Large Polaron Tunneling Research Articles

Phase transition and conduction mechanism study by overlapping large-polaron tunnelling model in Li2MoO4 ceramic compound

The Lithium-ionic conductor Li2MoO4 compound was prepared by the conventional solid-state reaction method. A Rietveld analysis of powder X-ray diffraction at room temperature has exhibited the title compound crystallizes in the trigonal system (space group) R3¯ with a phenacite-like structure. The calorimetric and Raman spectroscopy-studies revealed a single-phase transition at 635 K. The electrical conductivity was measured in the frequency range from 200 Hz to 1 MHz and temperatures between 578 and 723 K using impedance spectroscopy. Besides, the analysis of Nyquist plots revealed the contribution of two electrically active regions corresponding to the bulk mechanism and distribution of grain boundaries. The electrical conductivity for the grain (σg) follows an Arrhenius behavior with two different activation energies Ea(I) = 1.038 eV for T < 635 K and Ea(II) = 0.481 eV for T > 635 K. The frequency dependence of the conductivity was interpreted in terms of Jonscher's law. Temperature dependence of the power law exponent s suggests that the overlapping large-polaron tunneling (OLPT) model is the dominant transport process in this material. The optimum hopping length of the polaron (1.8 <Rω < 3.4 Å), is large compared with the interatomic spacing (LiO = 1.964 Å; MoO = 1.8 Å).

Effect of annealing temperature on structural, morphology and dielectric properties of La0.75Ba0.25FeO3 perovskite

The effect of annealing temperature on the structure, morphology and dielectric properties of La0.75Ba0.25FeO3 compound prepared by the sol-gel method was investigated. The increase of the annealing temperature from 900 to 1100 °C, promotes an increase of the average grain size value. Two dielectric relaxations are detected using the dielectric modulus formalism, attributed to grain and grain boundary relaxations. This behavior was confirmed by both Nyquist and Argand's plots of dielectric impedance and Modulus results at different measuring temperatures. The ac conductivity could be described by Jonscher's power law revealing the presence of both overlapping large polaron tunneling and non-overlapping small polaron tunneling mechanisms.

Effect of Impedance Relaxation in Conductance Mechanisms in TiO2/ITO/ZnO:Al/p-Si Heterostructure

The electrical conduction of a TiO2/ITO/ZnO:Al/p-Si structure under alternating-current excitation was investigated in the temperature range of 80 K to 300 K. The frequency dependence of the capacitance and conductance revealed the response of a thermally activated trap characterized by activation energy of about 140 meV. The frequency dependence of the conductance obeyed the universal dynamic response according to the common relation $$ G = A\omega^{s} $$ . The temperature dependence of the frequency exponent s illustrates that, in the low frequency range, conduction is governed by the correlated barrier hopping (CBH) mechanism involving two distinct energy levels for all investigated temperatures. For the high frequency region, conduction takes place according to the overlapping large-polaron tunneling mechanism at low temperatures but the CBH mechanism becomes dominant in the high temperature region. This difference in electrical behavior between low and high temperatures can be attributed to the dominance of dielectric relaxation at low compared with high temperatures.

Theoretical and experimental study of AC electrical conduction mechanism in the low temperature range of p-CuIn3Se5

In the present work, an attempt has been made to study theoretically and experimentally the AC electrical conduction mechanism in disordered semiconducting materials. The key parameter considered in this analysis is the frequency exponent s(ω,T)=(∂ln(σAC(ω,T))∂ln(ω))T, where σAC is the AC electrical conductivity that depends on angular frequency ω and temperature T. In the theoretical part of this work, the effect of the barrier hopping energy, the polaron radius and the characteristic relaxation time is considered. The theoretical models of Quantum Mechanical Tunneling (QMT), Non overlapping Small Polaron Tunneling (NSPT), Overlapping Large Polaron Tunneling (OLPT) and Correlated Barrier Hopping (CBH) are considered to fit experimental data of σAC in p-CuIn3Se5 (p-CIS135) in the low temperature range up to 96 K. Some important parameters, as the polaron radius, the localization length and the barrier hopping energies, are estimated and their temperature and frequency dependence discussed.

Effect of doping of alkaline metal ions on structural and electrical properties of Bi0.8M0.2FeO3-modified Na0.5Bi0.5TiO3 ceramics (M=Ca, Sr, and Ba)

In the present work the electrical properties of polycrystalline 0.9(Na0.5Bi0.5TiO3)–0.1(BiFeO3) and Bi0.8M0.2FeO3-modified Na0.5Bi0.5TiO3, (where M = Ca, Sr, and Ba) lead-free ceramics were investigated through impedance spectroscopy. X-ray diffraction analyses indicate the incorporation of alkaline earth metal ions at A-site of perovskite type ceramics revealing the conservation of crystal structure (rhombohedral structure with R3c space group) in the synthesized compositions. The substitution of alkaline earth metal ions at the A-site in perovskite-structured ceramics led to an enlargement of unit cell. The appearance of depressed semicircle arcs in Nyquist plots suggests non-Debye nature of relaxation occurring in studied samples. An equivalent circuit model comprising of parallel combination of resistance and constant phase element was used for the simulation of Nyquist plots. The mismatching of peaks in the combined spectroscopic plots of Z″ and M″ versus frequency also confirms the occurrence of non-Debye relaxation in BBFO sample. The ac conductivity studies show that the conduction mechanism changes from correlated barrier hopping model (CBH) to overlapping large polaron tunneling, when alkaline metal ions are substituted in place of Bi3+ ions. Activation energy values calculated from conductivity data suggest the presence of oxygen vacancies. The dielectric, impedance and conductivity behaviors are significantly influenced by the distortion of Ti/FeO6 octahedron.

Frequency and temperature dependent electrical and dielectric properties of LaCrO3 and Ir doped LaCrO3 perovskite compounds

Perovskite oxide materials, especially perovskite lanthanum chromium oxide, LaCrO3 (LCO) have taken considerable attention among the scientists due to the changing optical, electrical, dielectric and magnetic properties depending on doping via various elements. In this study, the LCO and iridium (Ir) doped LCO (LaCr1-xIrxO3) (x = 0.10) compounds were synthesized using solid-state reaction method. Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) have been employed to study particle morphologies and oxidation states of synthesized powders, respectively. Frequency-depended electrical/dielectric properties of perovskite oxide samples were performed in wide range frequency (1Hz-10 MHz) and temperature (−100 °C to +100 °C) using dielectric/impedance spectrometer. Electrical measurements have underscored that the dielectric constant of LCO compound dramatically decreased after doped with Ir. Furthermore, it was observed that ac conductivity and capacitance value of LCO decreased while impedance increased after Ir doping due to i) Ir occupation in Cr sites and ii) lattice distortion owing to doping larger ion Ir4+ into smaller Cr3+ ion in the LCO lattice. Finally, overlapping large polaron tunneling (OLPT) and quantum mechanical tunneling (QMT) conduction mechanisms were suggested for LCO while the correlated barrier hopping (CBH) and QMT conduction mechanisms were observed for Ir doped LCO compound. Moreover, the activation energy values have been calculated by employing both fmaxvs. 1/T and σdcvs. 1/T plots. It has been shown those energy values are in good agreement with each other for LCO and Ir doped LCO samples at various temperature ranges.

Investigation into defect chemistry and relaxation processes in niobium doped and undoped SrBi4Ti4O15 using impedance spectroscopy.

The aurivillius family of compounds SrBi4Ti4O15 (SBTi) and SrBi4Ti3.8Nb0.2O15 has been prepared using solid state reaction techniques. The niobium doping enhances the value of the dielectric constant, but decreases the phase transition temperature and grain size of SBTi. Grain conductivity evaluated from the impedance data reveals that Nb doping increases the resistance of grains which indicates the decrease in oxygen vacancies. The negative temperature coefficient of resistance shown by the grain boundary conductivity is explained using the Heywang–Jonker model. The variation of ac conductivity with frequency is found to obey Jonscher’s universal power law. The frequency exponent (n), pre-exponential factor (A), and bulk dc conductivity (σdc) are determined from the fitting curves of Jonscher’s universal power law. From the frequency exponent (n) versus temperature curve, we conclude that the conduction mechanism of SBTi changes from large-polaron tunneling (300–475 °C) to small-polaron tunneling (475–550 °C), and in that of the niobium doped it is small-polaron tunneling (300–375 °C) to correlated band hopping (375–550 °C). Activation energies have been calculated from different functions such as loss tangent, relaxation time, grain and grain boundary conductivities, and ac and dc conductivity. The activation energies reveal that conductivity in the sample has contributions from migrations of oxygen vacancies, bismuth ion vacancies, electrons ionized from strontium vacancies, strontium ion vacancies and valence fluctuations of Ti4+/Ti3+ ions.

Dielectric relaxation and conduction mechanism of complex perovskite Ca0.90Sr0.10Cu3Ti3.95Zn0.05O12 ceramic

In this work, Sr and Zn co-doped calcium copper titanate having nominal formula Ca0.90Sr0.10Cu3Ti3.95Zn0.05O12 has been synthesized and systematically studied for structure, dielectric and electrical properties. The phase purity of the prepared composition has been confirmed by X-ray diffraction (XRD) pattern and various structural parameters are presented here with Rietveld refinement data. The Scanning electron microscopy (SEM) image revealed the presence of homogeneous fine-grained microstructure investigated in the prepared ceramic. The impedance data well fitted with 2 R-CPE model confirmed the non-ideality of the system. Dielectric studies have shown the existence of two relaxation processes in low and high frequency domains. Low frequency dielectric response proposed as a combination of Maxwell-Wagner relaxation and intrinsic defect (Vo++) while high frequency behavior has been interpreted in terms of polaronic relaxation caused by hopping process of an electron between Ti3+ and Ti4+ states. The frequency dependent behavior of conductivity has been well fitted to Dong’s model and interpreted in term of Overlapping Large Polaron Tunneling (OLPT) mechanism. The various parameters; relaxation angular frequency (ω1) of Debye process, ionic hopping frequency (ω2), dc conductivity (σo), cole- cole parameter (α) and frequency exponent (s) have been determined by Dong’s Model fitting.

Dielectric relaxation and predominance of NSPT and OLPT conduction processes in Ba0.9Sr0.1TiO3

We investigate the relaxation and conduction mechanism of Ba0.90Sr0.10TiO3 (BST) ceramic, synthesized by the solid state reaction method. The dielectric and relaxation properties are analyzed in the temperature range of 380–450 °C with alternative current in the frequency range of 20 Hz-1MHz. Variation of dielectric constant, ε′, with temperature shows a normal ferroelectric transition at Tc = 95 °C with a weak degree of diffuseness. The modified Cole-Cole equation is used to describe all contributions to the relaxation mechanism. The frequency exponent m(ω,T) deduced from experimental data of the dielectric loss (ε″) as m(ω,T)=(∂lnε″/∂lnω)T shows a temperature and frequency dependence. Two conduction process are observed: non-overlapping small-polaron tunneling (NSPT) at low frequencies and overlapping large polaron tunneling (OLPT) at high frequencies. The analysis of Nyquist plots reveals also the presence of two contributions, who which the activation energies have been calculated.

Electrical properties and conduction mechanism study by OLPT model of NaZnPO4 compound

A polycrystalline sample with NaZnPO4 stoichiometry was synthesized by the conventional solid-state reaction and then identified and characterized by X-ray powder diffraction, Raman spectroscopy and impedance spectroscopy. The X-ray diffraction pattern revealed that the composite represents a single phase that crystallizes in the monoclinic system with a P21/n space group. All vibration modes of tetrahedral PO4 appeared in the Raman spectrum. Besides, the electrical conductivity behavior of the title compound was analysed as a function of frequency ranging from 209Hz to 1MHz and at a temperature interval ranging from 578K to 728K by means of impedance spectroscopy to elucidate the conduction mechanism in this compound. The temperature dependence of frequency exponent ′s′ showed that the conduction in NaZnPO4 is insured by the overlapping large polaron tunneling (OLPT) model. A correlation between electrical and structural properties was also depicted.

Crystal structure characterization and AC electrical conduction behavior of sodium cadmium orthophosphate

Sodium cadmium orthophosphate NaCdPO4 was prepared by solid-state reaction method at high temperature under air atmosphere. Structural, morphology and electrical properties have been investigated. X-ray powder diffraction analysis indicated that the sample is well crystallized in the orthorhombic maricite structure with Pmnb space group. The composition and morphology of the compound were performed by energy dispersive spectroscopy coupled with scanning electron microscope. Electrical properties were carried out in the frequency range 209 Hz–1 MHz at various temperatures 595–700 K using impedance spectroscopy technique. Nyquist plots analysis revealed the presence of two contributions at different temperatures associated with grain and grain boundary. In order to explain the impedance results an equivalent circuit has been proposed. The AC conductivity was analyzed by Jonscher’s power law of frequency (σ ac (ω) ∞ Aω s ). At high frequency, the conduction mechanism was well interpreted in term overlapping large polaron tunneling.

Study of complex impedance spectroscopic properties of the α-NaCuPO4 compound

In the present study, α-NaCuPO4 compound was prepared by solid-state reaction method and characterized by X-ray powder diffraction and infrared spectroscopy. The AC electrical conductivity and dielectric relaxation properties of this compound have been investigated by means of impedance spectroscopy measurements over a wide range of frequencies and temperatures 209 Hz–1 MHz and 598–708 K, respectively. Both impedance and modulus analysis exhibit the grain and grain boundary contribution in the electrical response of the sample. It was found that the data of the AC measurements follow the overlapping large polaron tunneling model and the model’s parameters were determined.

AC conductivity evolution in bulk and grain boundary response of sodium tungstate Na2WO4

The Na2WO4 compound has been obtained by the conventional solid-state reaction and characterized by X -ay powder diffraction. The title material crystallizes in the cubic system with Fd-3m space group. The electrical properties of the compound have been studied using complex impedance spectroscopy in the frequency range 200 Hz–5 MHz and temperature range 586–679 K. Two semicircles are observed in impedance plot indicating the presence of two relaxation processes in the compound associated with the grain and grain boundary. The relaxation behavior of the grain and grain boundary of the Na2WO4 are also obtained from the analyzed electrical modulus data. AC conductivity measured follows the power-law dependence σAC∼ωs typical for charge transport. Therefore, the experimental results are analyzed with various theoretical models. Temperature dependence of the power law exponent s strongly suggests that tunneling of large polarons is the dominant transport process. The mechanism of conduction is probably due from the displacements of the Na+ ion in the tunnel-type cavities along [111] direction.

Zinc chloride modified electronic transport and relaxation studies in barium-tellurite glasses

The ac conductivity of halide based tellurium glasses having composition 70 TeO2-(30-x) BaO-x ZnCl2; x = 5, 10, 15, 20 and 25 has been investigated in the frequency range 10-1 Hz to 105Hz and in the temperature range 453 K to 553 K. The frequency and temperature dependent ac conductivity show mixed behaviour with increase in halide content and found to obey Jonscher’s universal power law. The values of dc conductivity, crossover frequency and frequency exponent have been estimated from the fitting of experimental data of ac conductivity with Jonscher’s universal power law. For determining the conduction mechanism in studied glass system, frequency exponent has been analyzed by various theoretical models. In presently studied glasses, the ac conduction takes place via overlapping large polaron tunneling (OLPT). The values of activation energy for dc conduction (W) and the one associated with relaxation process (E R) are found to increase with increase in x up to glass sample with x = 15 and thereafter it decrease with increase in zinc chloride content. DC conduction takes place via variable range hopping (VRH) as proposed by Mott with some modification suggested by Punia et al. The value of real part of modulus (M') is observed to decrease with increase in temperature. The value of stretched exponent (β) obtained from fitting of M'' reveals the presence of non-Debye type of relaxation in presently studied glass samples. Scaling spectra of ac conductivity and values of electric modulus (M' and M'') collapse into a single master curve for all the compositions and temperatures. The values of relaxation energy (E R) for all the studied glass compositions are almost equal to W, suggesting that polarons have to overcome same barrier while relaxing and conducting. The conduction and relaxation processes in the studied glass samples are composition and temperature independent.

Alternating current conductivity and dielectrical properties of -5-sulfono-7-(4-x phenyl azo)-8-hydroxy quinoline derivatives

The influence of NO2, CH3 and Cl substitutions on structure, dielectric constants and alternating current conductivity, σac, of -5-sulfono-7-(4-x phenyl azo)-8-hydroxy quinoline (SAHQ) ligands is investigated in the frequency range 1–100kHz and in the temperature range 200–373K. SAHQ-x; x = -NO2 or -CH3 or -Cl ligands have monoclinic crystal system with different lattice parameters and crystallite size depending on the substitutions. The thermal behavior of SAHQ-x depends on the substitution. The AC electrical conduction mechanism for (SAHQ- NO2) and (SAHQ- Cl) ligands are found to follow the correlated barrier hopping model (CBH). The parameters such as barrier height, hopping distance, density of states near Fermi level and relaxation time have been reported. Overlapping large polaron tunneling model has been applied for explaining conduction behavior of SAHQ-CH3 ligand. The hopping energy and the polaron radius have been determined.

Impedance spectroscopy and ac conductivity mechanism in Sm doped Yttrium Iron Garnet

Abstract Polycrystalline samples of Y 3- x Sm x Fe 5 O 12 for x =0.0 to 3.0 were prepared in single phase form with cubic structure. The analysis of XRD patterns using Reitveld refinement reveals that these samples crystallize in cubic structure with Ia 3 d space group and lattice constant increases systematically with Sm concentration. The depressed and asymmetric semicircle of complex impedance spectra suggests that the relaxation of charge carriers is of non-Debye type and it is controlled by thermally activated charge carriers. Both grains and grain boundaries are found to contribute for the measured complex impedance spectrum, however at higher temperature (T ≥ 543 K), the relaxation process is dominated by the grain boundaries. The temperature dependence of relaxation frequency and conductivity exhibit a change of slope in the vicinity of ferrimagnetic transition temperature which is attributed to the magneto-electric coupling. The analysis of ac conductivity spectra in terms of Jonscher power law suggests that the conduction process is governed by small polaron hopping model for x ≤ 0.5 and for higher concentration overlapping of large polaron tunneling model is found to play a dominant role. The dielectric constant is found to increase with increase with increase in Sm concentration.

An enhancement on electrical properties of carboxymethyl cellulose-NH4Br based biopolymer electrolytes through impedance characterization

ABSTRACTIn this work, the formulation of biopolymer electrolytes (BEs) system has been accomplished by incorporating various plasticizers with carboxymethyl cellulose–NH4Br through solution casting technique. The ionic conductivity at room temperature of BEs system was achieved at ∼10−4 S cm−1 with addition of 25 wt% NH4Br and enhanced to ∼10−3 S cm−1 when plasticizers were added. The temperature-dependence of the BEs system exhibits Arrhenius behavior. Jonschers power law was used to study the electrical properties and shows that the highest conducting BEs system can be represented by overlapping overlapping a large polaron tunneling model for poly(ethylene glycol) system a, small polaron hopping model for glycerol system, and a quantum mechanical tunneling model for ethylene carbonate system.

Observation of semiconductor to metallic transition and polaron hopping in double perovskite Pr2CoTiO6 ceramics

This paper describes semiconductor to metal transition and polaron conduction in double perovskite Pr2CoTiO6 (PCTO) ceramics. The XRD pattern recorded at room temperature confirmed the pure phase, single crystalline structure. The semicircle arc in the impedance plot at each temperature can be attributed to the grain boundary contribution, indicating one dominating response in the measurement frequency range. The semiconductor to metallic transition was also confirmed by the variation of grain boundary resistance (Rgb) with temperature. The activation energy estimated from the imaginary part of electrical modulus and impedance are found to be the characteristic of polaron conduction in PCTO. Ac conductivity followed power law dependence σac = Bωn. The observed variation of the exponent ‘n’ with temperature suggests the typical of charge transport assisted by a hopping process. The observed minimum in the temperature dependence of frequency exponent ‘n’ strongly suggests that the large polaron tunneling is the dominant transport process.

A comparative of structural, optical, mechanical and electrical properties of alkaline earth metal substituted tellurite glasses

In the present work, the glass systems 0.70TeO2–0.30MO and 0.65TeO2–0.35MO (M = Ba, Sr) were prepared and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, UV–vis spectroscopy and differential scanning calorimetry. The structural studies show that amorphous alkaline tellurite glasses are made up of vibrational units of molecular water group, hydroxyl bonding, symmetrical stretching vibration of Te–Oax bonds in the deformed TeO4 and stretching and bending vibration of Te–O–Te linkages in TeO4/TeO3 + d polyhedra/TeO3. In the X-ray diffractograms, shifting of amorphous peak was observed towards higher angle side with the strontium substitution in comparison to barium substitution due to lower ionic radii. The optical band gap of investigated glasses was found to lie in the range 2.94–3.31 eV and was found to be lower in SrO added glasses than that of BaO added glasses. This may be due to the presence of more number of non-bridging oxygen in Sr substituted glasses than Ba substituted glasses. The Ba substituted samples were observed to be denser than Sr substituted glasses. Ultrasonic longitudinal and sheer velocities were found in the range of 3160–3210 and 1785–1850 m/s, respectively and were observed to be higher in Ba substituted glasses. The ac conductivity spectra were found to follow Jonscher power law and conduction mechanism is governed by overlapping large polaron tunneling model created with non-bridging oxygen and Tellurium ions.

Temperature and composition dependent density of states extracted using overlapping large polaron tunnelling model in MnxCo1−xFe2O4 (x=0.25, 0.5, 0.75) nanoparticles

We report detailed ac electrical and structural characterization of manganese cobalt ferrite nanoparticles, prepared by coprecipitation technique. X-ray diffraction (XRD) confirmed single-phase cubic spinel structure of the nanoparticles. Tetrahedral (A) and octahedral (B) group complexes were present in the spinel lattice as determined by Fourier Transform Infrared Spectroscopy (FTIR). Scanning Electron Microscope (SEM) images revealed presence of spherical shape nanoparticles having an average diameter ~50–80nm. Composition, temperature and frequency dependent ac electrical study of prepared nanoparticles interpreted the role of cationic distribution between A and B sites. Overlapping large polaron tunnelling (OLPT) conduction mechanism was observed from 290 to 200K. Frequency exponent s was fitted theoretically using OLPT model. High values of Density of States (DOS) of the order of 1022–1024eV−1cm−3 were extracted from ac conductivity for different compositions. We found that DOS was dependent on distribution of cations in the tunnel-type cavities along the a and b axis.