Change In Electric Polarization Research Articles

THEORETICAL BASICS OF MONITORING THE CONDITION OF THE ELECTRODES OF CHEMICAL CURRENT SOURCES BY THE METHOD OF ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

On the basis of the conducted critical review of modern physical models of the porous electrode, it сan be stated that under the conditions of non-uniform mass transfer taking into account the depth of the electrode, it is possible not only to develop new porous electrodes for a certain application, but also to control the state of electrochemical systems as a whole using the non-destructive method of electrochemical impedance spectroscopy. The presence of a macroscopic model of porous electrode allows one to use the integration of parameters over the surface of the electrode and obtain the average values of current, resistance and capacity within the electrode using the method of averaging in the volume ele­ment within the electrode, where porosity is the volume fraction of the void within the element, which is filled with electrolyte solution. This is the theoretical basis for using electrochemical impedance spectroscopy to assess the state of electrodes in electrochemical current sources. To take into account the influence of the aqueous electrolyte, it is possible to use a model taking into account the area of the effective wetted surface, which makes it possib­le to relate the wetting of the electrode pores with the electrolyte solution to the change in electrical conductivity and polarization of the electrode surface. In this case, when usingelectrochemical impedance spectroscopy, it is possible to obtain information about the following changes in primary current sources: 1– the effect of temperature, which leads to a decrease in the areas of the electrode wetted by the electrolyte, which affect the value of the capacity of the DEL, 2 – chemical processes that lead to the destruction of hydrophilic pores and pores with hydrophilic-hydrophobic walls, an increase in the hydrophobic component on the surface of the electrode, 3 – mechanical destruction of the electrodes. The use of models that take into account the geometry of pores makes it possible to obtain correct data for the analysis of the porous surface in the presence of an electrolyte and in cases of gas phase adsorption in presence of closed pores, as well as to use the value of the capacity on the surface of electrodes to assess the state of their performance.

Near-Room-Temperature Magnetoelectric Coupling via Spin Crossover in an Iron(II) Complex.

Magnetoelectric coupling is achieved near room temperature in a spin crossover FeII molecule-based compound, [Fe(1bpp)2 ](BF4 )2 . Large atomic displacements resulting from Jahn-Teller distortions induce a change in the molecule dipole moment when switching between high-spin and low-spin states leading to a step-wise change in the electric polarization and dielectric constant. For temperatures in the region of bistability, the changes in magnetic and electrical properties are induced with a remarkably low magnetic field of 3 T. This result represents a successful expansion of magnetoelectric spin crossovers towards ambient conditions. Moreover, the observed 0.3-0.4 mC m-2 changes in the H-induced electric polarization suggest that the high strength of the coupling obtained via this route is accessible not just at cryogenic temperatures but also near room temperature, a feature that is especially appealing in the light of practical applications.

Enhancement of giant magnetoelectric effect in Ni-doped CaBaCo4O7

The polar ferrimagnet CaBaCo${}_{4}$O${}_{7}$ exhibits the largest magnetic field driven electric polarization change \ensuremath{\Delta}$P$ reported so far in a narrow temperature range. Here, the authors demonstrate that only 2.5% of Ni substitution for Co increases the magnitude of \ensuremath{\Delta}$P$ by about 30%, as well as expanding the temperature range in which the giant \ensuremath{\Delta}$P$ appears down to a low temperature. This is attributable to the switching of the low-temperature zero magnetic field state from a ferrimagnetic to an antiferromagnetic state.

Successive electric polarization transitions induced by high magnetic field in the single-crystal antiferromagnet Co2Mo3O8

The polar antiferromagnet ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ was recently reported as a new multiferroic material exhibiting remarkable second-order magnetoelectric (ME) coupling effect while no other metamagnetic transitions occurred at low magnetic field. Herein, we have investigated the ME phenomena under high magnetic field (up to 60 T) in ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ single crystals and observed unique ME response associated with the changes in the hidden magnetic moment on the honeycomb lattice. Two spin-flop transitions are unambiguously defined at ${H}_{c1}\ensuremath{\sim}27\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ and ${H}_{c2}\ensuremath{\sim}31\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ under $H$ along the $c$ axis at 1.7 K, accompanied by two successive colossal changes of electric polarization. The results on the anisotropy of magnetoelectricity as well as the angular-dependent polarization are well consistent with the ME tensor prediction, providing a better approach to understand the evolution of magnetic structures under high magnetic field. Therefore, the hidden magnetic transitions and distinctive magnetoelectricity provide a unique platform on which the ME coupling mechanism in the presence of rich magnetic phase transitions can be explored in this 238 family.

Magnetoelectric coupling in Sr3Co2Fe23.04Al0.96O41 single crystal near room temperature

We have grown Z-type hexaferrite Sr3Co2Fe23.04Al0.96O41 single crystal with a little Al3+ ion doping on the Fe3+ ion sites by the Na2CO3-Fe2O3 flux method. The magnetic properties and magnetoelectric (ME) coupling effects were investigated systematically. With Al3+ ion doping, the transverse conical magnetic phase could be maintained up to 359 K under small magnetic field (H). The magnetization increases with two steps to saturation at 300 K as shown in the M-H curve, implying the existence of a spiral spin order. The ME coupling effects occur near room temperture. Besides, the sign of the electric polarization (P) is independent of the H and the intensity of the electric polarization changes without any decays at 200 K under the oscillating magnetic field. Both of them are crucial for device application, such as information storage devices and non-volatile memory.

Domain Wall Dynamics in a Ferroelastic Spin Crossover Complex with Giant Magnetoelectric Coupling.

Pinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn3+ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [MnIII(3,5-diCl-sal2(323))]BPh4 reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc → Pc → P1 → P1(1/2). The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn–Teller-active Mn3+ complex. On the basis of the change in symmetry from that of the parent structure, Cc, the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc → P1 and P1 → P1(1/2) transitions, with opposite signs. All these phases are polar, while P1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P1→ P1(1/2) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled.

Magnetic-order-induced ferroelectric polarization in the polar antiferromagnets RFeWO6 ( R = Sm, Gd, Er, and Tm)

We report the synthesis, structure, and magnetism-induced multiferroic properties of the polar magnets $R\mathrm{FeW}{\mathrm{O}}_{6}$ (R = Tm, Sm, Gd, and Er). All these compounds crystallize in the orthorhombic structure with the polar symmetry $Pna{2}_{1},$ which results from the ordering of ${\mathrm{Fe}}^{3+}$ and ${\mathrm{W}}^{6+}$ ions at different crystallographic sites. DC magnetization and specific heat measurements confirm the antiferromagnetic order of ${\mathrm{Fe}}^{3+}$ spins at ${T}_{\mathrm{N}1}=14\ensuremath{-}18\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and magnetic ordering of R ions at low temperatures. The magnetic order of ${\mathrm{Fe}}^{3+}$ ions in these compounds is accompanied by a dielectric anomaly and a change in electric polarization. Intriguingly, a second ferroelectric transition occurs at the magnetic ordering temperature (${T}_{\mathrm{N}2}=5.5\phantom{\rule{4pt}{0ex}}\mathrm{K}$) of ${\mathrm{Tm}}^{3+}$ ions in $\mathrm{TmFeW}{\mathrm{O}}_{6}$. The magnetic field dependent behavior of electric polarization varies with R ion, indicating the coupling between $4f\ensuremath{-}3d$ spins. The emergence of change in ferroelectric polarization at the magnetic ordering temperatures demonstrates the multiferroic nature of the polar magnets $R\mathrm{FeW}{\mathrm{O}}_{6}$ (R = Tm, Sm, Gd, and Er). Our study indicates that the aeschynite type family of compounds with polar symmetry can be an excellent platform to understand the role of $4f\ensuremath{-}3d$ coupling on multiferroicity.

Magnetic excitations of the hybrid multiferroic (ND4)2FeCl5·D2O

We report a comprehensive inelastic neutron scattering study of the hybrid molecule-based multiferroic compound (ND4)2FeCl5D2O in the zero-field incommensurate cycloidal phase and the high-field quasi-collinear phase. The spontaneous electric polarization changes its direction concurrently with the field-induced magnetic transition, from mostly aligned with the crystallographic a-axis to the c-axis. To account for such change of polarization direction, the underlying multiferroic mechanism was proposed to switch from the spin-current model induced via the inverse Dzyalloshinskii-Moriya interaction to the p-d hybridization model. We perform a detailed analysis of the inelastic neutron data of (ND4)2FeCl5D2O using linear spin-wave theory to quantify magnetic interaction strengths and investigate possible impact of different multiferroic mechanisms on the magnetic couplings. Our result reveals that the spin dynamics of both multiferroic phases can be well-described by a Heisenberg Hamiltonian with an easy-plane anisotropy. We do not find notable differences between the optimal model parameters of the two phases. The hierarchy of exchange couplings and the balance among frustrated interactions remain the same between two phases, suggesting that magnetic interactions in (ND4)2FeCl5D2O are much more robust than the electric polarization in response to delicate reorganizations of the electronic degrees of freedom in an applied magnetic field.

Giant magnetostriction and nonsaturating electric polarization up to 60 T in the polar magnet CaBaCo4O7

Giant magnetostriction in insulating magnetic materials is highly required for applications but is rarely observed. Here we show that giant magnetostriction (> 1500 ppm) can be achieved in an insulating transition metal oxide CaBaCo4O7 where the ferrimagnetic ordering at TC ~ 62 K is associated with a huge change in the lattice. Moreover, because this material is pyroelectric with a non-switchable electric polarization (P), the giant magnetostriction results in a record-breaking magnetoelectric effect - a gigantic change of electric polarization (deltaP ~ 1.6 {\mu}C/cm2) in response to the applied magnetic field up to 60 T. Geometric frustration as well as the orbital instability of Co2+/Co3+ ions is believed to play a crucial role in the giant magnetostriction. Our study provides new insights on how to achieve both giant magnetostriction and pronounced magnetoelectric effect in insulating transition metal oxides.

Remarkable effects of dopant valency – a comparative study of CaBaCo3.96Cr0.04O7 and CaBaCo3.96Ni0.04O7

Distinctive effects of dopant valency is discussed using an impurity level 1% doping each of Cr3+ and Ni2+ in CaBaCo4O7. Through a comparative study of the magnetic and dielectric properties of magnetoelectric CaBaCo3.96Cr0.04O7 and CaBaCo3.96Ni0.04O7, we highlight that Cr doping does not significantly alter the properties in spite of differences in magnetic spin and ionic radius compared to Co3+ and Co2+ while Ni doping induces spectacular changes. Particularly, about 4 times larger electric polarization change (~650 μC/m2 at 5.6 kV/cm) is observed in CaBaCo3.96Ni0.04O7 compared to CaBaCo3.96Cr0.04O7 and a stronger magnetoelectric coupling leading to a polarization change of ~12% in 15 T magnetic field, below 40 K. Further, magnetodielectric effects hint to competing magnetic phases over the temperature range of magnetic transitions. The article discusses the observed disparity in the light of a possible site selective doping of Cr3+ and Ni2+ in the triangular and kagomé layers, respectively, of the parent compound.

Synthesis and multiferroic properties of high-purity CoFe2O4–BiFeO3 nanocomposites

CoFe2O4–BiFeO3 (CFO–BFO) nanocomposites of three different molar ratios (CFO:BFO = 2:2, 1.5:2, and 1:2) were synthesized by a combination hydrothermal/citric acid sol-gel method. This methodology allowed us to synthesize high-purity crystalline nanocomposites at a lower calcination temperature of 450 °C. Transmission electron microscopy (TEM) images revealed that the CFO and BFO nanocrystals are inter-embedded in each other. Room-temperature magnetic characterizations showed that the sintered sample of the 2:2 ratio had a high magnetization value of 40.5 emu/g at 20 kOe. The values of magnetization and coercivity indicated the presence of dipolar interaction between CFO and BFO. The electric-polarization change (ΔP) was estimated to be ~295 μC/m2 at a magnetic field strength of ~30 kOe at room temperature, although the polarization-electric field (P–E) loop measurements indicated that all of the samples manifested leaky behavior. The results presented herein offer a means of reliable magnetic-field control of electric polarization in high-purity multiferroic CFO–BFO nanocomposites.

Giant Magnetoelectric Coupling and Magnetic-Field-Induced Permanent Switching in a Spin Crossover Mn(III) Complex.

We investigate giant magnetoelectric coupling at a Mn3+ spin crossover in [MnIIIL]BPh4 (L = (3,5-diBr-sal)2323) with a field-induced permanent switching of the structural, electric, and magnetic properties. An applied magnetic field induces a first-order phase transition from a high spin/low spin (HS-LS) ordered phase to a HS-only phase at 87.5 K that remains after the field is removed. We observe this unusual effect for DC magnetic fields as low as 8.7 T. The spin-state switching driven by the magnetic field in the bistable molecular material is accompanied by a change in electric polarization amplitude and direction due to a symmetry-breaking phase transition between polar space groups. The magnetoelectric coupling occurs due to a γη2 coupling between the order parameter γ related to the spin-state bistability and the symmetry-breaking order parameter η responsible for the change of symmetry between polar structural phases. We also observe conductivity occurring during the spin crossover and evaluate the possibility that it results from conducting phase boundaries. We perform ab initio calculations to understand the origin of the electric polarization change as well as the conductivity during the spin crossover. Thus, we demonstrate a giant magnetoelectric effect with a field-induced electric polarization change that is 1/10 of the record for any material.

A-Site and B-Site Cation Ordering Induces Polar and Multiferroic Behavior in the Perovskite NaLnNiWO6 (Ln = Y, Dy, Ho, and Yb)

Octahedral distortion in ABO3 perovskite materials is ubiquitous because of the ionic size mismatch between A and B cations, leading to various kinds of crystal symmetry. However, such a distortion always results in centrosymmetric structures except for the covalent bond formation because of the second-order Jahn–Teller (SOJT) effect that occurs with d0 or lone-pair cations. Here, we report that an unusual combination of the layered A-site cation ordering and B-site rock salt ordering in NaYNiWO6 prepared under high-pressure and high-temperature conditions results not only in a polar (P21) structure, as revealed by the neutron diffraction analysis, but exhibits multiferroic properties below the magnetic ordering of Ni2+ ions (TN = 21 K). Analysis of the neutron diffraction data at 20 K reveals an incommensurate sinusoidal spin ordering with the propagation vector, ki = (0.471(2), 0, 0.491(4)), and a commensurate collinear spin structure with kc = (0.5, 0, 0.5) below 18 K. X-ray diffraction data confirm the polar structure in Dy, Ho, and Yb compounds. All four compounds exhibit a switchable change in electric polarization (ΔP) at the magnetic ordering temperatures, demonstrating coupling between ferroelectricity and magnetism.

Separation of Phases and Charge States in Relaxor Ferroelectric PbCo1/3Nb2/3O3

The permittivity, conductivity, electric polarization, and features of high-resolution X-ray diffraction scattering of a relaxor ferroelectric PbCo1/3Nb2/3O3 have been investigated in the temperature range 5–350 K. Continuous correlated temperature changes in dielectric properties and electric polarization have been revealed, which were not typical of relaxor ferroelectrics. These changes can be attributed to local polar domains, which were induced in the original crystal matrix. In such domains, the charges (valences) of Co and Nb ions were continuously changed.

Photoexcitation of a one-dimensional polarization-inverted domain from the charge-ordered ferroelectric ground state of (TMTTF)2PF6

We theoretically revealed that a weak photoexcitation achieves the electric polarization-inversion with approximately $18\%$ of all the charges, which was interpreted as a superimposition of multi-exciton states, from the charge-ordered ferroelectric ground state of (TMTTF)$_{2}$PF$_{6}$ at absolute zero temperature. Regarding a relative change of electric polarization ($\Delta P/P$), the photoexcitation corresponds to $36\%$, which is much larger than $\Delta P/P$ of other typical organic materials. The value of $\Delta P/P\sim 36\%$ can be enlarged by a strong photoexcitation. This fact is useful not only for applications of this material and other analogous materials in optical devices but also for researches toward controlling electric polarizations by light, which is one of the recent attracting issues on photoinduced phase transition phenomena. The photoexcitation of $\Delta P/P\sim 36\%$ corresponds to the single peak of the optical conductivity in the low-energy region, which was also observed at 10 K. Theoretical calculations are based on a quarter-filled one-dimensional effective model with appropriate parameters and 50 unit cells.

(Invited) what Limits the Intrinsic Carrier Mobility of Two Dimensional Metal Dichalcogenides?

Two-dimensional (2D) metal dichalcogenides (MX2) are the most common type of 2D semiconductors and have shown great potential for a wide range of chemical and physical applications. However, they are limited by a low electron/hole mobility, which has been recognized as one of the major challenges impeding their further developments, and urges efforts to understand the mobility-limiting factors and discovery of higher-mobility alternatives. Here using density functional perturbation theory and Wannier interpolation of the electron–phonon matrix to study a wide range of MX2, we find that the intrinsic carrier mobility, in contrast to common belief, neither correlates with the effective mass nor can be assessed by the widely used deformation potential theory; instead it is limited by the longitudinal optical (LO) phonon scattering for most MX2, while for MoS2 and WS2, the mobility is limited by the longitudinal acoustic (LA) phonon scattering. Furthermore, we find that the LO scattering strength is strongly correlated with the magnitude of the Born effective charge, suggesting that the carrier transport is greatly affected by the electric polarization change induced by the atomic vibration. This finding enables us to use the Born effective charge to rapidly screen the 2D MX2 database for high-mobility semiconductor candidates. Our work reveals the underlying factors governing the intrinsic carrier mobility of 2D MX2, and offers a practical descriptor for discovering high-mobility candidates. Ref: L. Cheng, Y. Liu, JACS, 2018, DOI: 10.1021/jacs.8b07871

What Limits the Intrinsic Mobility of Electrons and Holes in Two Dimensional Metal Dichalcogenides?

Two-dimensional (2D) metal dichalcogenides (MX2) are the most common type of 2D semiconductors and have shown great potential for a wide range of chemical and physical applications. However, they are limited by a low electron/hole mobility, which has been recognized as one of the major challenges impeding their further developments, and urges efforts to understand the mobility-limiting factors and discovery of higher-mobility alternatives. Here using density functional perturbation theory and Wannier interpolation of the electron-phonon matrix to study a wide range of MX2, we find that the intrinsic carrier mobility, in contrast to common belief, neither correlates with the effective mass nor can be assessed by the widely used deformation potential theory; instead it is limited by the longitudinal optical (LO) phonon scattering for most MX2, while for MoS2 and WS2, the mobility is limited by the longitudinal acoustic (LA) phonon scattering. Furthermore, we find that the LO scattering strength is strongly correlated with the magnitude of the Born effective charge, suggesting that the carrier transport is greatly affected by the electric polarization change induced by the atomic vibration. This finding enables us to use the Born effective charge to rapidly screen the 2D MX2 database for high-mobility semiconductor candidates. Our work reveals the underlying factors governing the intrinsic carrier mobility of 2D MX2, offers a practical descriptor for discovering high-mobility candidates, and serves as a paradigm to accurately assess the carrier mobility in 2D semiconductors, thereby paving critical steps toward the development of 2D materials.

Insights into the Role of Ferroelectric Polarization in Piezocatalysis of Nanocrystalline BaTiO3.

Piezoelectric effect, commonly known as a change in electric polarization in piezoelectric/ferroelectric materials under mechanical stress, is extensively employed as a driving force for the catalytic degradation of organic pollutants. However, the relationship between electric polarization and piezocatalytic activity is still unclear. In this work, we investigated the role of ferroelectric polarization in the piezocatalytic activity of BaTiO3 nanoparticles through annealing BaTiO3 at different temperatures or poling BaTiO3 at different electric fields. The BaTiO3 nanoparticles annealed at 800 °C exhibit effectively enhanced piezocatalytic activity compared with those annealed at other temperatures. The polycrystalline particles annealed at higher temperatures exhibit a greatly reduced catalytic activity. After poling, the piezocatalytic activity of the polycrystalline BaTiO3 particles was obviously improved. In addition, we identified the free radical species and the intermediate products of the catalytic reaction. We also well-explained the dependence of electric polarization in the BaTiO3 piezocatalyst on annealing temperature and ultrasonic vibration theoretically. Our study indicates that increasing ferroelectric polarization (but not crystallite size) can effectively enhance the piezocatalytic activity. We believe that the present work provides a clear understanding of the role of ferroelectric polarization in piezocatalysis.

Electromagnon with Sensitive Terahertz Magnetochromism in a Room-Temperature Magnetoelectric Hexaferrite.

An electromagnon in the magnetoelectric (ME) hexaferrite Ba_{0.5}Sr_{2.5}Co_{2}Fe_{24}O_{41} (Co_{2}Z-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on the electric field (E^{ω}) of the THz light parallel to the c axis (∥ [001]), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410K. The resonance frequency of 1.03THz at 20K changes -8.7% and +5.8% under external magnetic field (H) of 2kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from antiphase motion of spins which are coupled with modulating electric dipoles through the exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of the Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different H.

Strain-induced polar discontinuities in two-dimensional materials from combined first-principles and Schrödinger-Poisson simulations

The local application of mechanical stress in piezoelectric materials gives rise to boundaries across which the electric polarization changes. Polarization charges appear along such polar discontinuities and the ensuing electric fields drive a charge reconstruction with the accumulation of free carriers at the boundaries. This is particularly relevant for two-dimensional materials that can sustain very large strains and display record piezoelectric responses. Here we show by first-principles simulations the emergence of one-dimensional wires of free electrons and holes along strain interfaces, taking SnSe as a paradigmatic material. We complement this by developing a Schr\"odinger-Poisson approach specifically designed for two-dimensional materials that it is able to reproduce the ab-initio results and also to extend them to regimes of parameters and system sizes that would be unaffordable in first principles calculations. This model allows us to assess the degree of tunability for the free charge in the wires coming from strain values and profiles, and to obtain the critical size at which the interfaces start to be metallic.