Impedance response of polycrystalline tungsten oxide

Polycrystalline tungsten oxide (WO₃) pellets were prepared by conventional ceramic processing technology. The ac small-signal electrical data acquired in the frequency (f) range 100 Hz≤f≤1 MHz at temperature (T) ranging the 31-100 °C revealed distinct semicircular relaxation in the impedance plane. This relaxation indicates device behavior originating from the grain boundaries. The lumped grain impedance associated with the device action remained too small to detect when the large resistance scale is realized. The semicircular relaxation is thermally activated indicating 0.58 eV as the activation energy for the relaxation time.     

Structural and ac electrical properties of a newly synthesized single phase rare earth double perovskite oxide: Ba2CeNbO6

A single phase rare earth double perovskite oxide Ba₂CeNbO₆ (BCN) is synthesized by solid-state reaction technique for the first time. The X-ray diffraction pattern of the sample at room temperature shows monoclinic structure, with the lattice parameters, a=5.9763 Å, b=5.975 Å and c=8.48 Å and β=90.04°. Impedance spectroscopy is used to study the ac electrical behavior of this material as a function of frequency (10²-10⁶ Hz) at various temperatures (30-450 °C). A relaxation is observed in the entire temperature range. Conduction mechanism is investigated by fitting the complex impedance data to Cole-Cole equation. Complex impedance plane plots show only one semicircular arc, indicating only the grain contribution of dielectric relaxation. The scaling behavior of imaginary part of electric modulus (M″) and imaginary part of electrical impedance (Z″) suggests that the relaxation describes the same mechanism at various temperatures. The frequency dependence of conductivity is interpreted in terms of the jump relaxation model and is fitted to Jonscher's power law. The values of dc conductivities extracted from the Jonscher power law varies from 2.79×10⁻⁷ to 3.5×10⁻⁵ Sm⁻¹ with the increase in temperature from 100 to 450 °C. The activation energies (0.37 eV) extracted from M″(ω) and Z″(ω) peaks are found to follow the Arrhenius law.     

Analysis of the AC electrical data in the Davidson-Cole dielectric representation

The representation of the AC electrical data in the complex plane reveal two major classifications of the relaxation processes known as ideal (Debye) and non-ideal (non-Debye) types. The non-Debye relaxation has been empirically observed via Cole-Cole (C-C), Davidson-Cole (D-C), and Havriliak-Negami (H-N) responses. Each of these non-Debye relaxations is visualized with an equivalent circuit similar to the ideal relaxation. Both ideal and C-C relaxations reveal semicircular behavior in the complex plane while D-C and H-N relaxations deviate from the usual semicircular loci known as skewed behavior. The extracted equivalent circuit elements are essentially non-Debye for both D-C and H-N relaxations possessing complexity in the relaxation time. The analytical method of extracting these elements in conjunction with the empirical parameters of the D-C relaxation is described using conventional (real) domain and complex domain. The curve fitting procedure provided extremely small error for the complex domain analysis. The behavior of the D-C relaxation function and the depression parameter β are also discussed using ωτ=1 and ωτ≠1 corresponding to the maximum of the imaginary part of the impedance (Z^*) or permittivity (ε^*).     

Complex impedance spectroscopic analysis of Mn-modified Pb(Zr0.65Ti0.35)O3 electroceramics

The polycrystalline samples of Pb(Zr_0.65−xMn_xTi_0.35)O₃ (PZMT) (x=0, 0.05, 0.10, 0.15) were prepared by a high-temperature solid-state reaction technique. Detailed studies on the effect of compositional variation of manganese (Mn) on the electrical behavior (complex impedance Z*, complex modulus M*, electrical conductivity and relaxation mechanisms) of the PZMT systems have been carried out by a nondestructive complex impedance spectroscopy (CIS) technique at 400 °C. The Nyquist plots suggest that the grains only are responsible in the conduction mechanism of the materials. The occurrence of single arc in the complex modulus spectrum of all the compositions of Mn confirms the single-phase characteristics of the PZMT compounds, and also confirms the presence of non-Debye type of multiple relaxation in the material.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).     

Complex impedance spectroscopy studies of (La0.70−xNdx)Sr0.30Mn0.70Cr0.30O3 (x≤0.30) perovskite compounds

The transport properties of Nd-doped perovskite materials (La_0.7−xNd_x)Sr_0.3Mn_0.7Cr_0.3O₃ (x≤0.30) were investigated using impedance spectroscopy techniques over a wide range of temperatures and frequencies. AC conductance analyses indicate that the conduction mechanism is strongly dependent on temperature and frequency. The DC conductance plots can be described using the small polaron hopping (SPH) model, with an apparent reduction of the polaron activation energy below the Curie temperature T_C. Complex impedance plots exhibit semicircular arcs described by an electrical equivalent circuit. Off-centered semicircular impedance plots show that the Nd-doped compounds obey to a non-Debye relaxation process. The conductivity of grains and grain-boundaries has been estimated. The activation energies calculated from the conductance and from time relaxation analyses are comparable. This indicates that the same type of charge carriers is responsible for both the electrical conduction and relaxation phenomena.     

Dielectric characteristics of cation deficient TbMnO3

Cation deficient polycrystalline Tb_1−xMnO₃ (x= 0.05, 0.10) and TbMn_1−yO₃ (y =0.05, 0.10) samples were fabricated by conventional solid-state reaction. The complex dielectric properties of the cation deficient TbMnO₃ were investigated as the function of temperature (77 K≤T≤350 K) and frequency (100 Hz≤ f≤ 200 kHz) separately. Compared to the parent TbMnO₃, the cation deficient TbMnO₃ samples exhibit not only high dielectric constant but also low dissipation factor. Nyquist plots of complex impedance show that the dielectric properties originate from two main relaxation sources, i.e. bulk contributions and grain boundary effects.     

Electrical analysis of a newly synthesized rare earth double perovskite oxide: Sr2CeNbO6

A polycrystalline rare earth double perovskite oxide, strontium cerium niobate, Sr₂CeNbO₆ (SCN) is synthesized by solid state reaction technique for the first time. Impedance spectroscopy is employed to determine the electrical parameters (resistance (R), capacitance (C) and relaxation time (τ)) of SCN in a temperature range from 303 to 703 K and in a frequency range from 100 Hz to 1 MHz. The spectrum of imaginary part of complex impedance (Z″) at each temperature exhibits one relaxation peak. The modified Cole-Cole equation is used (experimental data is fitted with this model) to describe these relaxation peaks. Scaling behaviour of Z″ suggests that the relaxation describes the same mechanism at the entire temperature range. Impedance data of SCN that have capacitive and resistive components is represented by Nyquist diagram. The experimental impedance data is fitted using equivalent RC circuit at various temperatures. The grain conduction and τ follow an Arrhenius law associated with activation energy 0.87 and 0.88 eV, respectively.     

Studies of dielectric and impedance properties of KCa2V5O15 ceramics

A polycrystalline sample, KCa₂V₅O₁₅, with tungsten bronze structure was prepared by a mixed-oxide method at low temperature (i.e., at 630 °C). A preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Surface morphology of the compound was studied by scanning electron microscopy (SEM). Two dielectric anomalies at 131 and 275 °C were observed in the temperature dependency of dielectric response at various frequencies, which may be attributed to the ferroelastic-ferroelectric and ferroelectric-paraelectric transitions, respectively. The nature of variation of the electrical conductivity, and value of activation energy of different temperature regions, suggest that the conduction process is of mixed-type (i.e., ionic-polaronic and space charge generated from the oxygen ion vacancies). The impedance plots showed only bulk contributions, and non-Debye type of relaxation process occurs in the material. A hopping mechanism of electrical transport processes in the system is evident from the modulus analysis. The activation energy of the compound (calculated both from loss and modulus spectrum) is same, and hence the relaxation process may be attributed to the same type of charge carriers.     

AC conductivity and dielectric relaxation in Ba(Sm1/2Nb1/2)O3 ceramic

The complex perovskite oxide a barium samarium niobate (BSN) synthesized by solid-state reaction technique has single phase with cubic structure. The scanning electron micrograph of the sample shows the average grain size of BSN∼1.22 μm. The field dependence of dielectric response and loss tangent were measured in the temperature range from 323 to 463 K and in the frequency range from 50 Hz to 1 MHz. The complex plane impedance plots show the grain boundary contribution for higher value of dielectric constant in the low frequency region. An analysis of the dielectric constant (ε′) and loss tangent (tan δ) with frequency was performed assuming a distribution of relaxation times as confirmed by the scaling behaviour of electric modulus spectra. The low frequency dielectric dispersion corresponds to DC conductivity. The logarithmic angular frequency dependence of the loss peak is found to obey the Arrhenius law with an activation energy of 0.71 eV. The frequency dependence of electrical data is also analyzed in the framework of conductivity and electric modulus formalisms. Both these formalisms show qualitative similarities in relaxation times. The scaling behaviour of imaginary part of electric modulus M″ and dielectric loss spectra suggest that the relaxation describes the same mechanism at various temperatures in BSN. All the observations indicate the polydispersive relaxation in BSN.     

Complex capacitance in the representation of modulus of the lithium niobate crystals

The lithium niobate (LiNbO₃ or LN) single crystal is grown in-house. The ac small-signal electrical characterization is conducted over a temperature range 35≤T≤150 °C as a function of measurement frequency (10≤f≤10⁶ Hz). Meaningful observation is noted only in a narrow temperature range 59≤T≤73 °C. These electrical data when analyzed via complex plane formalisms revealed single semicircular relaxation both in the complex capacitance (C^?) and in the modulus (M^?) planes. The physical meaning of this kind of observation is obtained on identifying the relaxation type, and then incorporating respective equivalent circuit model. The simplistic non-blocking nature of the equivalent circuit model obtained via M^?-plane is established as the lumped relaxation is identified in the C^?-plane. The feature of the eventual equivalent circuit model allows non-blocking aspect for the LN crystal attributing to the presence of the operative dc conduction process. Identification of this leakage dc conduction via C^?-plane is portrayed in the M^?-plane where the blocking nature is removed. The interacting interpretation between these two complex planes is successfully presented.     

Modification of dielectric relaxations in sodium bismuth titanate with samarium doping

Na_0.5Bi_(0.5−x) Sm_xTiO₃ (NBST) ceramics with x=0.05, 0.1, and 0.15 are prepared through chemical route. The X-ray diffraction studies confirmed the formation of single phase. Dielectric measurements in the temperature region ranging from room temperature (∼30 °C) to 600 °C at different frequencies (10 kHz-1 MHz) showed anomalies at 130, 306, and 474 °C (at 10 kHz frequency) for x=0.05 sample. Other samples showed only two peaks. To establish the electrical nature of these relaxations, impedance measurements are done at different temperatures and frequencies. The relaxation time, obtained from both impedance and modulus data, is found to decrease with increase in temperature. The relaxations observed are of non-Debye type. Increase in samarium content increases the activation energy for relaxation.     

Phase stability,elastic, electronic,thermal and optical properties of Ti3Al1−xSixC2 (0≤x≤1): First principle study

The structural parameters with stability upon Si incorporation and elastic, electronic, thermodynamic and optical properties of Ti₃Al_1−xSi_xC₂ (0≤x≤1) are investigated systematically by the plane wave pseudopotential method based on the density functional theory (DFT). The increase of some elastic parameters with increasing Si-content renders the alloys to possess higher compressive and tensile strength. The Vickers hardness value obtained with the help of Mulliken population analysis increases as x is increased from 0 to 1. The solid solutions considered are all metallic with valence and conduction bands, which have a mainly Ti 3d character, crossing the Fermi level. The temperature and pressure dependences of bulk modulus, normalized volume, specific heats, thermal expansion coefficient, and Debye temperature are all obtained through the quasi-harmonic Debye model with phononic effects for T=0−1000 K and P=0−50 GPa. The obtained results are compared with other results available. Further an analysis of optical functions for two polarization vectors reveals that the reflectivity is high in the visible–ultraviolet region up to ∼10.5 eV region showing promise as a good coating material.     

Ordered fluorite phases in the Bi2O3-Ta2O5 system: A structural and electrical investigation

Structure and electrical behaviour are reported for the system Bi_1-xTa_xO_1.5 + x (0.167 ≤ x ≤ 0.250). In the compositional range 0.200 < x ≤ 0.250 an incommensurately modulated pseudo-cubic phase (type II) is observed, with the appearance of a larger pseudo-cubic phase in the region 0.167 ≤ x ≤ 0.200. Structural analysis of the type II phases by neutron diffraction reveals subtle changes in the oxide ion distribution with temperature, associated with changes in the incommensurate modulation parameter. Analysis of the defect structure of the type II phase reveals chains of tantalate octahedra as a likely structural motif. It is proposed that these chains facilitate an electronic contribution to total conductivity at low temperatures through electron hopping along the chains. Changes in oxide ion vacancy ordering may explain the observed non-linear behaviour in the thermal expansion of lattice parameter and Arrhenius plots of total conductivity.     

Thermal variation of structure and electrical conductivity in Bi14WO24

Structure and electrical conductivity of Bi₁₄WO₂₄ as a function of temperature have been examined by X-ray and neutron powder diffraction, a.c. impedance spectroscopy and differential thermal analysis. The room temperature structure was successfully refined using a monoclinic subcell model in space group I2/m. However, additional reflections in the neutron data are consistent with a large supercell of dimensions a = 17.3780(1) Å, b = 17.3891(1) Å, c = 26.1785(2) Å and β = 90.270(1)°, as previously proposed. Transitions to tetragonal and cubic phases are observed at ca. 35 °C and 780 °C, respectively. The structure of the high temperature polymorph is confirmed as a fully disordered δ-Bi₂O₃ type phase. Analysis of the defect structure is consistent with a predominantly tetrahedral environment for tungsten, as seen at low temperatures. The conductivity behaviour is correlated with the appearance of the δ-phase at high temperatures and exhibits a value of 0.97 S cm^− 1 at 800 °C.     

The impedance spectroscopic study and dielectric relaxation in A(Ni1/3Ta2/3)O3 [A=Ba,Ca and Sr]

We present the results of impedance spectroscopic study with its analytical interpretations in the framework of electric modulus formalism for Barium Nickel Tantalate Ba(Ni_1/3Ta_2/3)O₃ (BNT), Calcium Nickel Tantalate Ca(Ni_1/3Ta_2/3)O₃ (CNT) and Strontium Nickel Tantalate Sr(Ni_1/3Ta_2/3)O₃ (SNT) synthesized by the solid-state reaction technique. The results of powder X-ray diffraction study reveal that BNT and SNT crystallize in cubic structure with lattice parameter a=4.07 Å and 3.98 Å respectively, whereas CNT crystallizes in monoclinic structure having lattice parameters, a=5.71 Å, b=13.45 Å and c=5.47 Å with β=118.3°. The logarithmic angular frequency dependence of the real part of complex dielectric permittivity and loss tangent as a function of temperature indicate significant dielectric relaxation in the samples, which have been explained by the Debye theory. The frequency dependence of the loss peak and the imaginary part of electrical modulus are found to obey the Arrhenius law. The relaxation mechanism of these samples is modeled by the Cole–Cole equation. This confirms that the polarization mechanism in BNT, CNT and SNT is due to the bulk effect arising in semiconductive grains. The scaling behavior of imaginary part of electric modulus M″ suggests that the relaxation describes the same mechanism at various temperatures but relaxation frequency is strongly temperature dependent. The normalized peak positions of tan δ/tan δ_m and M″/M″_m versus log ω for BNT, CNT and SNT do not overlap completely and are very close to each other. These indicate the presence of both long-range and localized relaxation. Due to their high dielectric constant and low loss tangent, these materials may find several technological applications such as in capacitors, resonators, filters and integrated circuits.     

Synthesis and ion conductivity of (Bi2O3)0.75(Dy2O3)0.25 ceramics with grain sizes from the nano to the micro scale

Using (Bi₂O₃)_0.75(Dy₂O₃)_0.25 nano-powder synthesized by reverse titration co-precipitation method as raw material, dense ceramics were sintered by both Spark Plasma Sintering (SPS) and pressureless sintering. According to the predominance area diagram of Bi-O binary system, the sintering conditions under SPS were optimized. (Bi₂O₃)_0.75(Dy₂O₃)_0.25 ceramics with relative density higher than 95% and an average grain size of 20 nm were sintered in only 10 min up to 500 °C. During the pressureless sintering process, the grain growth behavior of (Bi₂O₃)_0.75(Dy₂O₃)_0.25 followed a parabolic trend, expressed as D² − D₀² = Kt, and the apparent activation energy of grain growth was found to be 284 kJ mol^− 1. Dense (Bi₂O₃)_0.75(Dy₂O₃)_0.25 ceramics with different grain sizes were obtained, and the effect of grain size on ion conductivity was investigated by impedance spectroscopy. It was shown that the total ion conductivity was not affected by the grain size down to 100 nm, however lower conductivity was measured for the sample with the smallest grain size (20 nm). But, although only the δ phase was evidenced by X-ray diffraction for this sample, a closer inspection by Raman spectroscopy revealed traces of α-Bi₂O₃.     

Growth temperature dependence of the hysteretic behavior of Ni0.5Zn0.5Fe2O4 thin films

Herein, a discussion of the effect of deposition temperature on the magnetic behavior of Ni_0.5Zn_0.5Fe₂O₄ thin films. The thin films were grown by r.f. sputtering technique on (1 0 0) MgO single-crystal substrates at deposition temperatures ranging between 400 and 800 °C. The grain boundary microstructure was analyzed via atomic force microscopy (AFM). AFM images show that grain size (φ∼70-112 nm) increases with increasing deposition temperature, according to a diffusion growth model. From magneto-optical Kerr effect (MOKE) measurements at room temperature, coercive fields, H_c, between 37and 131 Oe were measured. The coercive field, H_c, as a function of grain size, reaches a maximum value of 131 Oe for φ ∼93 nm, while the relative saturation magnetization exhibits a minimum value at this grain size. The behaviors observed were interpreted as the existence of a critical size for the transition from single- to multi-domain regime. The saturation magnetization (21 emu/g<M_s<60 emu/g) was employed to quantify the critical magnetic intergranular correlation length (L_c≈166 nm), where a single-grain to coupled-grain behavior transition occurs. Experimental hysteresis loops were fitted by the Jiles-Atherton model (JAM). The value of the k-parameter of the JAM fitted by means of this model (k/μ_o∼50 A m²) was correlated to the domain size from the behavior of k, we observed a maximum in the density of defects for the sample with φ∼93 nm.     

Impedance spectroscopy study and ground state electronic properties of In(Mg1/2Ti1/2)O3

The complex perovskite oxide In(Mg_1/2Ti_1/2)O₃ (IMT) is synthesized by a solid state reaction technique. The X-ray diffraction of the sample at 30 °C shows a monoclinic phase. The dielectric properties of the sample are investigated in the temperature range from 143 to 373 K and in the frequency range from 580 Hz to 1 MHz using impedance spectroscopy. An analysis of the dielectric constant ε′ and loss tangent (tan δ) with frequency is performed assuming a distribution of relaxation times. The Cole-Cole model is used to explain the relaxation mechanism in IMT. The scaling behavior of imaginary part of electric modulus (M″) shows that the relaxation describes the same mechanism at various temperatures. The electronic structure and hence the ground state properties of IMT is studied by X-ray photoemission spectroscopy (XPS). The valence band XPS spectrum is compared with the electronic structure calculation. The electronic structure calculation indicates that the In-5s orbital introduces a significant density of states at the Fermi level, which is responsible for a high value of conductivity in IMT.     

Dielectric properties of Bismuth Titanate (Bi4Ti3O12) synthesized using solution combustion route

Ferroelectric Bismuth Titanate (Bi₄Ti₃O₁₂) was prepared by solution combustion route with glycine as fuel. The single phase Bismuth Titanate was obtained after calcination at 800 °C, which was confirmed with the help of X-ray diffraction studies and EDS analysis. SEM micrographs of the calcined powders show agglomerated particles, which is typical of combustion synthesis. Behavior of dielectric constant and dielectric loss as a function of temperature of as prepared sample are reported here. Ferroelectric to paraelectric phase transition occurs at the temperature T_c∼650 °C. Impedance studies were made in the frequency range from 1 KHz to 1 MHz. The semicircles observed in the complex impedance diagrams indicate deviation from the Debye behavior. Activation energy of the sample around T_c is found to be ∼0.35 eV and below T_c is ∼0.13 eV, which was calculated using the Arrhenius plots.