vacancy control in piezoresponse force microscopy (PFM). A persistence in peizoelectricity past the bulk Curie temperature and an enhanced electromechanical response due to a created internal electric field that further enhances the intrinsic electrostriction are explicitly demonstrated. The findings are supported by a series of temperature dependent band excitation PFM in ultrahigh vacuum and a combination of modeling techniques including finite element modeling, reactive force field, and density functional theory. This study shows the pivotal role that dynamics of vacancies in complex oxides can play in determining functional properties and thus provides a new route toward- achieving enhanced ferroic response with higher functional temperature windows in ferroelectrics and other ferroic materials.

Interplay of domain structure and phase transitions: theory, experiment and functionality

Domain walls and phase boundaries are fundamental ingredients of ferroelectrics and strongly influence their functional properties. Although both interfaces have been studied for decades, often only a phenomenological macroscopic understanding has been established. The recent developments in experiments and theory allow to address the relevant time and length scales and revisit nucleation, phase propagation and the coupling of domains and phase transitions. This review attempts to specify regularities of domain formation and evolution at ferroelectric transitions and give an overview on unusual polar topological structures that appear as transient states and at the nanoscale. We survey the benefits, validity, and limitations of experimental tools as well as simulation methods to study phase and domain interfaces. We focus on the recent success of these tools in joint scale-bridging studies to solve long lasting puzzles in the field and give an outlook on recent trends in superlattices.

Understanding physical chemistry of BaxSr1-xTiO3 using ReaxFF molecular dynamics simulations

Barium strontium titanate Ba_xSr_1-xTiO₃ (BSTO) has been widely used in nano devices due to its unique ferroelectric properties and can be epitaxially grown on a SrTiO₃ (STO) support, with a reduced lattice and thermal mismatch. In this work, we developed a ReaxFF reactive force field verified against quantum mechanical data to investigate the temperature and composition dependency of BSTO in non-ferroelectric/ferroelectric phases. This potential was also explicitly designed to capture the surface energetics of STO with SrO and TiO₂ terminations. Our molecular dynamics simulations indicate that when the percentage of Sr increases, the phase transition temperature and the polarizations of the Ba_xSr_1-xTiO₃ system decrease monotonically. In addition, as the oxygen vacancy concentration enhances, the initial polarization and the phase transition temperature of the system drop significantly. Furthermore, our simulation results show that charge screening induced by adsorption of water molecules on TiO₂ terminated surfaces leads to an increased initial polarization.

Machine Learning-Assisted Hybrid ReaxFF Simulations

We have developed a machine learning (ML)-assisted Hybrid ReaxFF simulation method ("Hybrid/Reax"), which alternates reactive and non-reactive molecular dynamics simulations with the assistance of ML models to simulate phenomena that require longer time scales and/or larger systems than are typically accessible to ReaxFF. Hybrid/Reax uses a specialized tracking tool during the reactive simulations to further accelerate chemical reactions. Non-reactive simulations are used to equilibrate the system after the reactive simulation stage. ML models are used between reactive and non-reactive stages to predict non-reactive force field parameters of the system based on the updated bond topology. Hybrid/Reax simulation cycles can be continued until the desired chemical reactions are observed. As a case study, this method was used to study the cross-linking of a polyethylene (PE) matrix analogue (decane) with the cross-linking agent dicumyl peroxide (DCP). We were able to run relatively long simulations [>20 million molecular dynamics (MD) steps] on a small test system (4660 atoms) to simulate cross-linking reactions of PE in the presence of DCP. Starting with 80 PE molecules, more than half of them cross-linked by the end of the Hybrid/Reax cycles on a single Xeon processor in under 48 h. This simulation would take approximately 1 month if run with pure ReaxFF MD on the same machine.

Recent Advances for Improving the Accuracy, Transferability, and Efficiency of Reactive Force Fields

Reactive force fields provide an affordable model for simulating chemical reactions at a fraction of the cost of quantum mechanical approaches. However, classically accounting for chemical reactivity often comes at the expense of accuracy and transferability, while computational cost is still large relative to nonreactive force fields. In this Perspective, we summarize recent efforts for improving the performance of reactive force fields in these three areas with a focus on the ReaxFF theoretical model. To improve accuracy, we describe recent reformulations of charge equilibration schemes to overcome unphysical long-range charge transfer, new ReaxFF models that account for explicit electrons, and corrections for energy conservation issues of the ReaxFF model. To enhance transferability we also highlight new advances to include explicit treatment of electrons in the ReaxFF and hybrid nonreactive/reactive simulations that make it possible to model charge transfer, redox chemistry, and large systems such as reverse micelles within the framework of a reactive force field. To address the computational cost, we review recent work in extended Lagrangian schemes and matrix preconditioners for accelerating the charge equilibration method component of ReaxFF and improvements in its software performance in LAMMPS.

Dynamics of the Chemically Driven Densification of Barium Titanate Using Molten Hydroxides

Molten hydroxides, often used for crystal growth and nanoparticle synthesis, have recently been applied for the single step densification of several inorganic materials under moderate uniaxial pressures and 1000 °C below their usual sintering temperatures. The latter approach, termed cold sintering process (CSP), is a mechanochemically driven process that enables the densification of inorganic materials through a dissolution-precipitation creep mechanism. In this study, we report the main densification mechanisms of BaTiO₃ in a NaOH-KOH eutectic mixture. A chemical insight at the atomistic level, investigated by ReaxFF molecular dynamics simulations, offers plausible ionic complex formation scenarios and reactions at the BaTiO₃/molten hydroxide interface, enabling the dissolution-precipitation reactions and the subsequent cold sintering of BaTiO₃.

Polarization Spinodal at Ferroelectric Morphotropic Phase Boundary

Here, we report a new phenomenon of uniform and continuous transformation of a single polarization domain into alternating nanodomains of two polarization vectors with the same magnitude but different directions at ferroelectric morphotropic phase Boundary (MPB). The transformation is fully reversible and could enhance the piezoelectric coefficient d_{33}. Further free energy calculations illustrate that such a polarization "decomposition" process occurs within the region on the Landau free energy curve with respect to the polarization direction where the second derivative becomes negative, which is similar to spinodal instability in phase transformations such as spinodal ordering or isostructural phase separation (e.g., spinodal decomposition). This "polarization spinodal" uncovers a new mechanism of polarization switching that may account for the ultrahigh ahysterestic piezoelectric strain at the MPB. This work could shed light on the development of phase transition theory and the design of novel ferroelectric memory materials.

DFT calculation of hydrothermal mechanism on preparation of MoS2

Basing on the simplest hydrothermal system containing deionized water, hexa-ammonium molybdate, and thiourea, hydrothermal mechanism on preparation of MoS₂ was studied by DFT calculation. Hydrothermal process was divided into four steps which covered ionization equilibrium, the hydrolysis of CS(NH₂)₂, the formation of intermediates, and the formation of MoS₂. Ionization equilibrium occurs at normal condition and determines the existence of Mo in the form of molybdic acid. Thiourea hydrolysis is rate-determining step in the process of hydrothermal which contains 10 elementary reactions. The formation of intermediates includes hydrogen transfer, dehydration, and vulcanization three steps which contain 18 elementary reactions, and the energy barrier of vulcanization is the highest. The formation of MoS₂ is divided into two steps, the first step is that MoO(OH)(SH)₃.H₂O reacts with MoO (SH)₄.H₂O to form layer MoS₂, and the second step is a very fast process that can affect the morphology of the products.

Water distillation modeling by disjoint CNT-based channels under the influence of external electric fields

Using molecular dynamics method, the ion rejection and water flow inside flexible disjoint carbon-based channels were examined in the presence of electric fields. The effects of the carbon nanotube diameters and field magnitude on the nano-channel efficiency were investigated. It was observed that water flow through the filter was modified by increasing the radius of nanotubes, while the salt rejection was reduced. The particles' behaviors inside the channel were described in view of Van der Waals interactions between the water molecules, ions, and carbon atoms. Furthermore, the results indicated that the ion rejection and water flow were increased under the application of proper magnitude of electric fields. Graphical abstract Using MD simulation method, a disjoint CNT-based filter was designed to produce freshwater from a NaCl solution by the aid of external electric field. It was observed that the filter operation was significantly affected by channel structural parameters and amount of applied electric fields.

Full-Scale Ab Initio Simulation of Magic-Angle-Spinning Dynamic Nuclear Polarization

Theoretical models aimed at describing magic-angle-spinning (MAS) dynamic nuclear polarization (DNP) NMR have great potential in facilitating the in silico design of DNP polarizing agents and formulations. These models must typically face a trade-off between the accuracy of a strict quantum mechanical description and the need for using realistically large spin systems, for instance, using phenomenological models. Here, we show that the use of aggressive state-space restrictions and an optimization strategy allows full-scale ab initio MAS-DNP simulations of spin systems containing thousands of nuclei. Our simulations are shown to reproduce experimental DNP enhancements quantitatively, including their MAS rate dependence, for both frozen solutions and solid materials. They also reveal the importance of a previously unrecognized structural feature found in some polarizing agents that helps minimize the sensitivity losses imposed by the spin diffusion barrier.

Atomistic investigation on effect of Ca doping ratio on mechanical behaviors of nanocrystalline Mg-Ca alloys

The effects of doping ratio of calcium (Ca) on mechanical behaviors are investigated using molecular dynamics (MD) and the second nearest-neighbor modified embedded-atom method (2NN-MEAM) formalism for nanocrystalline (NC) Mg-Ca alloys system. Research results indicate that mechanical behaviors of Mg-Ca alloys are independent of lower strain rate (under 1.0 × 10⁹ s^-1). In addition, we observe that Ca doping can affect the mechanical properties of the Mg-Ca alloys, and the optimal 2.0 at% of Ca atoms, which has excellent plasticity, is revealed. When the doping ratio is lower than critical atomic percent (CAT) of Mg₂Ca, Young's modulus and yield stress decrease increasing at% of substitutional Ca. The pyramidal <c + a > dislocations are observed frequently at more active grain boundary (GB) with higher Ca doping ratios. In contrast, with doping ratio above CAT, Mg₂Ca reinforcement dominates brittleness Mg/Mg₂Ca nanocomposites to obtain high strength. By calculating, a significant increase of strength is discovered when at% of Mg₂Ca is above 18.85 (5.34 at% Ca). Intergranular fractures are more likely to nucleate and propagate along weaker Mg/Mg₂Ca interfaces. These results are instrumental in design and improving the mechanical properties of Mg-Ca alloys.

Understanding the anticorrosive mechanism of a cross-linked supramolecular polymer for mild steel in the condensate water: comprehensive experimental, molecular docking, and molecular dynamics investigations

A supramolecular polymer (CDP-DA) was prepared through the crosslinking reaction among the assembled complexes (CDDA) based on β-cyclodextrin (β-CyD) and octadecylamine (ODA). The structural properties of CDP-DA were clarified by experimental techniques such as Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscope, and thermal analysis. Based on the results of molecular docking, the crosslinking unit, CDDA, in the CDP-DA structure favors to exhibit the configuration that narrow rims of two host compounds (β-CyD) are opposite to each other leaving the amino group of ODA outside the host cavity. The corrosion inhibition performance of CDP-DA for mild steel in industrial condensate water was evaluated by electrochemical measurements and surface analyses, the mechanism of which was disclosed by molecular dynamics simulations in the aspects of adsorption equilibrium and ions diffusion models. The results of electrochemical tests indicate that CDP-DA effectively retards the anodic and cathodic reactions and improves the interfacial charge transfer resistance of mild steel in condensate water, which can be categorized as the mixed-type inhibitor. Surface analyses reveal that CDP-DA adsorbs on the steel surface in the integral form showing a monolayer nature, which is consolidated by molecular dynamics simulations. The diffusion behavior of in situ ions in the adsorbed layer is prominently suppressed as compared with those in bulk solution. The robust barrier layer and the mitigated diffusion of ions may contribute to the effective inhibition for CDP-DA against steel deterioration in the condensate water. Anticorrosive mechanism of a cross-linked supramolecular polymer for mild steel in the condensate water.

Molecular dynamics simulation of reversible electroporation with Martini force field

Background

After the discovery of membrane-reversible electroporation decades ago, the procedure has been used extensively in biology, biotechnology and medicine. The research on the basic mechanism has increasingly attracted attention. Although most research has focused on models that consider all atomic and molecular interactions and much atomic-level information can be obtained, the huge computational demand limits the models to simulations of only a few nanometers on the spatial scale and a few nanoseconds on the time scale. In order to more comprehensively study the reversible electroporation mechanism of phospholipid membrane on the nanoscale and at longer time intervals of up to 100 ns, we developed a dipalmitoylphosphatidylcholine (DPPC) phospholipid membrane model with the coarse-grained Martini force field. The model was tested by separately examining the morphology of the phospholipid membrane, the hydrophilic channel size, the distribution of the voltage potential on both sides of the membrane, and the movement of water molecules and ions during electroporation.

Results

The results showed that the process went through several stages: (1) the formation of the pore with defects originating on the surface. (2) The maintenance of the pore. The defects expanded to large pores and the size remains unchanged for several nanoseconds. (3) Pore healing stage due to self-assembly. Phospholipid membrane shrunk and the pore size decreased until completely closed. The pores were not circular in cross-section for most of the time and the potential difference across the membrane decreased dramatically after the pores formed, with almost no restoration of membrane integrity even when the pores started to close.

Conclusions

The mechanism of the reversible electroporation process on the nanoscale level, including defects, expansion, stability, and pore closing stages on a longer time scale of up to 100 ns was demonstrated more comprehensively with the coarse-grained Martini force field, which took both the necessary molecular information and the calculation efficiency into account.

The thermal-mechanical properties of functionally graded membrane electrode assembly of PEMFC

Proton exchange membrane fuel cell (PEMFC) is one of the most promising clean energy technologies in the future because of its advantages of having zero pollution and high-power generation efficiency. However, the commercialization of PEMFC is difficult because of the constraints of operational lifetime and cost. Membrane electrode assembly (MEA) is the core component, and its durability determines the performance and life of PEMFC. Owing to the different expansion properties of each layer in MEA, stress concentration and uneven distribution are easily occurred in the process of dynamic cycling of PEMFC, causing the electrode crack and delamination and highly dropping the cell performance. We established the sandwich molecular model of functionally graded membrane electrode assembly (FG-MEA) and investigated the coefficient of thermal expansion and elastic modulus by molecular dynamics simulation. The relationship between gradient structure of FG-MEA and thermomechanical properties was discussed. Three FG-MEA models were established by adding different volume fraction of platinum (Pt) particles along the thickness direction of the membrane. It was found that with the decrease of gradient value, the coefficient of volumetric thermal expansion decreases and elastic modulus along gradient direction slightly increases. The results were in agreement with the estimation of empirical formula of exponential function. The research provides an idea and theoretical reference for the design of FG-MEA materials.