Paraelectric State Research Articles

Synthesis, atomic structure and electronic properties of ferroelectric AgBiP2Se6 ultrathin flakes

Layered transition metal thiophosphate ABP2X6(A = Ag or Cu, B = V, Cr, In or Bi, X = S or Se) as a promising 2D ferroelectric material, has attracted great attention nowadays. In this work, ultrathin AgBiP2Se6 sheets down to 6 nm were synthesized for the first time. Atomic force microscopy, double spherical aberration scanning TEM and Raman spectra were performed to investigate the morphology, atomic structure and vibration modes of the samples. The first-principles calculations utilizing density-functional theory(DFT) were also used to analyze the electronic properties, band structures and projected density of states (PDOS) for both ferroelectric (FE) and paraelectric(PE) states of AgBiP2Se6 monolayer.

DFT and Monte Carlo simulations of Optical, Structural, electronic and Electrocaloric properties of Ca3Ti2O7 compound

Using the DFT we investigated the optical, structural and electronic properties of Ca3Ti2O7 with the full-potential linearized augmented plane-wave (FP-LAPW) approach with the code “Wien2k”. The lattice constants of Ca3Ti2O7 have been derived by the GGA-PBE approximation. We obtained an indirect energy band gap value of Eg = 1.774 eV between N and Γ points. The thermodynamic properties have also been calculated and debated. In addition, the absorption coefficient, reflectivity, optical conductivity, dielectric function, refractive index and extinction coefficient are achieved. In some cases, the electro-caloric properties of the Ca3Ti2O7 compound have been examined by Monte Carlo simulations. The polarization with different electric field values is also demonstrated. This polarization shows a phase transition from a ferroelectric to a para-electric state close to the Curie temperature TC equal to 1104.001 K. At last, the electric entropy changes of the temperatures have been inferred. To complete this study, we have calculated the relative cooling power (RCP) parameter for the corresponding Ca3Ti2O7 and the electrical hysteresis loops.

Electrical characterization of BaBi4Zr2Sn2O15 complex ferroelectric system

BaBi4Zr2Sn2O15, a modified ceramic sample belonging to the oxide family of layered ferroelectrics, is synthesized using the mixed oxide process after firing for calcination at 810 °C for 6 hrs. XRD study of the compound exhibits the growth of an orthorhombic single-phase structure at atmospheric temperature. The electrical properties of the sample are investigated in a broad temperature range (30–500 °C) and frequency range (100 Hz - 106 Hz). The dielectric properties recommend the transition from ferro to paraelectric state at 410 °C which is much more than room temperature. The impact of frequency and temperature on complex immittance parameters (impedance and modulus) are analyzed by an impedance analyzer in broad frequencies and temperature ranges. The electrical properties show; (i) the existence of semiconductor-like negative temperature coefficients of resistance behavior, (ii) evidence of electrical relaxation phenomena as a function of temperature, and (iii) the existence of single electrical relaxation ascribed to the existence of grain contribution to the electric properties.

Electrical transport properties of Mn-modified Ba0.5Bi0.5Ti0.5Fe0.5O3

Ba0.5Bi0.5Ti0.5Fe0.5Mn0.1O3, a Fe-based Mn-modified BFO-BT ceramic, is processed by solid-state reaction route, calcined at 950 °C temperature. Formation of single-phase orthorhombic structure is observed from room temperature XRD analysis. Dielectric behavior is investigated over a broad range of temperature (30 °C–500 °C) and frequency (0.1 kHz–10 MHz). Broadening of transition temperature is noticed at 460 °C, where the material undergoes a change from ferro- to paraelectric state. From dielectric plots the effect of Mn on capacitive behavior of the material is studied. The conductivity study exhibits negative temperature coefficient of resistance (NTCR) behavior showing semiconducting behavior of the material.

Photoinduced Negative Differential Resistivity and Gunn Oscillations in SrTiO3.

SrTiO3 , a perovskite oxide, holds significant potential for application in the field of oxide electronics. Notably, its photoelectric activity in the low temperature regime, which overlaps with the quantum paraelectric state, exhibits remarkable characteristics. In this study, it is demonstrated that when photo-excited with above band gap energy photons, SrTiO3 exhibits non-linear transport of photocarriers and voltage-controlled negative resistance, resulting from an intervalley transfer of photo-induced electrons. As a consequence of the negative resistance, the photocurrent becomes unstable and spontaneously gives rise to low frequency Gunn-like oscillations.

Rigidity of dipolar coupled H2O molecular network in external electric field

We have performed the first study of the effect of an external electric field on the quantum paraelectric state of the electric dipolar lattice of polar water molecules confined within nano-sized cages of beryl crystal lattice. The complex dielectric permittivity ε* = ε'+iε" of a water subsystem in hydrous beryl is measured at frequencies 1 Hz-1 MHz in the temperature interval 10–300 K both in zero and 7 kV/cm external field. For comparison, we measure dielectric response of conventional quantum paraelectric KTaO3. In KTaO3, the application of electric field of several kV/cm suppresses quantum fluctuations and leads to a shallow maximum of ε′(T) around 15 K that could indicate a diffuse phase transition. In hydrous beryl, however, a bias electric field of up to 7 kV/cm has no effect on the dielectric response of the water molecules network. This result is confirmed by Monte Carlo simulations. We attribute the enhanced rigidity of the water dipolar subsystem to the strong long-range dipole-dipole interaction between separate H2O molecules. The field that could affect the polarization state of the dipoles is estimated to be of the order of 100 kV/cm.

Dielectric response in the antiferromagnetic phase of Fe1.10Te

Fe1.10Te was studied by scanning tunneling microscopy (STM) and dielectric response to elucidate the charge fluctuation. The cleaved a-b surface was clearly imaged by UHV-STM at T = 8.3 K. From the line profiles, each Fe site was thought to be equivalent suggesting no static charge order in the antiferromagnetic phase. The temperature dependence of the dielectric permittivity was obtained by measuring the complex impedance. The dielectric permittivity increased with decreasing temperature below the Neel temperature TN. The temperature dependence followed the Curie law, indicating the paraelectric state. It suggests charges fluctuate dynamically in the antiferromagnetic state. We conclude that the dynamical charge fluctuation cooperates with the antiferromagnetic order in Fe1.10Te.

Theory of criticality for quantum ferroelectric metals

Although ferroelectricity in metals was long considered to be impossible, a large family of materials hosting both states has been discovered in recent years, and these properties are often strongly coupled. The authors investigate here the quantum phase transition between the ferroelectric and paraelectric states in metals. They find unique quantum critical properties stemming from unconventional inversion-breaking fluctuations, along with enhanced tendencies to both spin singlet and triplet superconductivity, and provide protocols for strain control of the quantum state of these systems.

Temperature‐Dependent Anharmonic Phonons in Quantum Paraelectric KTaO3 by First Principles and Machine‐Learned Force Fields

AbstractUnderstanding collective phenomena in quantum materials from first principles is a promising route toward engineering materials properties and designing new functionalities. This work examines the quantum paraelectric state, an elusive state of matter characterized by the smooth saturation of the ferroelectric instability at low temperature due to quantum fluctuations associated with anharmonic phonon effects. The temperature‐dependent evolution of the soft ferroelectric phonon mode in the quantum paraelectric KTaO3 in the range 0–300 K is modeled by combining density functional theory (DFT) calculations with the stochastic self‐consistent harmonic approximation assisted by an on‐the‐fly machine‐learned force field. The calculated data show that including anharmonic terms is essential to stabilize the spurious imaginary ferroelectric phonon predicted by DFT in the harmonic approximation, in agreement with experiments. Augmenting the DFT workflow with machine‐learned force fields allows for efficient stochastic sampling of the configuration space using large supercells in a wide temperature range, inaccessible to conventional ab initio protocols. This work proposes a robust computational workflow capable of accounting for collective behaviors involving different degrees of freedom and occurring at large time/length scales, paving the way for precise modeling and control of quantum effects in materials.

Weyl nodal lines, Weyl points and the tunable quantum anomalous Hall effect in two-dimensional multiferroic metal oxynitride: Tl2NO2.

The investigation of two-dimensional (2D) multiferroic and topological quantum phases is a significant topic in current condensed matter physics. In this study, we discover quantum topological phases in the multiferroic material Tl2NO2. We observe that its ferroelectric (FE) phase displays a ferromagnetic ground state with magnetization favoring in-plane orientation. In the absence of spin-orbit coupling (SOC), a Weyl nodal loop around the Fermi level is evident, representing a 1D band crossing between spin-up and spin-down states. When spin-orbit coupling is taken into account, setting the magnetization in-plane, the Weyl nodal loop becomes gapped. Additionally, a pair of 2D Weyl nodes appear on the high-symmetry path, protected by a vertical mirror symmetry allowed by the magnetization. Remarkably, we prove that the Weyl nodes are situated at the topological phase transition between two quantum anomalous Hall (QAH) phases with opposite Chern numbers. Therefore, by adjusting the magnetization, it is possible to switch the propagation direction of chiral edge states. Furthermore, from its ferroelectric state to a paraelectric state, the time-reversal symmetry breaking nodal line is transformed into a Weyl point, achieving 100% spin polarization. Particularly, the Weyl points remain robust against SOC when the vertical mirror symmetry is preserved. Importantly, we also demonstrate that the Weyl point also represents the transition point where the QAH phase changes the sign of its Chern number. Overall, our study provides new insights into the study of multiferroic and topological phenomena in 2D materials and offers a potential avenue for controlling QAH phases.

Temperature-dependent properties of the antiferroelectric model PbZrO3 : An effective Hamiltonian study

A novel atomistic effective Hamiltonian scheme, incorporating an original and simple bilinear energetic coupling, is developed and used to investigate the temperature-dependent physical properties of the prototype antiferroelectric ${\mathrm{PbZrO}}_{3}$ (PZO) system. This scheme reproduces very well the known experimental hallmarks of the complex Pbam orthorhombic phase at low temperatures and the cubic paraelectric state of $\mathrm{Pm}\overline{3}\mathrm{m}$ symmetry at high temperatures. Unexpectedly, it further predicts a novel intermediate state also of Pbam symmetry, but in which antiphase oxygen octahedral tiltings have vanished with respect to the Pbam ground state. Interestingly, such a new state exhibits a large dielectric response and thermal expansion that remarkably agrees with previous experimental observations and the x-ray experiments we performed. We also conducted direct first-principles calculations at 0 K, which further support such a low-energy phase. Within this fresh framework, a reexamination of the properties of PZO is thus called for.

Large-scale atomistic simulation of quantum effects in SrTiO3 from first principles

Quantum effects of lattice vibration play a major role in many physical properties of condensed-matter systems, including thermal properties, such as specific heat, structural phase transition, as well as phenomena, such as quantum crystal and quantum paraelectricity that are closely related to zero-point fluctuations. However, realizing atomistic simulations for realistic materials with a fully quantum-mechanical description remains a great challenge. Here, we propose a first-principles strategy for large-scale molecular dynamics simulation, where a high-accuracy force field obtained by deep-potential (DP) is combined with quantum thermal bath (QTB) method to account for quantum effects. We demonstrate the power of this $\text{DP}+\text{QTB}$ method using the archetypal example $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$, which exhibits several phenomena induced by quantum fluctuations, such as the suppressed structure phase-transition temperature, the quantum paraelectric ground state at low temperatures and the quantum critical behavior $1/{T}^{2}$ law of dielectric constant. Our $\text{DP}+\text{QTB}$ strategy is efficient in simulating large-scale system and is first principles. More importantly, quantum effects of other systems could also be investigated as long as the corresponding DP model is trained. This strategy would greatly enrich our vision and means to study quantum behavior of condensed-matter physics.

BaTiO3 induced modification on the dielectric, ferroelectric and electrical conductivity properties of Pb(Fe0.5Nb0.5)O3-BaTiO3 solid solution

ABSTRACT In this report, we have investigated the effect of BaTiO3 on the preliminary structural analysis, ferroelectric phase transition and electrical properties of 0.05, and 0.1) solid solutions. Conventional solid-state reaction technique is adopted to synthesize solid solution. X-ray diffraction (XRD) analysis is used to study the preliminary structural analysis, which suggests the formation of the single-phase materials. The surface morphology is studied using field emission scanning electron microscope (FESEM). The temperature-dependent dielectric properties display a broad diffuse phase transition from ferroelectric to paraelectric state and the phase transition temperature was found to be decreased with an increase in BT concentrations. Ferroelectric property of the solid solutions is confirmed by the polarization vs. electric field (P–E) loops study at room temperature. Variation of AC conductivity with frequency and temperature is also studied. The addition of BT leads to a decrease in the dielectric loss and phase transition temperature.

Electric field-induced phase transition from the glasslike to paraelectric phase and dielectric spectra hardening in PMN single crystal

One of the key points in the physics of the relaxors is their response to the applied DC field. Many studies of this topic were made, in particular on the influence of the field on the dielectric properties. However, practically, in all the cases, the measurements were performed at a fixed frequency and usually with the change in the temperature at the fixed field strength. In this paper, we report the evolution of the dielectric spectra at low frequencies (0.1 Hz [Formula: see text] 1 kHz) at fixed temperature 246 K on changing the DC electric field applied in (111) from 1 kV to 7 kV. Cole-Cole function was used to describe the spectra and the field dependences of the mean relaxation time [Formula: see text], the oscillation strength [Formula: see text] and the width parameter [Formula: see text] were determined. The obtained [Formula: see text]([Formula: see text]) and [Formula: see text] [Formula: see text]([Formula: see text]) provide evidence of the field-induced transition from the nonpolar glass-like phase to the nonpolar paraelectric phase at around 1.5 kV/cm. In the paraelectric phase, very fast hardening of the spectra was observed with [Formula: see text] changing from 10 s to about [Formula: see text]s. The performed analysis demonstrated that the earlier reported positive C-V effect is completely determined by the spectra hardening, while [Formula: see text][Formula: see text] does not show any change in the glass-like phase and monotonously decreases with a field increase in the paraelectric state. For complete understanding of the microscopic origin of the observed phenomena, a detailed study on the short-and long-range structures at the same condition is necessary.

Effect of hydrostatic pressure on the quantum paraelectric state of dipolar coupled water molecular network

We measure the real part ${\ensuremath{\epsilon}}^{\ensuremath{'}}$ of the dielectric permittivity of beryl crystals with heavy water molecules ${\mathrm{D}}_{2}\mathrm{O}$ confined in nanosized cages formed by an ionic crystal lattice. The experiments are performed at a frequency of 1 MHz in the temperature interval from 300 down to 4 K under different hydrostatic pressures up to $P=6.3$ GPa. At high temperatures, a Curie-Weiss-like increase of ${\ensuremath{\epsilon}}^{\ensuremath{'}}(T)$ is observed upon cooling. Application of pressure leads to flattening of ${\ensuremath{\epsilon}}^{\ensuremath{'}}(T)$ at low temperatures due to quantum effects, i.e., tunneling of deuterium atoms in the hexagonal localizing potential. Analyzing the temperature behavior of ${\ensuremath{\epsilon}}^{\ensuremath{'}}$ with the Barrett expression allows us to obtain pressure dependencies of the quantum temperature ${T}_{1}$, the Curie-Weiss temperature ${T}_{C}$, and the Barrett constant $C$. The increase of ${T}_{1}$ observed up to 4 GPa is associated with an enhanced azimuthal tunneling of the confined water molecules through the barriers of the potential. For $P>4$ GPa, ${T}_{1}(P)$ levels off since the barriers disappear. Any further pressure increase does not affect the tunneling rate because of the absence of a barrier. The behavior is modeled by solving the Schr\"odinger equation for the water molecule in the azimuthal potential numerically. Small negative values of ${T}_{C}\ensuremath{\approx}\phantom{\rule{4pt}{0ex}}\ensuremath{-}10$ K obtained for $P<4$ GPa indicate the antiferroelectric ordering tendency of the water dipoles localized in the crystalline nanochannels. For higher pressure, a strong decrease of ${T}_{C}$ toward negative values is observed that would correspond to the enhanced interdipole coupling strength, which is however hard to explain in the present case, and thus calls for additional theoretical and experimental studies.

Characterization of dielectric relaxation, electrical conductivity and impedance spectroscopy of lead-free Li0.5Bi0.5Ti0.8Zr0.2O3 ceramic

This article focuses on dielectric, electrical, and impedance spectroscopy analysis of lead-free ceramic Zr modified Lithium Bismuth Titanate (Li0.5Bi0.5Ti0.8Zr0.2O3), possessing orthorhombic structure prepared by high-temperature mixed oxide method. The room temperature X-ray diffraction study confirms the formation of a single-phase orthorhombic compound. SEM micrograph shows the homogenous and non-uniform distribution of the grains with grain size 4.9 μm. The dielectric, electrical, and impedance properties of the compound are investigated in a broad frequency (103−106 Hz) and temperature (33 °C–500 °C) range. Dielectric anomaly is observed at 112 °C showing phase transition from ferroelectric to paraelectric state and the maximum value of the dielectric constant is nearly 225 at 10 kHz. Fitted Nyquist plots show the involvement of both bulk and bulk interface to the relaxation processes. The temperature-dependent ac conductivity informs the nature of the variation of conductivity and activation energy in various zones. The impedance loss spectra show the fall in resistance with a rise in temperature which confirms the negative temperature coefficient of resistance (NTCR) nature of the compound. Complex modulus Cole-Cole plots suggest that the presence of a non-Debye type relaxation process which is poly dispersive and in nature.

Dielectric behaviour of BZT-BCT nano pseudobinary system near TCP-assisted morphotopic phase boundary

The present study investigates the comparative dielectric studies in nano pseudobinary system synthesized by conventional and hybrid microwave sintering. The material was formed near Tri Critical Point (TCP)-assisted Morphotropic Phase Boundary (MPB) region from equal weight percentage of Lead Zirconate Titanate (BZT) and Barium Calcium Titanate (BCT) employing high energy ball milling. In the temperature dependent LCR study, a diffused phase transition (DPT) behaviour was identified for both conventional and hybrid microwave sintered samples. In each case, modified Currie-Weiss relation indicates an anomaly in the paraelectric state. Further, another approach using macroscopic composition-fluctuation model also exhibits the anomalous behaviour. Universal dielectric response (UDR) demonstrates that such behaviour is created by a temperature assisted relaxation phenomena by the formation of oxygen vacancies. Beyond 260 °C, microwave sintering creates more oxygen vacancies across grain boundary region as compared to the conventional sintering and in turn plays the key role for enhanced dielectric permittivity.

Effect of Surface Ionic Screening on Polarization Reversal and Phase Diagrams in Thin Antiferroelectric Films for Information and Energy Storage

Emergent behaviors in antiferroelectric thin films due to a coupling between surface electrochemistry and intrinsic polar instabilities are explored within the framework of the modified 2-4-6 Landau-Ginzburg-Devonshire (LGD) thermodynamic approach. By using phenomenological parameters of the LGD potential for a bulk antiferroelectric and a Stephenson-Highland (SH) approach, we study the role of surface ions with a charge density proportional to the relative partial oxygen pressure on the dipole states and their reversal mechanisms in antiferroelectric thin films. The combined LGDSH approach allows the boundaries of antiferroelectric, ferroelectriclike antiferroionic, and electretlike paraelectric states as a function of temperature, oxygen pressure, surface-ion formation energy and concentration, and film thickness to be delineated. This approach also allows the characterization of the polar and antipolar orderings dependence on the voltage applied to the antiferroelectric film, as well as the analysis of their static and dynamic hysteresis loops. The applications of the antiferroelectric films covered with a surface-ion layer for energy and information storage are explored and discussed.

Significance of considering pellet molding pressure and dielectric experimental conditions in the [dielectric constant—temperature] response of switchable dielectric molecular materials: illustration on dabcoHBF4 polycrystals

In this study, we investigate the temperature-dependent dielectric response of dabcoHBF4 molecular switchable dielectric materials. This compound made of polycrystals has been shaped into pellets under different controlled applied pressures [0.2–1 GPa]. The dielectric analysis carried out on these samples showed that the mechanical stresses induced by this shaping pressure had a significant effect on the value of the dielectric constant in both the ferroelectric and the paraelectric phase. However, the impact of this pressure remains secondary in the AC electrical conductivity, determined for the first time in these compounds. Thus, conductivity values around 2 × 10–7 S/cm in the ferroelectric phase and 2 × 10–6 S/cm in the paraelectric phase were found. The behavior of these compounds over wide frequency ranges [1–1 MHz] and temperature ramps [0.5–10 K/min] revealed that the frequency had no effect on the transition temperature at the difference of temperature rate as expected in most ferroelectric materials. When decreasing the frequency, the dielectric constant gains in value and can reach values higher than 50 in the ferroelectric phase and 1000 in the paraelectric phase with a frequency solicitation at 10 Hz. At lower frequencies, the Curie–Weiss law is lost in the paraelectric regime and an enhanced rise of the dielectric constant with increasing temperature probably reflects an interfacial polarization mechanism. Finally, this dielectric analysis in frequency, temperature (323–403 K) and along temperature ramps allowed to extract analytical laws of behavior associating some of these parameters. For example, one of these equations will be useful in the choice of the temperature ramp to be applied to optimize the switching time between a ferroelectric and a paraelectric state according to a condition on the value of the desired dielectric constant at the low and high states in the implementation of memories.

Quantum paraelectric phase of SrTiO3 from first principles

We demonstrate how the quantum paraelectric ground state of ${\mathrm{SrTiO}}_{3}$ can be accessed via a microscopic ab initio approach based on density functional theory. At low temperature the quantum fluctuations are strong enough to stabilize the paraelectric phase even though a classical description would predict a ferroelectric phase. We find that accounting for quantum fluctuations of the lattice and for the strong coupling between the ferroelectric soft mode and lattice elongation is necessary to achieve quantitative agreement with experimental frequency of the ferroelectric soft mode. The temperature dependent properties in ${\mathrm{SrTiO}}_{3}$ are also well captured by the present microscopic framework.