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The bulk photovoltaic effect observed in ferroelectric materials can enable photovoltaic performance beyond the Shockley–Queisser limit of efficiency. This requires the use of ferroelectrics with strong polarization and low bandgap (Eg) that are typically contradictory in the common perovskite oxides ferroelectrics. Here, we use first-principles calculations to study the KNbO3–K(Ti0.5Mo0.5)O3 (KNTM) solid solutions as possible narrow-gap ferroelectric materials. KTM, the end-member of the recently discovered KNTM solid solution system, maintains a ferroelectric polarization similar to that of other K-based systems due to its d0 configuration at the B-site. The substitution of Nb in KTM reduces Eg from 2.9 of KTM to 1.83 eV for an unstrained system and 1.7 eV for a compressively strained system, while maintaining ferroelectricity. The combination of narrow Eg, strong ferroelectricity, low toxicity, and abundance of the constituent elements make Nb-substituted KTM a promising candidate material for photoferroelectric applications. [ABSTRACT FROM AUTHOR]

Ferroelectric perovskite solid solutions are of interest due to their extensive use in modern electronic devices. Cation off-centering is the dominant mechanism of ferroelectricity in perovskite oxides, and it was shown that the average off-centering of these cations can be used to predict some of the essential properties of solid solutions. In this work, we use first-principles density functional theory to investigate the dependence of the cation displacements on the ionic size, amount of substitution, O6 tilt, and locations of the Bi and Me3+ cations in xBiMe3+O3–(1 − x)PbTiO3 (Me3+ = Ga, Sc, In) solid solutions. We carry out our calculations for the x = 0.125 and x = 0.25 BiMe3+O3 substitution concentrations and the ⟨100⟩, ⟨110⟩, ⟨111⟩, ⟨011⟩, and ⟨001⟩ arrangements of the BiMe3+O3 substituent units. We demonstrate that the substitution of larger ions leads to greater variation in the energy and cation displacement magnitudes of the different cation arrangements. Our study reveals that cation displacements are governed by the interplay of the volume expansion effect that favors higher displacements and the cooperative O6 tilt effect that decreases the displacements. Both of these effects increase with greater ionic radius and their relative strengths depend on the cation arrangement. We also illustrate how negative pressure can be achieved experimentally by the doping of large In cations in these solid solutions. Understanding the dependence of the different directional arrangements, O6 tilting, and the effect of ionic size is important for precise prediction of ferroelectric materials properties and enables rational design of new piezoelectric materials. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were carried out for different structural models of the Si/3C-SiC interface using the Tersoff SiC potential that can model both Si and SiC. We find that the bonding at the Si/3C-SiC interface has a strong effect on the crystallization of the Si phase and that a degree of intermixing is present between the two materials with some C atoms migrating from the 3C-SiC (hereinafter referred to as SiC) into the Si region. The degree of intermixing is likely to exhibit a strong dependence on the temperature and most likely also increases with time, which would lead to changes in the Si/SiC interface during the life of the Si/SiC composite. The inter-mixing also creates disorder and defects of threefold and fivefold bonded atoms in the vicinity of the interfaces. In particular, {111} 1 2 ⟨ 110 ⟩ misfit dislocations were formed at all three types of interfaces [(100), (110), and (111)] in order to relieve the local stress due to lattice mismatch. Additionally, the Si(110)/SiC(110) and Si(111)/SiC(111) interfaces prepared at higher temperatures show the formation of the {111} 1 6 ⟨ 112 ⟩ partial dislocation which arises due to intrinsic stacking faults. We find that the bonding at the crystalline(c) c-Si/SiC interface is weaker than that in bulk crystalline Si, whereas bonding at the amorphous(a)-Si/SiC interface is stronger than that in amorphous Si. Therefore, the rupture in the yield stress occurs at the vicinity of the Si/SiC interface and in the Si region for the a-Si/SiC systems, respectively. Finally, for both bulk and Si/SiC interface systems, a strong variation of the yield strength with temperature was observed. [ABSTRACT FROM AUTHOR]

When producing stable electrodes, polymeric binders are highly functional materials that are effective in dispersing lithium-based oxides such as Li4Ti5O12 (LTO) and carbon-based materials and establishing the conductivity of the multiphase composites. Nowadays, binders such as polyvinylidene fluoride (PVDF) are used, requiring dedicated recycling strategies due to their low biodegradability and use of toxic solvents to dissolve it. Better structuring of the carbon layers and a low amount of binder could reduce the number of inactive materials in the electrode. In this study, we use computational and experimental methods to explore the use of the poly amino acid poly-L-lysine (PLL) as a novel biodegradable binder that is placed directly between nanostructured LTO and reduced graphene oxide. Density functional theory (DFT) calculations allowed us to determine that the (111) surface is the most stable LTO surface exposed to lysine. We performed Kubo–Greenwood electrical conductivity (KGEC) calculations to determine the electrical conductivity values for the hybrid LTO–lysine–rGO system. We found that the presence of the lysine-based binder at the interface increased the conductivity of the interface by four-fold relative to LTO–rGO in a lysine monolayer configuration, while two-stack lysine molecules resulted in 0.3-fold (in the plane orientation) and 0.26-fold (out of plane orientation) increases. These outcomes suggest that monolayers of lysine would specifically favor the conductivity. Experimentally, the assembly of graphene oxide on poly-L-lysine-TiO2 with sputter-deposited titania as a smooth and hydrophilic model substrate was investigated using a layer-by-layer (LBL) approach to realize the required composite morphology. Characterization techniques such as X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), scanning electron microscopy (SEM) were used to characterize the formed layers. Our experimental results show that thin layers of rGO were assembled on the TiO2 using PLL. Furthermore, the PLL adsorbates decrease the work function difference between the rGO- and the non-rGO-coated surface and increased the specific discharge capacity of the LTO–rGO composite material. Further experimental studies are necessary to determine the influence of the PLL for aspects such as the solid electrolyte interface, dendrite formation, and crack formation. [ABSTRACT FROM AUTHOR]

In the last decade there has been a resurgence of activity in the field of photoferroelectrics with a focus on the studies of oxides. Ferroelectric (FE) materials have a spontaneous switchable polarization that can lead to interesting light‐matter interactions such as the bulk photovoltaic effect. While a wide range of compositions has been explored for ferroelectrics, until recently, FE oxides were considered to be capable of absorbing UV light only. Playing a key role in the advance of the field, first‐principles calculations have guided the experimental materials discovery efforts that have identified a wide variety of visible‐light‐absorbing oxides. First‐principles research efforts have also achieved considerable progress in revealing the interplay between the composition, structure and light absorption properties in these materials, and it is now understood that even small changes in the local environment can lead to large changes in band gap character and magnitude. Here, we survey the first‐principles efforts following different band‐gap engineering strategies and the advances in the computational methods that have been driven by these studies. [ABSTRACT FROM AUTHOR]

The ternary perovskite system PbTiO3-Bi(Zn1/2Ti1/2)O3-Bi(Mg1/2Ti1/2)O3 was investigated in an attempt to capture the enhanced tetragonality of the PbTiO3-Bi(Zn1/2Ti1/2)O3 binary system at a morphotropic phase boundary (MPB) to improve piezoelectric performance. Results from the (1-x)[0.75 PbTiO3-0.25Bi(Zn1/2Ti1/2)O3]-(x)Bi(Mg1/2Ti1/2)O3 pseudo-binary solution indicate the formation of an MPB-type phase boundary with a high TC (585 °C at x = 0.55). Multiple dielectric transitions were observed in a region of the ternary diagram characterized by high occupancy of the A-site by Bi3+ (>40%) and the B-site by ferroelectrically active cations (>70%). Temperature-dependent diffraction studies revealed the persistence of a tetragonal structure in at least a subset of the crystal to the higher temperature dielectric transition. [ABSTRACT FROM AUTHOR]

Using data obtained from first-principles calculations, we show that the position of the morphotropic phase boundary (MPB) and transition temperature at MPB in ferroelectric perovskite solutions can be predicted with quantitative accuracy from the properties of the constituent cations. We find that the mole fraction of PbTiO3 at MPB in Pb(B′B″)O3–PbTiO3, BiBO3–PbTiO3, and Bi(B′B″)O3–PbTiO3 exhibits a linear dependence on the ionic size (tolerance factor) and the ionic displacements of the B cations as found by density-functional-theory calculations. This dependence is due to competition between the local repulsion and A-cation displacement alignment interactions. Inclusion of first-principles displacement data also allows accurate prediction of transition temperatures at the MPB. The obtained structure-property correlations are used to predict morphotropic phase boundaries and transition temperatures in as yet unsynthesized solid solutions. [ABSTRACT FROM AUTHOR]

CO adsorption on the Pt(111) surface is studied using first-principles methods. As found in a recent study [Feibelman et al., J. Phys. Chem. B 105, 4018 (2001)], we find the preferred adsorption site within density functional theory to be the hollow site, whereas experimentally it is found that the top site is preferred. The influence of pseudopotential and exchange-correlation functional error on the CO binding energy and site preference is carefully investigated. We also compare the site preference energy of CO on Pt(111) with the reaction energy of formaldehyde formation from H[sub 2] and CO. We show that the discrepancies between the experimental and theoretical results are due to the generalized gradient approximation (GGA) treating different bond orders with varying accuracy. We can therefore expect that GGA results will contain significant error whenever bonds of different bond order are broken and formed. [ABSTRACT FROM AUTHOR]

Further improvement of the power conversion efficiencies of conventional perovskite ferroelectric oxides has been strongly impeded by their wide band gaps. Here, we use several band gap engineering strategies to design low band gap ferroelectric materials from first principles. We show that polarization rotation is useful for reducing the band gaps of strongly distorted perovskites. A variety of visible-light ferroelectric solid solutions are designed by combining Zn substitution into KNbO3with polarization rotation. Alternatively, the band gaps can be reduced by the introduction of low-lying intermediate bands through Bi5+substitution. With this strategy, two Bi5+-containing visible-light ferroelectric solid solutions are designed, which exhibit comparable bulk photocurrent to that of prototypical ferroelectric oxides, but with lower photon energies, as evidenced by the shift current calculations. [ABSTRACT FROM PUBLISHER]

The article focuses on a research on ferroelectric nature of tetragonal metal–organic hybrid perovskites by Y. Rakita and colleagues. Topics discussed include organometal halide perovskites as visible-light sensitizers for photovoltaic cells; role of bulk PV effect on photovoltaic (PV) performance of hybrid perovskites; and use the ferroelectric polarization.

Relaxor ferroelectrics have been a focus of intense attention due to their fascinating physical properties. Their diffuse phase transitions have been explained by the polar nanoregion model. Nevertheless, fundamental characterization of structure and dynamics in relaxors is still a long-standing challenge. Better scientific understanding of the microscopic origins of relaxor behavior is also required to improve efficiencies of relaxor based devices. Our molecular dynamics studies in 0.75PbMg1/3Nb2/3O3-0.25PbTiO3showed good agreement with experimental data and revealed conflicts with the current polar nanoregion model. Here, we review our work and propose an alternate model for structure and dynamics in the relaxor phase. [ABSTRACT FROM PUBLISHER]

We propose B-site Bi5+-doped ferroelectric perovskite materials as suitable candidates for the bulk photovoltaic effect and related solar applications. The low-lying 6s empty states of the electronegative Bi atom produce empty bands in the energy gap of the parent materials, effectively lowering the band gap by 1-2 eV, depending on the composition of the ferroelectric end member and the concentration of Bi5+ in the solid solution. The polarization decreases but survives upon doping, which enables the "shift-current" mechanism for photocurrent generation, while the decreased band gap allows absorption of much of the visible spectrum. The magnitude of shift-current response is calculated for 0.75Pb2InNbO6-0.25Ba2InBiO6 (PIN-BIB) and 0.75Pb2ScNbO6-0.25Ba2ScNbO6 (PSN-BSB) and is predicted to exceed the visible-light bulk photovoltaic response of all previously reported materials, including BiFeO3. Furthermore, the existence of their intermediate bands and multiple band gaps, combined with Fermi-level tuning by A-site co-doping, also allows for their potential application in traditional p - n junction-based solar cells as broad-spectrum photoabsorbers. [ABSTRACT FROM AUTHOR]

We demonstrate a new band engineering strategy for the design of semiconductor perovskite ferroelectrics for photovoltaic and other applications from first principles. We study six ferroelectric solid solutions created by partially substituting Zn2+ for Nb5+ into the parent KNbO3 material, combined with charge compensation at the A sites with different combinations of higher valence cations. Our first-principles calculations with the HSE06 functional yield a low band gap of only 2.1 eV for the 75%KNbO3-25%(Sr1/2La1/2)(Zn1/2Nb1/2)O3 solid solution, and this can be lowered further by 0.6 eV under strain through polarization rotation. The large polarization, especially under strain, of these materials provides a charge separation route by the bulk photovoltaic effect that could potentially allow power conversion efficiency beyond the Shockley-Queisser limit. This band engineering strategy is applicable to other perovskites and should be realizable by standard solid-state synthesis and thin film growth methods. [ABSTRACT FROM AUTHOR]

We propose a strategy to engineer the band gaps of perovskite oxide ferroelectrics, supported by first principles calculations. We find that the band gaps of perovskites can be substantially reduced by as much as 1.2 eV through local rhombohedral-to-tetragonal structural transition. Furthermore, the strong polarization of the rhombohedral perovskite is largely preserved by its tetragonal counterpart. The B-cation off-center displacements and the resulting enhancement of the antibonding character in the conduction band give rise to the wider band gaps of the rhombohedral perovskites. The correlation between the structure, polarization orientation, and electronic structure lays a good foundation for understanding the physics of more complex perovskite solid solutions and provides a route for the design of photovoltaic perovskite ferroelectrics. [ABSTRACT FROM AUTHOR]

The piezoelectric properties of tetragonal perovskites PbTiO3, BaTiO3, and KNbO3 under uniaxial strain along the [001] direction are calculated from first principles. In order to study the piezoelectric response of individual atomic sublattices, the total polarization P and piezoelectric constant d33 are decomposed into the contributions of A-site and B-site atoms. These three materials show different behaviors at the atomic scale. In PbTiO3, the Pb and Ti contribute almost equally to the polarization and d33. In KNbO3, most of the polarization and d33 are contributed by the ferroelectrically active Nb. But in BaTiO3, as the c axis increases, the contribution of Ba to d33 rises rapidly, reaching 50% of the total d33 at c = 4.30 Å. This means that the covalent bonding between Ba and O is significant and has an important impact on the piezoelectric response of BaTiO3. [ABSTRACT FROM AUTHOR]

The motion of ferroelectric domain walls (DWs) is critical for various applications of ferroelectric materials. One important question that is of interest both scientifically and technologically, is whether the ferroelectric DW has significant inertial response. To address this problem, we performed canonical ensemble molecular dynamics simulations of 180° and 90° DWs under applied electric fields. Examination of the evolution of the polarization and local structure of DWs reveals that they stop moving immediately after the removal of electric field. Thus, our computational study shows that ferroelectric DWs do not have significant intrinsic inertial response. [ABSTRACT FROM AUTHOR]

Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices, and for the coupling of light absorption with other functional properties. In these materials, the strong inversion symmetry breaking that is due to spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the bandgap, which may enable efficiencies beyond the maximum possible in a conventional p-n junction solar cell. Ferroelectric oxides are also stable in a wide range of mechanical, chemical and thermal conditions and can be fabricated using low-cost methods such as sol-gel thin-film deposition and sputtering. Recent work has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and ferroelectric-electrode interfaces can greatly increase the current harvested from ferroelectric absorber materials, increasing the power conversion efficiency from about 10−4 to about 0.5 per cent. Further improvements in photovoltaic efficiency have been inhibited by the wide bandgaps (2.7-4 electronvolts) of ferroelectric oxides, which allow the use of only 8-20 per cent of the solar spectrum. Here we describe a family of single-phase solid oxide solutions made from low-cost and non-toxic elements using conventional solid-state methods: [KNbO3]1 − x[BaNi1/2Nb1/2O3 − δ]x (KBNNO). These oxides exhibit both ferroelectricity and a wide variation of direct bandgaps in the range 1.1-3.8 electronvolts. In particular, the x = 0.1 composition is polar at room temperature, has a direct bandgap of 1.39 electronvolts and has a photocurrent density approximately 50 times larger than that of the classic ferroelectric (Pb,La)(Zr,Ti)O3 material. The ability of KBNNO to absorb three to six times more solar energy than the current ferroelectric materials suggests a route to viable ferroelectric semiconductor-based cells for solar energy conversion and other applications. [ABSTRACT FROM AUTHOR]

We present a modified bond-valence model of PbTiO3 based on the principles of bond-valence and bond-valence vector conservation. The relationship between the bond-valence model and the bond-order potential is derived analytically in the framework of a tight-binding model. An energy term, bond-valence vector energy, is introduced into the atomistic model and the potential parameters are reoptimized. This model potential can be applied both to canonical-ensemble (NVT) and isobaric-isothermal ensemble (NPT) molecular dynamics (MD) simulations. This model reproduces the experimental phase transition in NVT MD simulations and also exhibits the experimental sequence of temperature-driven and pressure-driven phase transitions in NPT simulations. We expect that this improved bond-valence model can be applied to a broad range of inorganic materials. [ABSTRACT FROM AUTHOR]

Relaxor ferroelectrics have been a focus of intense attention due to their anomalous properties, and understanding the structure and dynamics of relaxors has been one of the long-standing challenges in solid-state physics. We investigate the local structure and dynamics in 75%PbMg1/3Nb2/3O3-25%PbTiO3 using molecular dynamics simulations and the dynamic pair distribution function technique. We show that relaxor transitions can be described by local order parameters. The relaxor phase is characterized by the presence of highly anisotropic correlations between the local cation displacements that resemble the hydrogen bond network in water. This contradicts the current model of polar nanoregion inside a nonpolar matrix. We therefore suggest a new model of a homogeneous random network of anisotropically coupled dipoles. [ABSTRACT FROM AUTHOR]

Stable ferroelectric (FE) phases in nanometer-thick films would enable ultra-high density and fast FE field effect transistors (FelTiTs), and the stability of ferroelectricity in ultrathin films has been under intense theoretical and experimental investigation. Here we predict, using density functional theory calculations, that the low-energy epitaxial PbTiO3 (001)/Pt interface strengthens the electrode-oxide bonds by breaking in-plane symmetry and stabilizes a ground state with enhanced polarization in subnanometer oxide films, with no critical-size limit. Additionally, we show that such enhancement is related to large work function differences between the P- and P+ PbTiO3 surfaces, which gives rise to a net polarizing field in the oxide. [ABSTRACT FROM AUTHOR]

We use density functional theory calculations to study the lattice vibrations and electronic properties of the correlated metal LaNiO3. To characterize the rhombohedral-to-cubic structural phase transition of perovskite LaNiO3, we examine the evolution of the Raman-active phonon modes with temperature. We find that the A1g Raman mode, whose frequency is sensitive to the electronic band structure, is a useful signature to characterize the octahedral rotations in rhombohedral LaNiO3. We also study the importance of electron-electron correlation effects on the electronic structure with two approaches that go beyond the conventional band theory [local spin density approximation (LSDA)]: the local spin density + Hubbard U method (LSDA + U) and hybrid exchangecorrelation density functionals that include portions of exact Fock exchange. We find that the conventional LSDA accurately reproduces the delocalized nature of the valence states in LaNiO3 and gives the best agreement with the available experimental data on the electronic structure of LaNiO3. Based on our calculations, we show that the electronic screening effect from the delocalized Ni 3d and O-2p states mitigates the electronic correlations of the d7 Ni cations, making LaNiO3 a weakly correlated metal. [ABSTRACT FROM AUTHOR]

We examine band-structure engineering in extremely tetragonal ferroelectric perovskites (ABO3) to make these materials suitable for photovoltaic applications. Using first-principles calculations, we study how B-site ordering, lattice strain, cation identity, and oxygen octahedral cage tilts affect the energies and the compositions of the valence and conduction bands. We find that extreme tetragonality makes the band gap highly sensitive to the B-cation ordering, with a layered B-site arrangement exhibiting a small band gap. It also leads to a strong sensitivity of the band gap to the oxygen octahedral tilting. These effects only occur for cations with filled d states located near the valence-band maximum or empty d states at the conduction-band minimum; this criterion is explained by crystal field theory. In addition to a smaller band gap, the layered B-site ordering has a strong impact on the carrier mobility. We find that the excited electron effective mass is similar to that found in Si and other classic semiconductors. Moreover, the hole effective mass is strongly anisotropic, indicating a two-dimensional hole gas in the layered B-cation ordering. [ABSTRACT FROM AUTHOR]

The motion of domain walls is critical to many applications involving ferroelectric materials, such as fast high-density non-volatile random access memory. In memories of this sort, storing a data bit means increasing the size of one polar region at the expense of another, and hence the movement of a domain wall separating these regions. Experimental measurements of domain growth rates in the well-established ferroelectrics PbTiO3 and BaTiO3 have been performed, but the development of new materials has been hampered by a lack of microscopic understanding of how domain walls move. Despite some success in interpreting domain-wall motion in terms of classical nucleation and growth models, these models were formulated without insight from first-principles-based calculations, and they portray a picture of a large, triangular nucleus that leads to unrealistically large depolarization and nucleation energies. Here we use atomistic molecular dynamics and coarse-grained Monte Carlo simulations to analyse these processes, and demonstrate that the prevailing models are incorrect. Our multi-scale simulations reproduce experimental domain growth rates in PbTiO3 and reveal small, square critical nuclei with a diffuse interface. A simple analytic model is also proposed, relating bulk polarization and gradient energies to wall nucleation and growth, and thus rationalizing all experimental rate measurements in PbTiO3 and BaTiO3. [ABSTRACT FROM AUTHOR]

Bromine complexing agents (BCAs) are seen as a promising route to mitigate the potential health and environmental risks of the bromine-based redox-flow batteries, like the hydrogen bromine redox flow battery (H 2 –Br 2 RFB). The most studied BCAs are based on the pyridinium anion, which may adsorb and inhibit the Pt catalyst required in the H 2 –Br 2 RFB system for the hydrogen reactions. Herein the effect of two BCAs (ethyl-pyridinium bromide and hexyl-pyridinium bromide) on a Pt electrocatalyst are studied, along with a potential methodology to prevent adsorption of the BCA through a polydopamine (PDA) coating. The results show that the pyridinium anion is adsorbed on Pt throughout a large potential range (−0.02 to 1.0 V), reducing the availability of the surface for the adsorption of other species. The PDA coating prevented this adsorption, but itself experiences adsorption of the BCA leading to some reduction in its porosity. • Pt electrocatalysts are strongly affected by common bromine complexing agents. • H adsorption and HOR/HER kinetics are reduced due to aromatic ring adsorption. • Polydopamine coatings can be applied to Pt electrocatalyst without damaging HOR/HER. • PDA coatings are effective at preventing bromine complexing agent adsorption. [ABSTRACT FROM AUTHOR]

Using density functional theory calculations, we investigate the feasibility of using silver on the perovskite A-site in high-performance lead-free piezoelectrics. The calculations show that silver atoms are ferroelectrically active and can off-center by 0.5 Å. To induce Ag ferroelctric behavior absent in pure AgNbO3 we explore solid solutions of AgNbO3 with well-known simple perovskites PbTiO3, BaZrO3, and BaTiO3. For the AgNbO3–PbTiO3 solid solution, we predict a morphotropic phase boundary near (AgNbO3)3/8(PbTiO3)5/8. We find an unusual sequence of compositional phase transitions in AgNbO3–BaZrO3 and AgNbO3–BaTiO3 which are due to the effects of volume expansion in perovskites. For the AgNbO3–BaTiO3 solid solution we find two morphotropic phase boundaries at 12.5% and 37.5% AgNbO3 compositions with favorable piezoeletric properties and indications of high TC. [ABSTRACT FROM AUTHOR]

The Pb(Zr, Ti)O[sub 3] (PZT) disordered solid solution is widely used in piezoelectric applications owing to its excellent electromechanical properties. Six different structural phases have been observed for PZT at ambient pressure, each with different lattice parameters and average electric polarization. It is of significant interest to understand the microscopic origin of the complicated phase diagram and local structure of PZT. Here, using density functional theory calculations, we show that the distortions of the material away from the parent perovskite structure can be predicted from the local arrangement of the Zr and Ti cations. We use the chemical rules obtained from density functional theory to create a phenomenological model to simulate PZT structures. We demonstrate how changes in the Zr/Ti composition give rise to phase transitions in PZT through changes in the populations of various local Pb atom environments. [ABSTRACT FROM AUTHOR]

• The paper discusses the significant challenge in analytical chemistry of determining chirality in solid materials using electron paramagnetic resonance (EPR) spectroscopy. • The paper introduces a novel approach using electron paramagnetic resonance (EPR) spectroscopy on chiral crystals doped with Cu2+ ions to determine chirality of crystals. • The method is demonstrated using l - and DL-Histidine crystals doped with Cu2+, showing promising results in accurately determining chirality and enantiomeric composition. • This work provides an initial example of this technique and envisions its potential application to various other chiral organic crystal systems in the future. One of the most challenging tasks in analytical chemistry is the determination of the chirality (identification of an enantiomeric composition) in solids mainly because of the strict requirements of the pharmaceutical industry for enantiomerically pure drugs. Although there are a few methods available to accomplish enantio‑differentiation in solids, for example: X-ray diffraction (XRD), Thermogravimetric analysis (TGA), CD spectroscopy, and low-frequency (LF) Raman spectroscopy, this is still very challenging. In this work, we have developed a new method to measure the chirality of crystals, based on electron paramagnetic resonance (EPR) spectroscopy of chiral crystals doped with Cu2+ as the EPR active ion. Here, we demonstrate our approach using a model system of l - and DL-Histidine crystals doped with Cu2+. We show that EPR measurements of the Cu2+-doped Histidine crystals can accurately determine the chirality and enantiomeric composition of the crystals. We present a very preliminary example of this technique, and we hope that in the future it will be possible to refine and develop this method for many other chiral organic crystal systems. [Display omitted] [ABSTRACT FROM AUTHOR]

Power networks, especially distribution networks, have been undergoing substantial changes since the application of new technologies. Technology development in the early part of the 21st century has opened up new horizons for automated, efficient and reliable power grids. New technologies, while enhancing the capability of the electrical networks and providing opportunities and innovative solutions to the challenges of future networks, can also cause drawbacks that should be investigated and taken into account. Distributed generation (DG) is one of the new technologies that improves the operation of power grids. Despite tangible benefits that integration of DG units brings to electrical grids, their notable impacts on protection systems of power networks raise many challenges and concerns on how a fault should be detected and isolated in active distribution networks. Many attempts have been made to investigate the downside of the interconnection of DG units and methods to mitigate their impacts have been proposed. This study reviews the impact of DG integration on protection systems addressed in other research works and recapitulates suggested methods provided by scholars. [ABSTRACT FROM AUTHOR]

Using first-principles methods, we investigate the electronic properties of the [Ba(Mo 1 / 2 ,Mg 1 / 2 )O 3 ] x -[BaTiO 3 ] 1 − x solid solution derived from barium titanate as a potential candidate to be used in photovoltaic devices. Focusing on the bandgap and its origin, we study the effect of different possible Mo and Mg contents, arrangements, and phases of [Ba(Mo 1 / 2 ,Mg 1 / 2 )O 3 ] x -[BaTiO 3 ] 1 − x . We find that [Ba(Mo 1 / 2 ,Mg 1 / 2 )O 3 ] 0.25 -[BaTiO 3 ] 0.75 is a viable candidate for use in transparent photovoltaics due to its energy bandgap of 2.6 eV in the rhombohedral phase. In all cases, [Ba(Mo 1 / 2 ,Mg 1 / 2 )O 3 ] x -[BaTiO 3 ] 1 − x materials exhibit spontaneous polarization that allows the exploitation of the bulk photovoltaic effect and in principle may allow high power conversion efficiency exceeding the Shockley-Queisser limit for these materials. [ABSTRACT FROM AUTHOR]

Interconnected power systems are multi-loop structured. In such networks, the determination of settings for all overcurrent relays can be carried out in different forms and may be quite complicated. Optimal coordination of overcurrent relays is a solution to find optimal time setting of the relays in interconnected networks. However, the optimal coordination method without using correct and efficient starting points i.e. the location of initial relays in the procedure for settings may cause many miscoordinations or the solution may not converge. In this paper an innovative approach is proposed in which both optimal coordination and break points for initial relays to find the settings of overcurrent relays are utilized. The fault location is in front of main relay and network configuration is so that maximum current flows through backup relay. Therefore, network configuration is taken into consideration and the suggested approach reflects configuration changes. A comparison between optimal coordination of overcurrent relays with or without break points and the conventional procedure of overcurrent relays coordination is presented. The results show the advantages of the proposed method. [ABSTRACT FROM AUTHOR]

Magnesium ion (Mg2+) is one of the most significant cations in living systems with involvement in many biochemical reactions and cellular processes and hence, sensitive and specific detection of Mg2+ is therefore essential for various applications. Here, we report the solvothermal synthesis of boron‐doped carbon dots (BC10) with more oxygen surface states by using salicylaldehyde and naphthalene‐1‐boronic acid. The as‐prepared BC10 showed greenish‐white luminescence under 365 nm UV illumination with quantum yield (QY) of 5.5 % at optimum dilution with dimethyl sulfur oxide (DMSO) solvent. The BC10 in DMSO (DS‐BC10) have shown high selectivity and sensitivity towards Mg2+ ion through the increased PL intensity due to chelation‐enhanced photoluminescence (CHEP). The enhanced PL intensity was further supported by the increased QY by a factor of 12 after the addition of Mg2+ ions to 65.7 %. Moreover, the limit of detection of DS‐BC10 is calculated to be 0.4 μM, which is 100 times better than the recently reported carbon dot‐based Mg2+ sensor. Following that, DS‐BC10 is also explored for detection of magnesium ions in lab tap water, seawater, and drinking mineral water with good precision. In addition, density functional theory (DFT) calculations using B3LYP/6‐31G (d) method for the DS‐BC10 and their Mg2+ complexes support the experimental observations. [ABSTRACT FROM AUTHOR]

Switchable polarization makes ferroelectrics a critical component in memories, actuators and electro-optic devices, and potential candidates for nanoelectronics. Although many studies of ferroelectric switching have been undertaken, much remains to be understood about switching in complex domain structures and in devices. In this work, a combination of thin-film epitaxy, macro- and nanoscale property and switching characterization, and molecular dynamics simulations are used to elucidate the nature of switching in PbZr0.2Ti0.8O3 thin films. Differences are demonstrated between (001)-/(101)- and (111)-oriented films, with the latter exhibiting complex, nanotwinned ferroelectric domain structures with high densities of 90° domain walls and considerably broadened switching characteristics. Molecular dynamics simulations predict both 180° (for (001)-/(101)-oriented films) and 90° multi-step switching (for (111)-oriented films) and these processes are subsequently observed in stroboscopic piezoresponse force microscopy. These results have implications for our understanding of ferroelectric switching and offer opportunities to change domain reversal speed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Machine Learning methods were used to predict when solids lose magnetic properties. • The methods used ionic properties, composition, tolerance factor and other features. • Random forest method with a systemic feature selection were successful. • There is applicability for a wide array of solid solutions. In this work, we studied the application of machine learning for the prediction of the Curie temperature (T c) of ferroelectric BiMe′Me′′O3-PbTiO 3 systems (where Me′ and Me′′ are metal cations). We found that among the studied K-nearest neighbor (KNN), support vector regression (SVR) and random forest (RF) methods, RF obtains the best performance and is insensitive to the choice of hyperparameters. SVR results show a strong sensitivity to the choice of hyperparameters and obtained T c predictions with significantly lower accuracy than RF even after hyperparameter optimization. KNN results show poor accuracy and are essentially unusable with an incomplete feature set and are only qualitatively accurate with a complete feature set. With regard to the choice of features for accurate prediction of the Ferroelectric (FE) systems, we find that Bi content and B-cation valence, ionic radius and ionic displacements form the irreducible set of features such that these features or their equivalents must be used to obtain quantitatively accurate T c predictions. We also find that homovalent and heterovalent BiMe′Me′′O 3 -PbTiO 3 solid solutions form distinct classes of compounds with different behaviors so that both types must be included in the input data set to obtain high predictive accuracy. Our work confirms that for the small data sets typically available in materials science, careful selection of the input system, features and ML methods is required to enable accurate model construction and discovery of previously unknown relationships, but can be achieved in a systematic manner. [ABSTRACT FROM AUTHOR]

We use first-principles density functional theory within the local density approximation to ascertain the ground state structure of real and theoretical compounds with the formula ABS3 (A = K, Rb, Cs, Ca, Sr, Ba, Tl, Sn, Pb, and Bi; and B = Sc, Y, Ti, Zr, V, and Nb) under the constraint that B must have a d0 electronic configuration. Our findings indicate that none of these AB combinations prefer a perovskite ground state with corner-sharing BS6 octahedra, but that they prefer phases with either edge- or face-sharing motifs. Further, a simple two-dimensional structure field map created from A and B ionic radii provides a neat demarcation between combinations preferring face-sharing versus edge-sharing phases for most of these combinations. We then show that by modifying the common Goldschmidt tolerance factor with a multiplicative term based on the electronegativity difference between A and S, the demarcation between predicted edge-sharing and face-sharing ground state phases is enhanced. We also demonstrate that, by calculating the free energy contribution of phonons, some of these compounds may assume multiple phases as synthesis temperatures are altered, or as ambient temperatures rise or fall. [ABSTRACT FROM AUTHOR]

Using density functional theory (DFT) within the local density approximation (LDA), we calculate the physical and electronic properties of PbTiO3 (PTO) and a series of hypothetical compounds PbTiO3-xSx x = 0.2, 0.25, 0.33, 0.5, 1, 2, and 3 arranged in the comer-sharing cubic perovskite structure. We determine that replacing the apical oxygen atom in the PTO tetragonal unit cell with a sulfur atom reduces the x = 0 LDA calculated band gap of 1.47 eV to 0.43-0.67 eV for x = 0.2-1 and increases the polarization. PBE0 and GW methods predict that the compositions x = 0.2-2 will have band gaps in the visible range. For all values of x < 2, the oxysulfide perovskite retains the tetragonal phase of PbTiO3, and the a lattice parameter remains within 2.5% of the oxide. Thermodynamic analysis indicates that chemical routes using high-temperature gas, such as H2S and CS2, can be used to substitute ofor S in PTofor the compositions x = 0.2-0.5. [ABSTRACT FROM AUTHOR]

We show that light drives large-amplitude structural changes in thin films of the prototypical ferroelectric PbTi03 via direct coupling to its intrinsic photovoltaic response. Using time-resolved x-ray scattering to visualize atomic displacements on femtosecond time scales, photoinduced changes in the unit-cell tetragonality are observed. These are driven by the motion of photogenerated free charges within the ferroelectric and can be simply explained by a model including both shift and screening currents, associated with the displacement of electrons first antiparallel to and then parallel to the ferroelectric polarization direction. [ABSTRACT FROM AUTHOR]

Oxygen vacancies in perovskite oxide solid solutions are fundamentally interesting and technologically important. However, experimental characterization of the vacancy locations and their impact on electronic structure is challenging. We have carried out first-principles calculations on two Zr-modified solid solutions, Pb(Zn1/3Nb2/3)O3 and Pb(Mg1/3Nb2/3)O3, in which vacancies are present. We find that the vacancies are more likely to reside between low-valent cation-cation pairs than high-valent cation-cation pairs. Based on the analysis of our results, we formulate guidelines that can be used to predict the location of oxygen vacancies in perovskite solid solutions. Our results show that vacancies can have a significant impact on both the conduction and valence band energies, in some cases lowering the band gap by ≈0.5 eV. The effects of vacancies on the electronic band structure can be understood within the framework of crystal field theory. [ABSTRACT FROM AUTHOR]

We report scanned probe characterizations of the ferroelectric phase transition in individual barium titanate (BaTiO3) nanowires. Variable-temperature electrostatic force microscopy is used to manipulate, image, and evaluate the diameter-dependent stability of ferroelectric polarizations. These measurements show that the ferroelectric phase transition temperature (TC) is depressed as the nanowire diameter (dnw) decreases, following a 1/dnw scaling. The diameter at which TC falls below room temperature is determined to be ∼3 nm, and extrapolation of the data indicates that nanowires with dnw as small as 0.8 nm can support ferroelectricity at lower temperatures. We also present density functional theory (DFT) calculations of bare and molecule-covered BaTiO3 surfaces. These calculations indicate that ferroelectricity in nanowires is stabilized by molecular adsorbates such as OH and carboxylates. These adsorbates are found to passivate polarization charge more effectively than metallic electrodes, explaining the observed stability of ferroelectricity in small-diameter BaTiO3 nanowires. [ABSTRACT FROM AUTHOR]

Electrically tunable dielectric thin films in active circuits and systems are challenged by capacitance-induced delays and impedance matching requiring a lower dielectric constant. Here an approach to increasing the intrinsic tunability of compounds containing TiO 6 octahedra by considering the influence of different connectivity among these octahedra is presented. Such connectivity variants in nanocrystalline monoclinic BaTi 2 O 5 thin films enable a two orders of magnitude enhancement in Ti anharmonic interaction, thereby permitting a ≈ 65% decrease in dielectric constant to 70 at room temperature without sacrificing tunability. Edge-sharing TiO 6 octahedra possess a much shorter Ti-Ti distance of only 2.91 Å as compared to the perovskite structure (~4 Å), permitting large field-induced structural re-arrangement and intrinsic tunability. [Display omitted] [ABSTRACT FROM AUTHOR]

The phase transition evolution for [KNbO3]1-x[BaNi1/2Nb1/2O3-δ]x(x=0,0.1) is determined via complementary dielectric permittivity and Raman-scattering measurements. Raman scattering by optical phonons over the range of 100-1000cm-1 for 83 K