Low-temperature Dielectric Relaxation Research Articles

Dielectric Spectroscopy of Non-Stoichiometric SrMnO3 Thin Films

The dielectric properties of non-stoichiometric SrMnO3 (SMO) thin films grown by molecular beam epitaxy were systematically investigated. Especially, the effects of cation stoichiometry-induced diverse types and densities of defects on the dielectric properties of SMO films were revealed. Two anomalous dielectric relaxation behaviors were observed at different temperatures in both Sr-rich and Mn-rich samples. High-temperature dielectric relaxation, resulting from a short-range Mn-related Jahn–Teller (JT) polaron hopping motion, was reinforced by an enhancement of JT polaron density in the Sr-rich film, which contained abundant SrO Ruddlesden–Popper (R-P) stacking faults. However, an excessive number of disordered Sr vacancy clusters in Mn-rich thin film suppressed the hopping path of JT polarons and enormously weakened this dielectric relaxation. Thus, The Sr-rich film demonstrated a higher dielectric constant and dielectric loss than the Mn-rich film. In addition, low-temperature dielectric relaxation may be attributed to the polarization/charge glass state.

The magnetic ground state of Ba(Fe1/2Sn1/2)O3-δ: A potential giant dielectric material for technological applications

Ba(Fe1/2Sn1/2)O3-δ (BFSO) is a potential environmental friendly compound for miniaturization of electronic devices as it exhibits giant dielectric constant at room temperature. However, its magnetic ground state is not settled. We present here comprehensive investigation of the as-sintered and oxygen-annealed BFSO samples using x-ray diffraction, dielectric, x-ray photoelectron spectroscopy (XPS), dc magnetization (M(T, H, t)) and specific heat measurements. Both samples crystallize in a cubic Pm-3m symmetry. At room temperature, both samples show a giant dielectric constant of the order of ε' ∼104 and two low-temperature dielectric relaxations resulting from polaron-hoping and Maxwell-Wagner effect, respectively. The temperature-dependent dc magnetization and specific heat measurements on as-sintered and oxygen-annealed BFSO samples confirm the absence of long-range ordered antiferromagnetic transition at TN ∼55 K. Further, both samples show history-dependent irreversibility of zero-field cooled warming (ZFCW) and field cooled warming (FCW) M(T) curves at the irreversibility temperature (Tirr ∼18 K) with a peak in the ZFCW M(T) curve at the spin-glass freezing temperature Tf ∼16 K. In addition, the FCW M(T) curve of both samples show plateauing over a small temperature range before it starts increasing below Tf, a characteristic typically observed in cluster/spin-glass systems. The presence of long-time magnetic relaxation and aging effect in both samples further support the spin-glass behavior. We also discussed the impact of oxygen vacancies on the physical and magnetic properties and possible origin of the glassy ground state in BFSO.

Low-temperature dielectric relaxation mechanism and correlated barrier hopping transport in neodymium perovskite chromite

Herein, neodymium chromite (NdCrO3) has been synthesized by the sol–gel auto-combustion method. The Rietveld analysis of the X-ray diffraction pattern has shown that NdCrO3 crystalizes in the perovskite structure with orthorhombic unit cell and Pbnm space group symmetry. The electrical transport properties have been investigated by complex impedance spectroscopy over the experimental frequency and temperature ranging from 40 to 1 × 106 Hz and 158–283 K, respectively. The equivalent circuit modeling of Nyquist plots revealed the contribution of grains and grain boundaries to the charge conduction and the relaxation mechanism of NdCrO3, respectively. The AC conductivity analysis divulged the correlated barrier hopping transport mechanism in the NdCrO3. The imaginary electrical modulus spectra fitted by the Kohlrausch, Williams, and Watts function presented the non-Debye type nature of the relaxation mechanism in the material. The dielectric permittivity has exhibited the Maxwell–Wagner type polarization of the material, and the loss factor has exhibited an increasing trend with temperature.

Tailoring magnetic and dielectric properties of Yb2Ti2O7 pyrochlore through structural distortion

While the unique geometrical frustration of pyrochlore Yb2Ti2O7 has attracted attention, the dielectric and other properties of this pyrochlore beyond frustrated magnetism are not well understood. Here, we report on the fascinating low-temperature dielectric relaxation of Yb2-xBaxTi2O7-δ (x = 0–0.20) and demonstrate that this phenomenon is related to structural distortion. A-site Ba substitution, which increases the density of point defects and introduces a different atomic radius, enhances the chemical disorder and structural distortion. As a result, the increases in oxygen vacancies and nonmagnetic Ba2+ and Yb2+ ions dilute the ferromagnetic Yb3+–Yb3+ interactions, as indicated by the decrease in effective magnetic moment. On the other hand, the distorted octahedra facilitate the hopping of Yb ions, and the random distribution of Ba2+ ions at the Yb3+ sites gives rise to additional random fields, leading to significantly increased dielectric relaxation. The room-temperature dielectric constant is improved to ∼170, twice that of pristine Yb2Ti2O7. This work provides a comprehensive view of the structural, magnetic, and dielectric properties of Yb2Ti2O7 and lays the foundation for additional research into pyrochlore compounds.

Multiferroic property, dielectric response and optical behavior of filled tungsten bronze Ba4Nd2Fe2-xMnxNb8O30 (0 ≤ x ≤ 1) ceramics

In this paper, the structural, magnetic, dielectric, ferroelectric and optical properties of Mn doped tungsten bronze Ba4Nd2Fe2-xMnxNb8O30 (0 ≤ x ≤ 1) ceramics were investigated in detail. All the samples are of single phase with the tetragonal structure and the group space of P4bm. Compared with antiferromagnetism in pure sample, Mn-doped samples show a weak room-temperature ferromagnetic state. The observed ferromagnetism in Mn-doped ceramics can be understood in terms of the AFM coupling of Fe-based and Mn-based sublattices via Dzyaloshinskii-Moriya interaction. The relaxor ferroelectric nature can be demonstrated in the doped samples. The sample x = 0.5 exhibits a high storage energy density U of 1.06 J/cm3 and efficiency η of 94.3%. The low-temperature dielectric relaxation obeys well with Vogel-Fulcher relation and can be ascribed to the local polarization fluctuation, while the dielectric relaxation around 300 K follows with Arrhenius law and can be attributed to the localized hopping process of electrons. In addition, the optical band gap decreases from 2.64 eV to 2.49 eV with increasing the Mn concentration.

Exploration of the mechanisms enabling high performance in relaxor ferroelectric crystals

<p indent="0mm">Relaxor ferroelectric single crystals, exhibiting excellent dielectric, piezoelectric, electro-optic, and pyroelectric properties, are important functional dielectric materials in electronic technology. Since the 1990s, relaxor ferroelectric crystals have attracted considerable attention in the ferroelectric and piezoelectric research fields and improved the performance of numerous piezoelectric devices, especially medical ultrasonic imaging transducers. In this paper, we briefly introduce the history of relaxor ferroelectric crystals and then focus on the research progress of our group in recent years, as listed below. (1) To elucidate the contribution of polar nanoregions (PNRs)<italic> </italic>to piezoelectricity, we performed closely integrated experimental and computational studies on relaxor ferroelectric crystals. In this work, we obtained key experimental evidence (low-temperature dielectric relaxation) for the contribution of PNRs to the piezoelectric activity in relaxor-PbTiO₃ (PT) crystals via cryogenic measurements, and more importantly, for the first time, we quantified the contribution of PNRs accounting for 50%–80% of the room-temperature dielectric and piezoelectric properties. Based on the phase-field simulations, we proposed a mesoscale mechanism wherein the PNRs are aligned in a ferroelectric matrix facilitating the polarization rotation, which successfully explains the contribution of PNRs to dielectric/piezoelectric properties. This work opens up a new material design paradigm that introduces local heterogeneities to achieve enhanced macroscopic responses. (2) Based on the origin of high piezoelectric activity in relaxor ferroelectric crystals, we proposed a theoretical approach, i.e., judiciously introducing local structure heterogeneity, to engineer the interfacial energies, thereby increasing the piezoelectricity of ferroelectric materials. According to this method, we designed and fabricated a series of rare earth element-doped relaxor ferroelectric crystals with piezoelectric coefficients <italic>d</italic>₃₃ in the 3400–4100 pC/N, double the values of the undoped counterparts. The newly developed piezoelectric materials offer potential benefits for piezoelectric devices, especially high-frequency medical transducers, where ultrahigh clamped dielectric constant (~3000) improves the electrical impedance matching of the transducers, thereby increasing the sensitivity and decreasing the insertion loss in the ultracompact transducers. (3) Using phase-field simulations and experimental measurements, we transformed the originally opaque [001]-poled rhombohedral PMN-PT crystals into a transparent one by using an AC electric field to engineer the domain structure. These crystals possess ultrahigh piezoelectric coefficient <italic>d</italic>₃₃ (&gt;2100 pC/N), outstanding electromechanical coupling factor <italic>k</italic>₃₃ (~94%), and large electro-optical coefficient <italic>γ</italic>₃₃<sc>(~220 pm/V),</sc> far beyond the performance of the state-of-the-art transparent ferroelectric crystal LiNbO₃, showing great potential in various hybrid device applications, including photoacoustic medical transducers and self-energy-harvesting touchscreens. Furthermore, we discovered that the larger domain sizes lead to higher piezoelectricity in [001]-poled rhombohedral perovskite ferroelectric crystals, opposing the long-standing belief that a smaller domain size always results in higher piezoelectricity in ferroelectric crystals. This observation may benefit the future design of piezoelectric materials. Finally, the applications of our newly designed relaxor ferroelectric crystals in electromechanical and electro-optical devices are introduced, and the future perspectives of relaxor ferroelectric crystals are discussed.

Low-temperature dielectric relaxation associated with NbO6 octahedron distortion in antimony modified potassium sodium niobate ceramics

Although the antimony (Sb) has been widely used to modify potassium sodium niobate (KNN) ceramics for tailoring the phase structure and performance, the role of Sb still remains insufficiently understood, consequently hindering the understanding of the physical origin of high-performance KNN-based ceramics. Here, we combine the experiments and first-principles calculations to deeply reveal the effects of Sb on KNN ceramics. Our results reveal a re-entrant-like relaxation behavior near the rhombohedral-orthorhombic (R-O) phase transition at the low content of Sb, which transforms into a canonical one at higher content of Sb. First-principles calculations show a significantly decreased difference in the bond length of six B-O bonds of NbO6 octahedral in Sb-modified KNN ceramics compared to pristine KNN ceramics, responsible for the low-temperature re-entrant-like dielectric relaxation. Furthermore, the addition of Sb would soften the B-O repulsion and gradually break the long-range ferroelectric ordering, resulting in the occurrence of nanoscale domains and enhanced local heterogeneity. Finally, we find that the optimized piezoelectric properties are the trade-off between the long-range ferroelectric ordering and the local heterogeneity. Therefore, this work not only deeply reveals the effects of Sb on KNN ceramics from multi-scale perspectives but also helps the future composition design for achieving high piezoelectricity.

Structural Distortion-Modulated Magnetic and Dielectric Properties in Nonstoichiometric Yb2-xTi2O7-δ Pyrochlore.

Rare-earth titanate pyrochlores have attracted considerable attention for their unique magnetic frustration. Among those compounds, Yb2Ti2O7, a candidate for quantum spin ice, has been extensively studied in its magnetic ground state. However, works on its dielectric property and structure-property relationship lag far more behind. Here, by preparing and investigating nonstoichiometric Yb2-xTi2O7-δ (x = 0-0.15) ceramics, we demonstrate that the samples with x ≤ 0.05 maintain a single-pyrochlore phase, but the nonstoichiometry arouses significant structural distortion and increased oxygen vacancy. As a result, the ferromagnetism, indicated by a positive Curie-Weiss temperature, decreases almost linearly with increasing x value. Remarkably composition-dependent low-temperature dielectric relaxations have been observed. In addition, through introducing nonstoichiometry, the relaxor degree of dielectric behavior is enhanced, and the dielectric curve shows an altered shape. The origin of this dielectric relaxation is attributed to the increased structural distortion reflected by the changed bond length/angle, since there is no phase transition in 90-300 K. Our work gives a comprehensive view on the structural, magnetic, and dielectric properties of Yb2Ti2O7, which is instructive for further work on pyrochlores.

Acceptor-donor concentration dependent polaronic relaxation, pinned electron defect dipole characteristic and colossal permittivity in co-doped rutile (Zn1/3Nb2/3)xTi(1-x)O2 (x = 0.02, 0.04, 0.06 and 0,08) ceramics

Colossal permittivity with low loss tangent has been achieved by co-substitution of Zn and Nb in TiO2. Incorporation of the divalent and pentavalent ions has been confirmed by structural and Raman spectroscopic analysis. Specific ratio of acceptor-donor pairs led to observation of colossal permittivity (∼104) with a low loss tangent (<0.1). Evidence of localized donor electrons in the dielectric response shows the generation of electron pinned defect dipoles. Low-temperature dielectric relaxation has been observed which correspond to the polaronic conduction following Arrhenius law. As the acceptor-donor content increases, the polaronic relaxation dynamics occurs at higher temperatures. This also reveals that the defect complexes are strongly correlated. Permittivity values corresponding to the loss tangent minima have been compared as a function of acceptor-donor content. Finally, optimum sintering temperature, frequency, measurement temperature, and composition have been identified.

Effects of molecular structure on the dielectric relaxation of substituted cellulose derivatives

The paper discusses the temperature dependencies of the complex dielectric permittivity obtained at different frequencies for the substituted cellulose derivatives with various concentrations of the hydroxypropoxy group. The substituted cellulose derivatives are shown to undergo a low-temperature dielectric relaxation. The input of the dielectric relaxation into dielectric permittivity decreases when the OH groups in cellulose molecules are substituted by the methoxy groups. In contrast, when OH groups in cellulose molecules are substituted by hydroxypropoxy groups, the dielectric relaxation region shifts to higher temperatures, and its intensity increases. The dielectric spectroscopy method is shown to be a reliable method to estimate the degree of the substitutions of the hydroxyl groups with hydroxypropoxy groups in the cellulose derivatives, used in the pharmaceutical industry as drug carriers and drug delivery rate regulators.

Role of oxygen vacancies on the low-temperature dielectric relaxor behavior in epitaxial Ba0.85Ca0.15Ti0.9Zr0.1O3 thin films

Dielectric relaxation in ferroelectric perovskites can result from different inherent chemical and charge disorders within their crystal structures. Despite several theories on dielectric relaxations, the relationship between the concentration of oxygen vacancy (${V}_{\mathrm{O}}$) and dielectric relaxation has not been studied in perovskite oxide thin films. In this work, we report a systematic investigation on the influence of the ${V}_{\mathrm{O}}^{\ensuremath{'}}\mathrm{s}$ on the dielectric relaxation of ${\mathrm{Ba}}_{0.85}{\mathrm{Ca}}_{0.15}{\mathrm{Ti}}_{0.9}{\mathrm{Zr}}_{0.1}{\mathrm{O}}_{3}$ (BCZT) epitaxial thin films grown with ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ (LSMO) top and bottom electrodes on single-crystal ${\mathrm{SrTiO}}_{3}$ (100) substrates using pulsed laser deposition. The ambient oxygen pressures during film growth were systematically varied to control the concentration of ${V}_{\mathrm{O}}$ in the epitaxial BCZT thin films. Low ambient oxygen pressure was found to enhance the tetragonality of the BCZT films and a systematic decrease in the tetragonality was observed at high oxygen pressure conditions as evidenced from x-ray diffraction (XRD) studies. Temperature dependent XRD analyses indicated a low-temperature anomaly near $\ensuremath{\sim}160\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in the BCZT/LSMO heterostructures. Low-temperature dielectric measurements revealed relaxor dielectric response with broad frequency dependence of the dielectric constant. It is proposed that the low-temperature dielectric relaxation is possibly caused by dipolar clusters that are formed by the aggregation of correlated ${V}_{\mathrm{O}}$-induced dipolar defects within the films deposited under different ambient oxygen pressures. High-resolution transmission electron microscopy suggested the presence of the nanometer-sized ${V}_{\mathrm{O}}$ clusters as represented by the structural defects of average diameters of $\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ in the BCZT films grown at variable ambient oxygen pressure. Activation energies of the aforesaid dipolar clusters, obtained from the Vogel-Fulcher fitting of the dielectric dispersion, showed a gradual increase in magnitude, as well as decreasing freezing temperatures, with decreasing concentrations of ${V}_{\mathrm{O}}$ in the BCZT films. From the theoretical analysis of the experimental data, it is postulated that the presence of the ${V}_{\mathrm{O}}$-induced dipolar defects affects the low-temperature dielectric relaxation in the BCZT thin films. This work significantly extends the understanding of ${V}_{\mathrm{O}}$-mediated dielectric relaxor behavior in complex perovskite oxide films.

Polarization Mechanism in Filled Tungsten Bronze Ba4Eu2Ti4Nb6O30 with Pinched P – E Hysteresis LoopsSupported by the National Natural Science Foundation of China (Grant Nos. 51790493 and 51961145105).

Pinched P–E hysteresis loops have been observed in filled tungsten bronze Ba4Eu2Ti4Nb6O30, indicating the presence of novel polarization mechanisms. We investigate the evolution of polar order in filled tungsten bronze Ba4Eu2Ti4Nb6O30, together with its dielectric properties over a wide temperature range, from 50 K to 773 K. The temperature dependences of the dielectric properties exhibit two low-temperature dielectric relaxations, at around 300 K (P1), and 100 K (P2), and a high temperature peak at 588 K with no frequency dispersion, indicating the ferroelectric transition temperature T c. Pinched P–E loops are observed in the temperature range between the low temperature relaxation at P1, and the ferroelectric transition. On cooling, the pinched P–E hysteresis loops open gradually, with increasing remnant polarization (P r). Two pairs of reversal electric fields indicate two types of polar reversal mechanisms, with an activated energy of 1.41 eV (E 1), and 0.94 eV (E 2), respectively. One corresponds to the field-induced transition from a nonpolar to a polar state, which dominates at a high temperature close to T c, while the other relates to the reversal of ferroelectric domains which stabilize gradually on cooling. At temperatures below 300 K, the polarization exhibits an evident decrease, probably related to the disruption of the polar order due to the dielectric relaxation at P1.

Temperature and frequency dependence of dielectric relaxations in YFeO3

We report a dielectric study in wide frequency and temperature ranges of yttrium orthoferrite, YFeO3, in order to obtain the temperature-frequency dependence of the dielectric relaxation processes. A mixture of oxide powders (Y2O3 and Fe2O3) was activated by high-energy ball milling. The mechanically activated powders were pressed and sintered at 1073 K. The cylindrical test samples were characterized at room temperature by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Broadband Dielectric Spectroscopy (BDS) studies were carried out at several temperatures to the best understanding of dielectric properties. The X-ray diffraction patterns show an orthorhombic YFeO3 single phase. The magnetic hysteresis loop shows a weak ferromagnetic behavior, characteristic of the YFeO3. Dielectric spectroscopy analysis allows identifying two relaxation processes, the first at temperatures between 193 and 253 K and second above 333 K, defined as low-temperature dielectric relaxation (LTDR) and high-temperature dielectric relaxation (HTDR), respectively. It was determined the activation around 0.4 eV and 1.0 eV for the LTDR and HTDR, respectively. It indicates a single and doubly ionized oxygen vacancy. Moreover, it was proved that a hopping model is the dominating mechanism in the studied material, due to the conduction mechanism follows the Jonscher power law.

Identification and comparison of peculiarities in physical properties of multiferroic morphotrophic phase boundary sintered BiFeO3-xPbTiO3 nano-ceramics

In this paper a comprehensive study that includes structural, magnetic, ferroelectric, dielectric and electrical characterizations of as-prepared and quenched MPB 1-x(BiFeO3)-x(PbTiO3)(BF-xPT) solid solutions prepared by wet chemical method has been presented. Qualitative and quantitively structural analysis revealed coexistence of rhombohedral/monoclinic, tetragonal and orthorhombic phases. Sensitivity to the experimental variables such as composition, synthesis conditions, sintering temperature and particle size, confirms the existence of a complex energy phase diagram where minima associated with various polymorphs lies almost at same level for MPB compositions. Magnetic characterizations reveal the presence of three anomalies, i.e., originating from the Néel transition, iso-structural transition and ferroelectric phase transition. Second anomaly detected to be the most sensitive to the quenching. Enhancement of magnetic moment in quenched compositions is a manifestation of strong interaction between spins of Fe2+ ions with the pinned point defect dipoles. Interestingly, thermal quenching enhanced the ferroelectric properties by removing the domain wall pinning. Saturated ferroelectric loops were observed only for quenched samples which is a consequence of strong domain wall de-pinning. Low temperature dielectric relaxation has been observed while the high temperature dielectric data of as-prepared composition exhibited Maxwell-Wagner and iso-structural phase transition. In contrast, along with these two features, quenched samples showed additional Curie transition. Enhancement in large polaron hoping in the low frequency region for the quenched composition has been confirmed via frequency dependent profile of the ac conductivity. Low value of activation energy of charge carriers for quenched compositions indicates large polaron hoping characteristics in BF-xPT solid solution.

Origins of dielectric relaxations in AgNb7O18 ceramic

The origins of the relaxor behavior and low-temperature dielectric relaxation processes in AgNb7O18 ceramic were studied and a new relaxation mechanism was proposed through combined bond valence energy landscape (BVEL) calculation and equivalent circuit modeling (ECM). The BVEL calculation suggests that the Ag1 displacement subjects to a double-well potential that originates from the anisotropic coordination symmetry and low coordination number at Ag1 site. The relaxor ferroelectric behavior, observed at 200 K ≤ T ≤ 270 K, is sensitive to thermal history and is ascribed to the Ag1 hopping in double-well potential. The temperature dependence of the matrix capacitance (obtained by ECM) is very similar with that of b axis (obtained by in situ XRD) at 123 K < T < 174 K. Therefore, a phase transition being responsible for the low-temperature dielectric relaxation is assumed. The combined BVEL and ECM techniques performed in this work are therefore quite helpful to understand the complicated relaxor phenomena and explore new relaxors.

Preparation of Ba2FeNbO6 double perovskite ceramics from molten-salt synthesized powders and their dielectric and magnetic properties

Double perovskite Ba2FeNbO6 (BFN) ceramics were prepared from the molten-salt synthesized BFN powders. The BFN powders have a cubic structure with $$ Fm\bar{3}m $$ space group, confirmed by X-ray diffraction patterns. Their average particle sizes increased with the annealing temperature and dwell time whereas decreased with the molten-salt content. X-ray photoemission spectra revealed both Fe2+ and Fe3+ ions present in the BFN powders and oxygen in the forms of lattice oxygen and oxygen vacancies, while niobium was present as Nb5+. The BFN ceramics exhibited significant frequency-dependent dielectric behavior, and two kinds of dielectric relaxor processes were observed. Their activation energies were 0.16 eV and 0.47 eV, respectively. The electron hopping process (between Fe2+ and Fe3+ ions) with the activation energy of 0.16 eV contributes the low-temperature dielectric relaxation process, and an activation energy of 0.47 eV associated with the high-temperature dielectric relaxation is originally from the barrier energy of the thermal motion of the oxygen vacancies. The dielectric relaxation observed at high-temperature region can be explained by Maxwell–Wagner interfacial polarization in the BFN ceramics. M–H plots of the BFN ceramics at different temperatures revealed their antiferromagnetic nature and a weak ferromagnetic behavior at 2 K. The remanent magnetization (Mr) of the BFN ceramics was measured to be 1.75 emu/g at 2 K and coercive field (Hc) as 1.26 kOe. Magnetization splitting was observed at ~ 290 K between the temperature-dependent magnetizations of the BFN ceramics, which were measured under zero field-cooled (ZFC) and field-cooled modes with an external field of 1 kOe. In the ZFC curve, a maximum peak was observed at 14 K, corresponding to the Neel temperature (TN). The TN of the present BFN ceramics shifts toward low temperature as compared with those prepared by conventional solid-state reaction methods. Such a reduction of the Neel temperature is ascribed to the size effect.

Magnetism, magnetocaloric and magnetodielectric properties of DyVWO6: a new aeschynite-type polar antiferromagnet

We report magnetic, magnetocaloric and magnetodielectric properties of a new polar magnet DyVWO6 crystallizing in the orthorhombic structure (Pna21). It consists of edge-shared dimers of VO6 and WO6 octahedra, which are connected by corner-sharing to a form three-dimensional network structure with Dy3+ cations located in the channels. The ordering of V3+ and W6+ ions in the octahedral sites breaks the inversion symmetry of the parent-CaTa2O6 structure (Pnma). Magnetic and heat capacity measurements show that the V3+ and Dy3+ ions undergo antiferromagnetic transitions at ∼ 5 K and ∼ 2.5 K, respectively. Isothermal magnetization shows a metamagnetic transition at the applied magnetic field of 1 T at 2 K that corresponds to a transition from the antiferromagnetic (AFM) to ferromagnetic (FM) spin structure. Further, this compound exhibits significant magnetocaloric effect (MCE) with a maximum magnetic entropy change (−∆) of 8.45 J Kg−1 K−1 at 7 T. In addition to this, we observe a drop in dielectric constant below with a low temperature dielectric relaxation.

Dielectric relaxations in fine-grained SrTiO3 ceramics with Cu and Nb co-doping

SrTi1−3x(CuxNb2x)O3 (x = 0.05, 0.1, 0.15 and 0.2) ceramics were synthesized using a solid-state reaction method at 1400 °C in air. Their structures, valence states, conduction mechanisms and dielectric properties were investigated in detail. Fine grains and vibration modes of the samples related to the doping effects were observed. A distorted pseudo-cubic structure was confirmed by XRD and Raman spectroscopy. All ceramics exhibited a colossal dielectric constant and dielectric relaxations. The dielectric constant in the low- and high-frequency ranges for x = 0.05 and 0.1 was ascribed to the contribution of the grain boundary and grain, respectively, and the electrode polarization was significant at x = 0.15 and 0.2. The dielectric relaxation peaks obeyed the T−1/4 law at low temperature for all samples, confirming the polaron relaxation process. The electric modulus analysis confirmed that low-temperature dielectric relaxation was related to the grain response, and the electric conduction exhibited the same behavior with the dielectric relaxation. The variable-range-hopping conduction indicated a highly distorted structure and the localization of carriers in SrTi1−3x(CuxNb2x)O3, which was consistent with the XRD and Raman results. The mixed-valence structure of Cu was identified by XPS, and the polaron hopping between the mixed-valence Cu ions was supposed to be responsible for the dielectric relaxation and electric conduction.

Analysis of the low-temperature dielectric relaxation in $$\hbox {CH}_{3}\hbox {NH}_{3}\hbox {PbI}_{3}$$ CH 3 NH 3 PbI 3 partially substituted with Sn and Ba

In this work, we partially replaced Pb with non-toxic elements to form hybrid perovskites $$\hbox {CH}_{3}\hbox {NH}_{3} \hbox {Pb}_{0.9}\hbox {M}_{0.1}\hbox {I}_{3}$$ , where $$\hbox {M} = \hbox {Sn}^{2+}$$ and $$\hbox {Ba}^{2+}$$ and studied their dielectric properties. According to room-temperature Rietveld analysis, all compositions with 10 % substitution were tetragonal with a space group $$I4/\hbox {mcm}$$ . A notable dielectric relaxation was detected at $$T\sim 60$$ – $$100\hbox { K}$$ , far below the orthorhombic-to-tetragonal phase transition at $$T\sim 155\hbox { K}$$ . To clarify the conflicting literature reports, the dielectric relaxation was fitted to both Arrhenius and Vogel–Fulcher models and showed no indication of the finite (i.e. non-zero) dipolar freezing temperature. It is concluded, therefore, that the low-T dielectric dispersion is a signature of the Debye-type thermally activated re-orientation of the non-interacting electric dipoles with an activation energy of $$E_\mathrm{{a}} \sim 123-171\hbox { meV}$$ .

Ferroelectric transitions and relaxor behavior in Ba4Sm2(Ti1-xZrx)4Ta6O30 tungsten bronze ceramics

Ferroelectric transitions of Ba4Sm2(Ti1-xZrx)4Ta6O30 (x = 0, 0.1, 0.3, 0.5, 0.7, 1) tetragonal tungsten bronze ceramic samples were investigated systematically with the crystal structure, dielectric properties, and Raman vibrations. For x = 0 and x = 0.1, the tetragonal tungsten bronze structure with space group P4bm was determined. A first order ferroelectric transition with a sharp, frequency independent dielectric permittivity maximum was indicated for x = 0 and x = 0.1, while a strong low temperature dielectric relaxation was also determined for x = 0.1. For x = 0.3, 0.5, 0.7, and 1, a tetragonal tungsten bronze structure with space group P4/mbm was determined. Relaxor ferroelectric behavior with strong frequency dispersion becomes dominating for compositions of x ≥ 0.3. Compositions of x = 0 and 0.1 exhibited nonlinear polarization electric-field (P-E) hysteresis loops at room temperature, while other compositions only exhibited linear curves. Raman spectra presented distinct vibration modes for x = 0 and x = 1, and the intensity of some modes showed sudden rising or falling with increasing x from x = 0 to x = 0.1. The abrupt change from normal ferroelectric to relaxor behavior at x = 0.1 in this system is quite different from the gradual change in the Ba4Sm2Zr4(NbxTa1-x)6O30 system, and both the different electron configuration and ion size mismatch between Zr and Ti should play important roles in the crossover here.