Low-temperature Dielectric Anomaly Research Articles

Low-temperature dielectric anomaly arising from electronic phase separation at the Mott insulator-metal transition

Coulomb repulsion among conduction electrons in solids hinders their motion and leads to a rise in resistivity. A regime of electronic phase separation is expected at the first-order phase transition between a correlated metal and a paramagnetic Mott insulator, but remains unexplored experimentally as well as theoretically nearby T = 0. We approach this issue by assessing the complex permittivity via dielectric spectroscopy, which provides vivid mapping of the Mott transition and deep insight into its microscopic nature. Our experiments utilizing both physical pressure and chemical substitution consistently reveal a strong enhancement of the quasi-static dielectric constant ε1 when correlations are tuned through the critical value. All experimental trends are captured by dynamical mean-field theory of the single-band Hubbard model supplemented by percolation theory. Our findings suggest a similar ’dielectric catastrophe’ in many other correlated materials and explain previous observations that were assigned to multiferroicity or ferroelectricity.

High-Temperature Dielectric and Relaxation Behavior of Tantalum-Doped Sodium Bismuth Titanate-Barium Titanate Ceramics

Ta-doped 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 [(1 − x)(0.94BNT–0.06BT) − xTa (x = 0.00–0.02)] ceramics were prepared by a solid-state reaction process. X-ray diffraction (XRD) shows a rhombohedral symmetry with space group R3c for all samples. The introduction of Ta moves the low-temperature dielectric anomaly to lower temperature while the high-temperature dielectric anomaly becomes broader, although the dielectric permittivity is depressed slightly. Dielectric permittivity (ɛr) as a function of temperature for the sample x = 0.02 shows a stable and broad relative permittivity ∼ 3500 from 50 °C to 400 °C with low dielectric loss (tanδ) ≤ 0.02 from 80 °C to 340 °C (at 1 kHz). A Debye-like relaxation was found in the paraelectric region. The activation energy of relaxation frequency (Ea) of the samples x = 0, 0.005, 0.01, 0.02 is 0.62 eV, 0.84 eV, 1.15 eV and 1.20 eV and the activation energy of conduction (Econ) is 0.56 eV, 0.72 eV, 0.89 eV and 0.90 eV for x = 0.00, x = 0.005, x = 0.01 and x = 0.02, respectively, which are derived from isothermal impedance spectroscopy and admittance spectroscopy, respectively. The dielectric relaxation behavior is related to the migration of oxygen vacancies and cation vacancies dependence on the concentration of Ta.

Frequency dispersion and temperature dependence of electrical behaviours in 0.4Bi(Ni1/2Zr1/2)O3–0.6PbTiO3

In this study, the temperature dependence of dielectric behaviours and the polarisation and electrostrain responses of 0.4Bi(Ni1/2Zr1/2)O3–0.6PbTiO3 (0.4BNZ–0.6PT) were studied to better understand their electrical performance. A Rietveld refinement analysis of the XRD pattern, supported by bright-field TEM images, suggests the coexistence of tetragonal and pseudocubic phases. Above 398 K, the normalised piezoelectric coefficient (d33∗) increases due to heating, reaching 941 pm/V at 453 K. Most importantly, 0.4BNZ–0.6PT displays obvious frequency dispersion in the ferroelectric state. A low-temperature dielectric anomaly occurred at approximately 175 K that was related to the origin of the frequency dispersion. The planar electromechanical coupling constant (kp) also changed considerably after heating, increasing from 0.14 at 90 K to 0.43 at 300 K. This indicates an intrinsic relationship between the dielectric and piezoelectric behaviours. Our study demonstrates that there is a significant difference between typical ferroelectrics and the ferroelectric state of 0.4BNZ–0.6PT relaxor ferroelectrics.

Structural and electrical effects of Ag substitution in tungsten bronze Sr2AgxNa1−xNb5O15 ceramics

In this work, the correlation between the composition and structure of the Sr2AgxNa1−xNb5O15 (SANN, 0.00 ≤ x ≤ 1.00) ferroelectric ceramic was investigated. The SANN ceramic was synthesized via the solid-state reaction method and equivalently charged Ag+ cations were doped in the SANN matrix to investigate the relationship between the composition and structure. Investigation of the ceramic's properties mainly focused on two aspects: (1) enhancement of the dielectric and ferroelectric properties caused by the dense structure, increased orthorhombic distortion, and increased disorder distribution; (2) the low-temperature dielectric anomaly evolved from a slightly smeared out phase transition into an obvious ferroelastic/ferroelectric phase transition by Ag+ introduction in SANN. In addition, improved light transmittance was realized at x = 1.00, which may be related to the homogeneous equiaxed microstructure and uniform size of the ceramic grains.

Investigation of room temperature multiferroic properties in sol-gel derived gadolinium, cobalt doped BiFeO3 nanoceramics

Detailed studies of structural, dielectric, magnetic, ferroelectric, and optical properties of a chemically sol-gel synthesized series of Bi1 − xGdxFe1 − yCoyO3 (x = 0, 0.1 and y= 0, 0.1) nanoceramics are presented. These nanoceramics attract attention as promising candidates for application as room temperature multiferroics in spintronics devices, exhibiting an optimistic set of properties. The average particle sizes of all the samples are calculated, and it was found to be ∼45 nm from the Scanning Electron Microscopy image. The dielectric behavior of the prepared nanoceramics was investigated over a wide frequency (100 Hz–103 kHz) and a temperature (35–350 °C) range. Interestingly, a low temperature dielectric anomaly is observed at around 170 °C for Bi0.9Gd0.1Fe0.9Co0.1O3 due to the magnetoelectric coupling. A noticeably large value of dielectric constant (∼990) and a low tan δ loss have also been observed for this nanoceramic at room temperature. The complex impedance spectroscopic analysis was also performed by plotting Nyquist plots, and the corresponding activation energies are evaluated from the Arrhenius fittings. Magnetization measurements of the samples reveal the presence of weak ferromagnetism due to surface spins as well as a noticeable improvement in the magnetic properties with Gd and Co doping. Ferroelectric properties have also been observed to be slightly improved because of doping, though it becomes somewhat lossy for Gd and Co doped samples. All the promising improved properties directly depend on the subsequent increase in the bandgap from 2.29 eV to 2.45 eV, with doping in the parent BiFeO3 compound confirmed from UV–Vis spectroscopy.

The origin of weak coupling between polar clusters in Ta2O5-doped 0.94Bi0.5Na0.5TiO3-0.06BaTiO3

Ta2O5-doped 0.94Bi0.5Na0.5 TiO3-0.06BaTiO3 [()(0.94BNT-0.06BT)-xTa ()] ceramics were prepared by a solid-state reaction technique. The single perovskite structure with space group R3c of the ceramics was identified by X-ray diffraction (XRD). Raman spectroscopy revealed the evolution of the local structure with Ta2O5 concentration. The temperature dependence of dielectric permittivity of the ceramics were deconvoluted by three Gaussian distribution functions which suggests that three dielectric anomalies exist in this system. The low-temperature dielectric anomaly exhibits a typical relaxor behavior with strong frequency dispersion (reentrant relaxor behavior). The activation energy derived from the V-F law is 0.298 eV, 0.338 eV, 0.412 eV and 0.449 eV, for , 0.005, 0.01 and 0.02, respectively. The mid-temperature and high-temperature anomalies are attributed to two structure phase transitions. The increase of the activation energies suggests that the coupling between polar clusters or polar nanoregions (PNRs) becomes weaker. The origin of the interaction between PNRs and phase transition behavior has been proposed according to the average structure, local structure and defect compensation mechanism of the system.

Low-temperature dielectric anomalies in KSr2Nb5O15 ceramics with tetragonal tungsten bronze structure: The effect of microstructure

The dielectric behaviors of the tetragonal tungsten bronze (TTB) structured ceramics have been extensively studied by doping other elements due to their compositional flexibility. The microstructure of ceramics will inevitably vary with the change of chemical composition. In order to clarify the microstructural effect, the dielectric properties of the KSr2Nb5O15 (KSN) ceramics with various grain microstructures were investigated. Successful preparation of the KSN ceramics with same chemical compositions and different microstructures could be attributed to the usage of microcrystalline powder with different particle size distributions. It was found that the dielectric characteristics varied significantly with microstructures of the KSN ceramics. A new low-temperature dielectric anomaly appeared in the ceramics with a duplex structure. In addition, the peak shifted with the change of grain size distribution. These results suggested that the microstructural influence should not be neglected for the studies of the origin of the dielectric anomalies in TTB structured ceramics.

Low-temperature dielectric anomaly in Bi0.5K0.5TiO3

Here, we report our studies on Bi0.5K0.5TiO3 (BKT) prepared by conventional solid-state reaction method and two-step sintering method to further understand the dielectric dispersion around room temperature. The coexistence of the tetragonal and cubic phases is confirmed. Most importantly, we find an extra dielectric loss anomaly around –100 °C. The polarization response suggests the contribution of the hysteresis-free polarization. Correspondingly, a Raman-active mode around 75 cm−1 is frozen on cooling. A model is proposed to interpret the origin of the low-temperature dielectric anomaly.

High-temperature dielectric relaxations in (Ce, Y) codoped BaZrO3 ceramics

(Ce, Y) codoped BaZrO3 ceramics, BaZr0.1Ce0.7Y0.2O3-δ and BaZr0.4Ce0.4Y0.2O3-δ ceramics were synthesized via solution combustion based on the glycine nitrate process. The dielectric properties of the samples have been investigated in the temperature range from 300 to 1000 K and frequency range from 102 Hz to 106 Hz. It is observed that the samples exhibit colossal dielectric behavior in the temperature below ∼600 K. The colossal dielectric behavior is composed of two sets of relaxations with the low- and high-temperature relaxations being caused by the dipole pairs of -Y and -Y, respectively. In the temperature higher than ∼600 K, the samples show two dielectric anomalies with extremely large dielectric permittivity over 106. It is argued that the low-temperature dielectric anomaly is the typical pseudo-relaxor behavior caused by ion-hopping-motion. While the high-temperature dielectric anomaly results from the negative capacitance effect.

New Cu3TeO6 Ceramics: Phase Formation and Dielectric Properties

Targeting low temperature cofired ceramics (LTCC) applications and base-metal electrode multilayer ceramic capacitors (BME-MLCCs), ceramics of a new composition, tricopper tellurate (Cu3TeO6), are reported here. The crystal structure of Cu3TeO6 was determined to be cubic, Ia3, with the unit cell parameter a = 9.538 Å. The sequence of phase formation is proposed with the oxidation of tetravalent tellurium (Te(4+)) into hexavalent tellurium (Te(6+)) as a key step for the formation of Cu3TeO6. Ceramics sintered at 865 °C with densities of 94% exhibit two dielectric anomalies in the temperature dependence of the dielectric response, around -150 °C and +50 °C, respectively, accompanied by obvious frequency dispersion of the relative permittivity (ε(r)) and dielectric losses (tan δ), with an Arrhenius like behavior. A temperature stable dielectric region (near room temperature) formed between the two anomalies with ε(r) ∼ 12 and tan δ ∼ 0.01, and a very low positive temperature coefficient of the relative permittivity (TCε(r)), 2.07 × 10(-4) °C(-1), was obtained in the same region. The low temperature dielectric anomaly is associated with the possible mixed Cu(+)/Cu(2+) valence in Cu3TeO6 ceramics, while the high temperature anomaly is attributed to point defect ordering, including VO, CuCu2+(+)', which might be formed during sintering. Therefore, Cu3TeO6 ceramics are of interest in view of not only the possible applications in BME-MLCCs, LTCC, and related technologies, but also for their possible compatibility with low cost abundant Cu electrodes.

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.

Structural Effects Behind the Low Temperature Nonconventional Relaxor Behavior of the Sr2NaNb5O15 Bronze

An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr(2)NaNb(5)O(15) (SNN) compound has been performed in the 300-100 K temperature range, by combining X-ray, neutron diffraction, and transmission electron microscopy with dielectric measurements, in order to clarify the structural effects responsible for the observed low temperature dielectric properties. Interestingly, a relevant second anomaly in the dielectric constant, in addition to the ferroelectric (FE) to paraelectric (PE) transition at T(C) = 518 K is found at T ≈ 240 K, revealing a relaxor-like behavior of the material at low temperature. This phenomenon has been previously observed in FE perovskite-type phases and referred to as the re-entrant phenomenon. However, FE polarization tends to vanish below this low temperature dielectric anomaly and this fact is not expected for a classical relaxor-ferroelectric phase. Although there is no structural transition from RT to 100 K, there is a change in the elastic properties of the material in the considered temperature range and the intense anomaly at ~240 K could be associated to a smeared-out phase transition to a frustrated FE/ferroelastic (FEL) low temperature state in correlation with subtle structural effects.

On the phase identity and its thermal evolution of lead free (Bi1/2Na1/2)TiO3-6 mol% BaTiO3

Temperature-dependent dielectric permittivity of 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 (BNT-6BT) lead-free piezoceramics was studied to disentangle the existing unclear issues over the crystallographic aspects and phase stability of the system. Application of existing phenomenological relaxor models enabled the relaxor contribution to the entire dielectric permittivity spectra to be deconvoluted. The deconvoluted data in comparison with the temperature-dependent dielectric permittivity of a classical perovskite relaxor, La-modified lead zirconate titanate, clearly suggest that BNT-6BT belongs to the same relaxor category, which was also confirmed by a comparative study on the temperature- dependent polarization hysteresis loops of both materials. Based on these results, we propose that the low-temperature dielectric anomaly does not involve any phase transition such as ferroelectric- to-antiferroelectric. Supported by transmission electron microscopy and X-ray diffraction experiments at ambient temperature, we propose that the commonly observed two dielectric anomalies are attributed to thermal evolution of ferroelectric polar nanoregions of R3c and P4bm symmetry, which coexist nearly throughout the entire temperature range and reversibly transform into each other with temperature.

Phases and domain structures in tungsten-doped Pb(Mg1∕3Nb2∕3)1−xTixO3 (x=0.35) crystal

Dielectric permittivity, electric polarization, and domain structure have been carried out as a function of temperature in 0.5 mol % WO3-doped Pb(Mg1∕3Nb2∕3)0.65Ti0.35O3 (PMNT35%) single crystal. Two first-order-type temperature-dependent phase transitions, monoclinic (rhombohedral)→monoclinic→cubic, were observed near 310 K (290 K) and 440 K (435 K) upon heating (cooling), respectively. “Monoclinic (rhombohedral)” represents that dominant monoclinic domains coexist with a small fraction of rhombohedral domains. Reorientations of monoclinic domains are mainly responsible for the low-temperature steplike dielectric anomaly. This study shows that WO3 dopant can enhance monoclinic domains whose dynamics play an important role in temperature-dependent structural transitions.

Low Temperature Microwave Properties of Bismuth Based Dielectric Ceramics

The microwave properties of Bi2(Zn1/3Nb2/3)2O7 and Bi2(Zn1/3Ta2/3)2O7 pyrochlore dielectric ceramics at low temperature were investigated. The ceramic samples were prepared by conventional mixed oxide technology. The structures of the samples were identified as monoclinic pyrochlores. The microwave properties of the cylindrical samples were measured of TE01δ modes at the temperature range of 10 K to 300 K in a cylindrical resonant cavity. The occupying of Nb5+ and Ta5+ in B sites lead different dielectric constants and low temperature dependencies of Q value. A low temperature dielectric anomaly in BZN was observed at 69 K which may be associated with a slight phase transition of the monoclinic pyrochlore Bi2(Zn1/3Nb2/3)2O7 polymorphs. The correlations between the structure, morphology and microwave dielectric properties were discussed.

Polarization Switching in Heterophase Nanostructures: PLZT Relaxor Ceramics

The polarization switching is experimentally investigated in hot-pressed PLZT-x/65/35 ceramics with a lanthanum content from 5 to 12 at. %. The specific features in the temperature dependence of the polarization switching in a heterophase state are interpreted by analyzing the change in the switched charge measured over wide ranges of fields and temperatures. Particular emphasis is placed on the role of depolarization fields induced by interphase boundaries. A model of the evolution of polydomain nanostructures with a change in the temperature and in the response to an external field is considered. It is assumed that the low-temperature dielectric anomaly and the temperature hysteresis are associated with the loss of stability of a large-scale domain structure and its transformation into a nanodomain state.

The low temperature dielectric anomaly of bismuth-based ceramics

A series of bismuth-contained ceramics have been prepared and the dielectric properties of typical samples are investigated in the temperature range of 300 to 10K. There exists a dielectric anomaly occurred common to all the bismuth, zinc(nickel), niobium(tantalum) oxide pyrochlores. The possible polarization mechanism is suggested and discussed by carrying out the phenomenological model analysis. The tendency of the dielectric loss towards the high frequency of the giga hertz level is postulated. The evidence of the shifted maxima of dissipation factors caused by the substitution of cations in the concerned oxide pyrochlores provides a potential application for high microwave frequency with no degraded dielectric properties, especially the quality factor of the dielectric resonators.

Impurity effect on low-temperature polarisation of the charge-density-waves in o-TaS 3

The temperature dependence of the low-temperature dielectric response is studied in o-TaS 3 samples doped by Nb, Se, and Ni and for nominally pure ones. It is found that the low-temperature dielectric constant depends anomalously on doping and is higher for doped crystals, whereas the temperature dependence of the characteristic time of all samples follows the activation law with nearly the same activation energy ∼400 K ( T > 20 K). The observed behaviour is inconsistent with all available explanations of the low-temperature dielectric anomaly.

Synthesis and dielectric properties of new compounds with pyrochlore type structure

Abstract Abstract A series of new compounds with cubic pyrochlore structure and mixed occupation of lattice sites in the general formula of (Bi1.5B'xB”0.5yZn0.5-x-0.5y) (Zn0.5B'''yNb1.5-y)(O7-x-yFx+y)(where B'=Li,Na;B”=Ca,Mg, Ni,Zn; B'''=Sn,Ti,Zr; 0≤x, y<0.4) were synthesized. The structure of these compounds has been recognized by XRD. Dense and uniform microstructure of dielectric ceramic body could be obtained with sintering temperatures lower than 1000°C. A low temperature dielectric anomaly has been observed in these materials, which were influenced by the processing and the cation and anion substitutions.