Dipolar Glass Research Articles

Microscopic origin of giant piezoelectricity in ferroelectric xBi(Ni0.5Hf0.5)O3−(1−x)PbTiO3 beyond the morphotropic phase boundary

Local structural correlations in a novel ferroelectric solid solution [$x\mathrm{Bi}({\mathrm{Ni}}_{0.5}{\mathrm{Hf}}_{0.5}){\mathrm{O}}_{3}\text{\ensuremath{-}}(1\ensuremath{-}x){\mathrm{PbTiO}}_{3}$ at $x=0.39$] have been drawn out through atomistic modeling against neutron pair distribution functions and complementary Raman scattering measurements. By examining polar displacements of different cations from the refined structural models, we reveal the distributions of cation-specific dipolar fluctuations at the key composition where the maximum piezoelectric response was recorded. It becomes clear that the unusual structural modifications observed after the poling are simply a manifestation driven by the varying extent of the dipolar fluctuations and not a structural transformation caused by adaptive cationic displacements. Moreover, in addition to strong dipolar flexibility under an electric field, both poled and unpoled ceramics exhibit orientation-independent polarized Raman scattering, which is more typical of a dipolar glass with a responsive and polarizable matrix than a polycrystalline material composed of ferroelectric domains. Therefore, compared to other analogous $x\mathrm{Bi}(\mathrm{Me}){\mathrm{O}}_{3}\text{\ensuremath{-}}(1\ensuremath{-}x){\mathrm{PbTiO}}_{3}$ systems, the current material is a unique case where chemical substitution has led to a highly polarizable ionic-covalent matrix that seems to be the key ingredient to generate giant piezoelectricity at a composition beyond the morphotropic phase boundary.

Broadband Dielectric, Terahertz, and Infrared Spectroscopy of BaTiO3–BaZrO3 Solid Solution: From Proper Ferroelectric over Diffuse and Relaxor Ferroelectrics and Dipolar Glass to Normal Dielectric

Herein, the properties of broadband dielectric spectra up to THz–infrared range of the (1 − x)BaTiO3‐xBaZrO3 (BZT‐x) solid solution are reviewed and complemented. This includes the proper ferroelectric (x = 0), diffuse ferroelectrics (0 < x ≤ 0.2), relaxor ferroelectrics (0.4 ≤ x ≤ 0.8), dipolar glasses (x > 0.8), and the normal dielectric (x = 1). The spectra are characterized by an overdamped central mode in the microwave range which weakens on cooling. Except for BaTiO3, the soft mode and central mode do not soften appreciably and do not contribute substantially to the low‐frequency permittivity maximum. The most important dielectric contribution is brought by Cole–Cole relaxation assigned to hopping of Ti ions in the BaTiO3 clusters, which obeys the Arrhenius law with a common activation energy for the whole relaxor range of E a ≈ 181±17 meV and broadens on cooling as described by a temperature‐independent distribution of activation energies. It follows that the polar nanoregions within the BaTiO3 clusters remain very small with temperature‐independent size and frozen boundaries, even if the Ti ions inside them are hopping. This differs from the usual lead‐containing relaxors, in which the polar nanoregions grow on cooling and follow the glass‐like freezing.

Phase Diagram and Cation Dynamics of Mixed MA1–xFAxPbBr3 Hybrid Perovskites

We use a multi-technique approach to determine the phase diagram and molecular cation dynamics of mixed methylammonium-formamidinium MA1-xFAxPbBr3 (0 ≤ x ≤ 1) hybrid perovskites. The calorimetric, ultrasonic and X-ray diffraction experiments show a substantial suppression of the structural phase transitions and stabilization of the cubic phase upon mixing. We use the broadband dielectric and Raman spectroscopies to study the MA and FA cations dynamics in these compounds. The broadband dielectric spectroscopy indicates absence of the MA cation ordering and a gradual increase of the rotation barrier upon mixing. The room-temperature dielectric permittivity substantially decreases as the fraction of the FA cations is increased. No significant changes of the permittivity are detected at temperatures where the dielectric relaxations are absent. We also observe weak signatures of a dipolar glass phase for the highest mixing level (x = 0.5). The Raman spectroscopy supports the dielectric results and reveals additional subtle information about the FA cation dynamics.

STOCHASTIC PSEUDOSPIN NEURAL NETWORK WITH TRIDIAGONAL SYNAPTIC CONNECTIONS

Context. To reduce the computational resource time in the problems of diagnosing and recognizing distorted images based on a fully connected stochastic pseudospin neural network, it becomes necessary to thin out synaptic connections between neurons, which is solved using the method of diagonalizing the matrix of synaptic connections without losing interaction between all neurons in the network. Objective. To create an architecture of a stochastic pseudo-spin neural network with diagonal synaptic connections without loosing the interaction between all the neurons in the layer to reduce its learning time. Method. The paper uses the Hausholder method, the method of compressing input images based on the diagonalization of the matrix of synaptic connections and the computer mathematics system MATLAB for converting a fully connected neural network into a tridiagonal form with hidden synaptic connections between all neurons. Results. We developed a model of a stochastic neural network architecture with sparse renormalized synaptic connections that take into account deleted synaptic connections. Based on the transformation of the synaptic connection matrix of a fully connected neural network into a Hessenberg matrix with tridiagonal synaptic connections, we proposed a renormalized local Hebb rule. Using the computer mathematics system “WolframMathematica 11.3”, we calculated, as a function of the number of neurons N, the relative tuning time of synaptic connections (per iteration) in a stochastic pseudospin neural network with a tridiagonal connection Matrix, relative to the tuning time of synaptic connections (per iteration) in a fully connected synaptic neural network. Conclusions. We found that with an increase in the number of neurons, the tuning time of synaptic connections (per iteration) in a stochastic pseudospin neural network with a tridiagonal connection Matrix, relative to the tuning time of synaptic connections (per iteration) in a fully connected synaptic neural network, decreases according to a hyperbolic law. Depending on the direction of pseudospin neurons, we proposed a classification of a renormalized neural network with a ferromagnetic structure, an antiferromagnetic structure, and a dipole glass.

Dipole ordering of water molecules in cordierite: Monte Carlo simulations

Electric dipoles of water molecules, enclosed singly in regularly spaced nanopores of a cordierite crystal, become ordered at low temperature due to their mutual interaction and show the frequency dependence of their dielectric susceptibility, typical for relaxor ferroelectrics, according to recent experimental data. The corresponding phase transition is accompanied by anomalies in thermodynamic quantities, such as heat capacity and dielectric susceptibility, which are calculated here using the Monte Carlo method, and their agreement with the experimental data is discussed. Despite the increase in the correlation length, the partially filled dipole lattice at low temperatures, according to the calculations, does not have long-range order and corresponds to a dipole glass. This simulation gives a microscopical insight into the formation of polar nanoregions in relaxor ferroelectrics and the temperature dependence of their size.

Dynamical origins of weakly coupled relaxor behavior in Sn-doped (Ba,Ca)TiO3−BiScO3

The peculiar characteristics of relaxors, viz., a frequency-dependent dielectric permittivity peak and good functional properties (dielectric, electromechanical, electrocaloric, etc.), are attributed to nanoscale regions with correlated dipoles, or polar nanoregions (PNRs). However, the exact nature of PNRs and their contribution to relaxor behavior remains debatable. In recent years, solid solutions of $\mathrm{BaTi}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Bi}\mathrm{Me}{\mathrm{O}}_{3}$ (where Me is a metal), have emerged as an interesting system with characteristics in between that of relaxors and dipole glasses. Here, we have examined the atomistic origins of weakly coupled relaxor behavior, specifically with regard to formation of PNRs, in Sn-doped $(1\ensuremath{-}x)(\mathrm{Ba},\mathrm{Ca})\mathrm{Ti}{\mathrm{O}}_{3}\text{\ensuremath{-}}x\mathrm{BiSc}{\mathrm{O}}_{3}$ using macroscopic polarization and neutron dynamic pair distribution function measurements. We show that the short-range atomic correlations observed within the PNRs dynamically fluctuate with frequencies of the order of THz. Furthermore the composition-dependent dielectric and polarization behaviors are critically influenced by the relative stability of the atomic correlations near \ensuremath{\sim}1 THz, while the instantaneous atomic correlations are largely independent of $x$. The current results are discussed based on a model of intrinsic local modes distributed in a dielectrically soft matrix.

Revision of the freezing concept in relaxor ferroelectrics: the case of Na0.5Bi0.5TiO3-Sr0.7Bi0.2TiO3 solid solutions

Presence of a frequency-independent maximum and a frequency-dependent shoulder in temperature-frequency dependence of dielectric permittivity are characteristic features of NBT-based solid solutions, while interpretation of these features is insufficient. In the present work, dielectric dispersion is studied in a wide frequency range (20 mHz–30 GHz) in (1-x)NBT-xSr0.7Bi0.2TiO3 solid solutions, in which a continuous transfer from NBT-like to pure relaxor-like behaviour is observed upon increasing of Sr0.7Bi0.2TiO3 concentration. Temperature dependence of dielectric dispersion and static dielectric permittivity in different compositions of this solid solutions group is analysed. The observed behaviour of static dielectric permittivity upon increasing of x is interpreted as a diminishing of transition between the low and the high-temperature states, while the character of the dielectric dispersion remains the same in both sides of the transition. Upon increasing of x, the dielectric dispersion transforms from the behaviour characteristic to relaxor ferroelectrics into the behaviour resembling dipolar glass. Problems, arising from introducing of the concept of freezing, described by the Vogel-Fulcher law, are discussed.

High-temperature electrical energy storage performances of dipolar glass polymer nanocomposites filled with trace ultrafine nanoparticles

• PEI dipolar glasses with high T g and -CF 3 or -SO 2 - are synthesized rationally. • Trace ultrafine nanoparticles enhance permittivity markedly and keep low loss. • A mechanism of the enhancement is proposed in terms of mobility of dipolar groups. • The nanocomposites exhibit excellent high-temperature energy storage performances. The urgent demand for high-temperature dielectrics toward capacitive energy storage arises from numerous emerging harsh-environment high-temperature applications such as electric vehicles, aerospace power conditioning, downhole oil explorations. For these applications, it is imperative to enhance the discharge energy density of polymer-based dielectrics at elevated temperature. Herein, a trace composite approach is proposed and demonstrated. Two types of polyetherimides (PEIs)-based dipolar glasses are designed, synthesized and composited with trace amount of ultrafine Al 2 O 3 nanoparticles. PEIs-based dipolar glasses with high T g (~250 °C) and much enhanced electrical polarization are constructed by introducing dipolar groups (-CF 3 ) and sulfonyl groups (-SO 2 -) with high dipolar moment (~4.30 D) in the polymer chains. The incorporation of ultrafine Al 2 O 3 nanoparticles increases the electrical polarization and keeps low loss. The remarkable permittivity enhancement via the ultrafine filling could be ascribed to more active mobility of dipolar groups characterized by activation energy of movement and the correlation of arrangement of dipolar groups in the chains with this effect is also revealed. These results will contribute to deeply understanding and rationally designing high-performance polymer-based nanocomposites orienting to high-temperature electrical energy storage.

Magnetic and structural properties of magnetic colloids with a well-developed system of magnetized aggregates

The dynamical magnetic properties of a system of nanoparticle aggregates possessing an intrinsic resulting magnetic moment are studied for the first time. It is established that the processes of magnetization relaxation in magnetic colloids with a well-developed system of magnetized aggregates are similar to the processes of relaxation of magnetically ordered systems with short-range order, i.e. elastic dipole glasses. With decreasing temperature of a sample below the solidification point, the system transforms into dipole glass. The aggregates can acquire additional magnetization in the external field, which leads to the novel peculiarities of the structure formation in such a system. The results of microstructure investigation of a layer of magnetized aggregates exposed to an in-plane rotating magnetic field as well as to combined rotating and constant magnetic field acting perpendicularly to the layer are presented. Being exposed to a rotating magnetic field, the initially individual aggregates agglomerate into clusters, the rotation frequency of which depends as on the rotating field frequency as on the intensity of the additionally imposed constant magnetic field. The process of structural ordering in a layer of magnetized aggregates has been numerically simulated.

Emergence of a Magnetostructural Dipolar Glass in the Quadruple Perovskite Dy_{1-δ}Mn_{7+δ}O_{12}.

We show that a polar, pseudo-Jahn-Teller instability exists for the underbonded rare-earth A-site cations in the quadruple perovskite Dy_{1-δ}Mn_{7+δ}O_{12}, which leads to the spontaneous formation of a dipolar glass. This observation alone expands the applicability of pseudo-Jahn-Teller physics in perovskite-derived materials, for which it is typically confined to B-site cations. We demonstrate that the dipolar glass order parameter is coupled to a ferrimagnetic order parameter via strain, leading to a first order magnetostructural phase transition that can be tuned by magnetic field. This phenomenology may emerge in a broad range of perovskite-derived materials in which A-site cation ordering and octahedral tilting are mutually tied to meet the criteria of structural stability.

Increasing the temperature range of dipolar glass polymers through copolymerization: A first approach to dipolar glass copolymers

This work introduces, for the first time, the dipolar glass copolymer concept as a new strategy to develop polymer dielectrics. Methacrylic monomers containing sulfone and nitrile groups were copolymerized with N-vinyl-2-pyrrolidone, achieving the preparation of 9 new copolymers with higher dielectric properties than conventional polymer dielectrics. These copolymers, besides exhibiting dielectric constant values between 5.3 and 8.2 and dissipation factors below 0.022, compared to their respective poly(methacrylate) homopolymers, exhibit higher Tg values allowing to delay, on the temperature scale, the occurrence of highly dissipative phenomena (e.g. α relaxation and ionic conductivity). Therefore, these new dipolar glass copolymers can be considered as polymers with high dielectric constants, reasonably low loss values and broader working temperature ranges than conventional polymers dielectrics. Finally, results presented in this work reveal the significant effect that dipolar interactions within the material could have on the dielectric properties of these systems, opening a new research line based on the design of new dipolar glass copolymers with outstanding dielectric performance.

Dipolar glass state in BaCe0.3Ti0.7O3 perovskite solid solutions

An extensive broadband dielectric spectroscopy study of BaCe0.3Ti0.7O3 ceramics is presented. The distribution of relaxation times of the system derived from the frequency dependence of complex dielectric permittivity are similar to the ones observed in dipolar glasses. Therefore, it is demonstrated that the dipolar glass model, which is applied here for the first time for disordered perovskite oxides is able to properly describe the dielectric behavior of BaCe0.3Ti0.7O3 solid solution. The observed dipolar glass behavior in such BaTiO3-based solid solution is clearly different from the canonical Pb-based relaxors and the two types of dielectric behavior are comparatively discussed.

Tailoring the electric and magnetic properties of Ba0.8Sr0.2TiO3 ceramics by unsaturated Fe-doping

A special unsaturated Fe-doping method is applied to ferroelectric Ba0.8Sr0.2TiO3 ceramics. With the help of X-ray diffraction, Raman spectra, and electrical measurements, we recognize that paraelectric cubic phase and ferroelectric tetragonal phase coexist in the doped ceramics Ba0.8Sr0.2Ti0.9Fe0.1O3, Ba0.8Sr0.18Ti0.9Fe0.1O3−δ and Ba0.8Sr0.18Ti0.92Fe0.1O3−δ with Fe mainly occupy Ti sites. Meanwhile, obvious room temperature ferromagnetism and magnetodielectric effect present, attributed to the substitution of Fe ions (with variable valence) for both Ti and Sr. In addition, there appears dipole glass and spin glass at low-temperature. As more Fe ions enter into Sr sites, ferroelectricity, ferromagnetism and magnetodielectric effect increase slightly, which is related to the increase of dipoles formed between oxygen vacancies and low-valent Fe ions off-center at Sr sites, as well as the enhancement of ferromagnetic interaction between non-equivalent Fe ions. This work manifests the uncommonly used unsaturated Fe-doping as an effective method for tailoring the electric and magnetic properties and realizing magnetoelectric coupling in traditional ferroelectrics.

Dependence of the Magnetization Kinetics of Magnetic Dispersed Nanosystems on the Dispersed Phase Concentration and the Change of Its State

The paper presents the results of experimental investigation of the magnetization relaxation processes in an ensemble of magnetic nanoparticles. The experimental results show a nonlinear dependence of the magnetic susceptibility on the volume concentration of the magnetic nanoparticles. Dependences of the magnetization relaxation time on the concentration and temperature of the magnetic nanoparticles are determined from an analysis of the frequency and temperature dependences of the magnetic susceptibility. A complex nonmonotonic dependence of the magnetization relaxation time on the magnetic particle concentration and temperature dependence of the magnetic susceptibility close to exponential one characteristic for a system of interacting dipole particles in the state of the so-called dipole glass have been established. The special features of the magnetization relaxation are caused by the interaction of particles and the change of the structure of the magnetic particle ensemble.

Relaxor-like Behavior and Structure Features of Bi2Ti2O7 Pyrochlore Single Crystals

By slow cooling of the melt, large Bi2Ti2O7 single crystals with a composition close to stoichiometric were grown. No traces of impurity phases were found on the X-ray diffraction patterns. A structural model with the space group Fd3m and displacive disorder in Bi-sublattice was proposed based on X-ray single crystal diffraction data. The dielectric properties of Bi2Ti2O7 single crystals as a function of temperature at different frequencies were studied for the first time. As in the case of ceramic samples, a step-like frequency-dependent anomaly was detected at a temperature of about 220 K at a frequency of 1 kHz. It was found that attempts to describe the dielectric relaxation using the Arrhenius equation do not lead to physically significant values of the fitting parameters. However, the relaxation behavior is well described by the empirical Vogel–Fulcher relation, which is typical for many dipole and spin glasses, as well as relaxor ferroelectrics. Based on the value of fitting parameters, Bi2Ti2O7 occupies an intermediate position between the canonical relaxor PbMg1/3Nb2/3O3 on one hand and lead-free weakly coupled relaxors based on BaTiO3 on the other one. Possible mechanisms of the observed relaxor-like behavior of the Bi2Ti2O7 single crystal are discussed in terms of correlated hopping of bismuth cations and geometric frustration of the pyrochlore lattice.

Quantum paraelectricity to dipolar glass transition in Sc doped BaFe12O19 single crystals

We report a quantum paraelectric (QP) to dipolar glass (DG) phase transition induced by Sc substitution in ferrimagnetic hexaferrite BaFe12−xScxO19 (0≤x≤2.4) single crystals. The DG behavior is revealed via anisotropic dielectric spectroscopy, polarization, and specific heat measurements. The phase evolution from QP to DG (0.5≤x≤1.2) and finally to the nonpolar state (x≥ 2.4) with increasing x is found to be due to the competition between the enhanced polar instability by lattice volume expansion and disordered/frustrated dipole-dipole interaction induced by the partial replacement of Sc3+ for Fe3+ in the bipyramids. Our results suggest further experimental clues to realize possible multiferroicity with strong displacive ferroelectricity in BaFe12O19 and other magnetic materials.

Dipolar spin glass transition in three dimensions

Dilute dipolar Ising magnets remain a notoriously hard problem to tackle both analytically and numerically because of long-ranged interactions between spins as well as rare region effects. We study a new type of anisotropic dilute dipolar Ising system in three dimensions [Phys. Rev. Lett. {\bf 114}, 247207 (2015)] that arises as an effective description of randomly diluted classical spin ice, a prototypical spin liquid in the disorder-free limit, with a small fraction $x$ of non-magnetic impurities. Metropolis algorithm within a parallel thermal tempering scheme fails to achieve equilibration for this problem already for small system sizes. Motivated by previous work [Phys. Rev. X {\bf 4}, 041016 (2014)] on uniaxial random dipoles, we present an improved cluster Monte Carlo algorithm that is tailor-made for removing the equilibration bottlenecks created by clusters of {\it effectively frozen} spins. By performing large-scale simulations down to $x=1/128$ and using finite size scaling, we show the existence of a finite-temperature spin glass transition and give strong evidence that the universality of the critical point is independent of $x$ when it is small. In this $x \ll 1$ limit, we also provide a first estimate of both the thermal exponent, $\nu=1.27(8)$, and the anomalous exponent, $\eta=0.228(35)$.

High dielectric constant dipolar glass polymer based on sulfonylated poly(ether ether ketone)

In this work, a new kind of dipolar glass polymer, containing strongly polar sulfonyl groups, was synthesized by post functionalization of the corresponding poly(ether ether ketone) containing carboxyl groups (PEEK-COOH). The sulfonylated PEEK (PEEK-SO2) exhibited strong β transition at −100 °C and 1 Hz due to the large dipole moment (4.25 D) of the side-chain sulfonyl groups, which were attached to the backbone via flexible ester bonds. Because of this strong β dipole relaxation, the glassy PEEK-SO2 exhibited a high dielectric constant of 5.0 at 25 °C and 1 Hz, nearly 32% higher than that of PEEK-COOH. From an electric displacement-electric field loop study, PEEK-SO2 exhibited a relative high discharge energy density of 2.35 J/cm3 at 300 MV/m and room temperature, which was 1.4 times that of PEEK-COOH. However, the hysteresis loss was only 0.008 at 100 Hz and 25 °C. This work further reveals that dipolar glass polymers with high mobile dipolar side groups are suitable for high energy density and low loss dielectric applications.

Preparation, Structural and Electrophysical Studies of Ferroelectric Ceramic Samples of the System (1-2x)BiScO3-xPbTiO3-xPbMg1/3Nb2/3O3, 0 ≤ x ≤ 0.50

Ceramic samples of the (1-2x)BiScO 3 -xPbTiO 3 -xPbMg 1/3 Nb 2/3 O 3 system in the whole compositions region x = 0-0.50 were synthesized. According to the X-ray diffraction studies (XRD), the region of solid solutions with perovskite structure formation in the system was determined (x = 0.23-0.50). The presence of a morphotropic phase boundary between tetragonal and rhombohedral forms of solid solutions was established at x = 0.40-0.42. Refined XRD with profile peak analysis established the presence of the additional cubic phase with broadened X-ray reflexes in the solid solutions along with the main phase. It was concluded that the main and additional phases are located in the volume and in the surface layer of ceramic crystallites, respectively. The crystal structures of phases in the polarized and depolarized sample with x = 0.42 were determined by the Rietveld method. It was found that the monoclinic perovskite phase is present in the polarized sample. Temperature-frequency dependences of dielectric permittivity and losses of solid solutions were studied. It was found that the increasing content of BiScO 3 in samples causes a change in their dielectric properties from the usual ferroelectric (FE) in the region (1-2x) = 0-0.08 to ferroelectric-relaxor (FE-R) in the region (1-2x) = 0.08-0.40, and then to the properties of dipole glass with weak correlations in the region (1-2x) > 0.40. It was found that the samples with x = 0.434 and 0.446 below T c = 414 and 445 K spontaneously pass to the FE state. In the samples with x = 0.42 the application of an electric field of 2.0 kV/ cm induces a transition from FE-R to FE state with T c = 350 K, which remains after the field is removed.

Preparation and Dielectric Characterization of P(VDF–TrFE) Copolymer-Based Composites Containing Metal–Formate Frameworks

We report the synthesis and dielectric characterization of novel polyvinylidene fluoride-trifluoroethylene P(VDF-TrFE) composite films containing [(CH3)2NH2][Mg(HCOO)3] (DMAMg) and [NH4][Zn(HCOO)3] (AmZn) dense metal-organic frameworks (MOFs). The optical camera and Raman microscopies are used to map the distribution of the MOF fillers in the prepared films. The dielectric spectroscopy experiments of the DMAMg/P(VDF-TrFE) composite performed in a broad temperature range demonstrate rich dielectric behavior originating from the dipolar dynamics of the (CH3)2NH2+ molecular cations and glassy behavior of the copolymer matrix. An anomalous behavior of the complex dielectric permittivity is also observed because of the structural phase transition of DMAMg fillers. The dielectric properties of the AmZn/P(VDF-TrFE) composite film are mainly determined by the dipolar glass relaxation of the P(VDF-TrFE) polymer. The frequency-dependent dielectric spectra of both composites allow us to characterize the observed dipolar relaxation processes. The (CH3)2NH2+ cation dynamics follows the Arrhenius law, whereas the glassy behavior of P(VDF-TrFE) is described by the Vogel-Fulcher equation. For both composites, we observe a significant increase of the dielectric permittivity compared with the P(VDF-TrFE) film without MOF fillers.