Relaxor-like Dielectric Anomaly Research Articles

Significance of isostructural distortion and strong magnetoelastic coupling in the weak ferromagnet YFe0.9Cr0.1O3

We report the appearance of weak ferromagnetism above the long-range canted antiferromagnetic order TN = 553 K at which the Fe-Cr interactions become dominant in YFe0.9Cr0.1O3. The weak ferromagnetic order above TN appears due to the increasing magnetic interactions in Fe-sublattices resulting in a canted antiferromagnetic transition at TFe-Fe = 620 K. Electrical hysteresis loop studies and Positive-Up Negative-Down protocol measurements reveal spontaneous but weak ferroelectricity in the system. Dielectric measurements as a function of temperature and frequency reveal a relaxorlike dielectric anomaly within a temperature interval of 400–500 K and anomalous signatures at and temperatures above TN. Synchrotron x-ray diffraction studies over a broad temperature range, 298–873 K, illustrate a significant magnetoelastic coupling at TN and around TFe-Fe. Isostructural distortions are further revealed over the temperature range of 400–500 K from the x-ray diffraction studies. Analyses of the diffraction patterns reveal the delicate interplay between the in-plane and out-of-plane exchange interactions over TN and in isostructural distortion regions. We strongly argue that the ferroelectricity in YFe0.9Cr0.1O3 results from the magnetostriction and Dzyaloshinskii-Moriya interaction manifested through a significant magnetoelastic coupling at TN and TFe-Fe.

Electron diffraction study of crystal structures of (Sr1−xBax)2Nb2O7

Selected-area electron diffraction and convergent-beam electron diffraction (CBED) techniques are used to reveal the Ba doping effect on the crystal structure of (${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{)}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ [SBN(x)] in the temperature range from 293 to 693 K. Ba doping of ${\mathrm{Sr}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ (SN) causes weakening of the incommensurate modulation and deviation from the C-centered lattice. On the other hand, symmetries of fundamental reflections in CBED patterns of SBN(x) are seen to be the same as those of SN, showing a small structural deviation from Cmc${2}_{1}$. Crystal symmetries above and below the temperature of a relaxorlike dielectric anomaly in SBN ($x=0.32$) at 465 K remain unchanged. CBED experiments with a nanometer-sized electron probe do not show any indication of the presence of a nanodomain around this temperature. However, the presence of a few-micron-sized polar-inversion domains is found at around 573 K in SBN ($x=0.32$). The origin of the reported relaxorlike dielectric anomaly in SBN ($x=0.32$) is discussed on the basis of the experimental results of CBED.

Effect of Li+ ion occupancy on microstructure and dielectric characteristics in KSr2Nb5O15 tungsten bronze ceramics

Li-doped KSr2Nb5O15 (KSN) tungsten bronze ceramics were prepared by the conventional solid-state method. Fine KSN crystalline fabricated by a two-step molten salt synthesis were used as raw material. The effects of Li2CO3 content on microstructure and dielectric properties of the ceramics were investigated. It was found that a small addition of Li2CO3 was effective in inhibiting the abnormal grain growth and crack formation. Investigations of the microstructure indicated that grain size transformed from normal to bimodal distribution when Li2CO3 content was increased. For the case of 8 mol%, a unique keyboard structure was observed in coarse grains. Dielectric spectra of all compositions displayed temperature-stable high dielectric constant. A broad low temperature relaxor-like dielectric anomaly was observed in addition to the high temperature maximum around 70°C. It was interesting to note that the high temperature peak gradually became significant with the increase of Li2CO3 content.

Ferroelectric charge order enhanced by magnetic frustration in dimer Mott insulators

We investigate the effect of magnetic frustration on the dielectric properties of dimer Mott insulators. We adopt the two-dimensional quarter-filled extended Hubbard model for $\ensuremath{\kappa}\text{\ensuremath{-}}(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF}){}_{2}X$ with an additional coupling with the dimer lattice motions, and numerically obtain the ground state. In the parameter range for strong magnetic frustration, an interdimer bond-length alternation mode that lifts the quasidegeneracy of classical spin configurations is strongly coupled to the electron system; this alternation mode occurs even with an extremely small electron-lattice coupling constant. Furthermore, the bond-length alternation induces in the dimers electric dipoles, which align along the same direction. A dimer Mott insulator phase with the ferroelectric charge order is obtained in the region. These results are consistent with the relaxorlike dielectric anomaly in $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}_{2}{\mathrm{Cu}}_{2}{(\mathrm{CN})}_{3}$.

High-temperature dielectric properties of TbFeO3 ceramics

TbFeO3 ceramics were prepared via conventional solid-state reaction route. By means of the complex dielectric permittivity, electric modulus, and impedance analysis, the high-temperature dielectric properties of TbFeO3 were investigated in the temperature range from 300 to 1073K and frequency range of 102–107Hz. Rich dielectric phenomena, including three dielectric responses (R1–R3), a relaxor-like dielectric anomaly and negative capacitance were found in the order of ascending temperature. The low-temperature relaxation (R1) near room temperature is caused by the surface-layer effect. The high-temperature relaxations R2 and R3 were argued to be related to the dipolar and Maxwell–Wagner relaxation, The anomaly is confirmed to be caused by the negative capacitance effect.

Coexistence of the soft mode and sub-THz central peak in ferroelectric BaTiO3/(Ba,Sr)TiO3 superlattices

Near- and sub-Terahertz dynamics of soft and Debye-type central modes was studied by the polarized Raman spectroscopy in ferroelectric BaTiO3/Ba0.3Sr0.7TiO3 (BT/BST) superlattice in the temperature range of 80–400K where system undergoes the series of phase transitions: paraelectric-tetragonal–monoclinic in BT layers and paraelectric-orthorhombic-monoclinic in BST layers. It was shown that temperature evolution of these modes can be described within the model of coexisted damped harmonic oscillator and Debye relaxator. The occurrence of the pronounced central mode can explain the recently observed relaxor-like dielectric anomaly in such superlattice.

Dielectric Response of Multiorbital Molecular Compounds (TTM-TTP)X (X = AuI2 and I3)

The temperature dependence of the dielectric constant in the organic molecular compounds (TTM-TTP)X (X = AuI2, I3) was investigated, where the intramolecular charge degrees of freedom arising from the mixing of orbitals are discussed. We observed a large relaxor-like dielectric anomaly in single crystals along the TTM-TTP molecular long axis but not along the short axis as well as the suppression of such anomaly under dc bias fields. These results indicate the intramolecular charge fluctuation between two fragments of the molecular orbitals in one TTM-TTP molecule with inhomogeneity. The relations between the anomaly, natural defects, spin singlet transition, and unpaired localized spins are discussed.

High-temperature relaxations in CaF2 single crystals

Abstract The dielectric properties of CaF2 single crystals were investigated in the temperature range from room temperature to 773 K and the frequency range of 50 Hz–10 MHz. A Debey-like relaxation and a relaxor-like dielectric anomaly were observed. Raman spectrum and the differential scanning calorimeter measurement revealed the coexistence of native defects of fluorine vacancies ( V F ) and interstitial fluorine ions ( F ′ i ) and external defect of oxygen ions. Impedance analysis showed that the Debey-like relaxation results from the mobility of V F , while the relaxor-like behavior is associated with the relaxation caused by complexes of O ″ − V F .

Observation of a new cryogenic temperature dielectric relaxation in multiferroic Bi7Fe3Ti3O21

In this communication, we report details of structural, magnetic, and dielectric properties of multiferroic Bi7Fe3Ti3O21 at cryogenic temperatures. The low temperature impedance spectroscopic studies showed a relaxor-like dielectric anomaly which follows Vogel-Fulcher relation with freezing temperature (TVF) of 33 K. Temperature and field dependent magnetization indicate the onset of a possible antiferromagnetic ordering below 150 K. Variable temperature powder neutron diffraction studies indicate no structural change down to 22 K. The appearance of the low temperature dielectric anomaly along with evidence of magnetic ordering at low temperatures suggests the presence of magnetic domains of mesoscopic scale within the bulk matrix.

Collective Excitation of an Electric Dipole on a Molecular Dimer in an Organic Dimer-Mott Insulator

The terahertz response in 10-100 cm(-1) was investigated in an organic dimer-Mott (DM) insulator κ-(ET)(2)Cu(2)(CN)(3) that exhibits a relaxorlike dielectric anomaly. An ~30 cm(-1) band in the optical conductivity was attributable to collective excitation of the fluctuating intradimer electric dipoles that are formed by an electron correlation. We succeeded in observing photoinduced enhancement of this ~30 cm(-1) band, reflecting the growth of the electric dipole cluster in the DM phase. Such optical responses in κ-(ET)(2)Cu(2)(CN)(3) reflect an instability near the boundary between the DM-ferroelectric charge ordered phases.

Photoinduced Growth of Ferroelectric Charge Order in Organic Dimer-Mott insulator

Layered triangular organic dimer Mott (DM) insulator κ -(ET)2 Cu2 (CN)3 was shown to exhibit a relaxor-like dielectric anomaly below 40 K with strong dispersion relation, reflecting its electric dipole glass (ferroelectric charge order; FCO) nature[1, 2]. The dielectric anomaly in κ -(ET)2 Cu2 (CN)3 indicates that this compound is located in the vicinity of the DM-FCO phase boundary, where ferroelectric fluctuation such as the electric dipole glass state or the polar cluster is formed in the DM phase. Optical excitation of the DM-FCO competing state by an ultrashort light pulse enables us to achieve dramatic responses, such as photoinduced ferroelectricity, photoinduced growth of the electric dipole glass or the polar clusters.

Synthesis and Characterization of a New Sillenite Compound—Bi12(B0.5P0.5)O20

A new sillenite compound, Bi12(B0.5P0.5)O20, was synthesized using a solid‐state reaction method. The stoichiometry was confirmed by XRD analyses, microstructural investigations, and quantitative elemental analysis. An investigation of the dielectric properties at frequencies from 100 Hz to 1 MHz revealed a broad, highly frequency‐dispersive, relaxor‐like dielectric anomaly, which appeared in the temperature range of −80°–100°C. The permittivity, Q×f value, and temperature coefficient of the resonant frequency, measured at ∼5.5 GHz, were determined to be 37.4, 850 GHz, and −19 ppm/K, respectively.