Overlapping-large Polaron Tunneling Model Research Articles

Charge transport mechanism, dielectric relaxations and relaxor ferroelectric properties of Sm2MgMnO6 double perovskite

This study investigates the charge transport mechanism, dielectric relaxation and relaxor ferroelectric properties of Sm2MgMnO6 which exhibits monoclinic crystal structure with space group P21/n. The charge carriers conduction mechanism in the temperature range 150 K–239 K and 244 K–304 K are correlated barrier hopping (CBH) and overlapping large polaron tunneling (OLPT), respectively. The maximum barrier height associated to CBH model is 0.26 meV. The dc activation energy associated to CBH and OLPT model are 0.11 eV and 0.91 eV respectively. The polaron hopping energy associated to Mott's variable range hopping model is 0.11 eV at 244 K. The value of tanδ varies from 0.004 to 3.329 in temperature range 151.07 K–306.87 K and frequency range 1 kHz–800 kHz. The temperature variation of ε′, ε″ and tanδ shows a single relaxation which follows Vogel–Fulcher (VF) law. The VF law and the shape of P–E hysteresis loop confirms the relaxor ferroelectric nature of the studied material. The saturated polarization (PMax), remnant polarization (Pr) and coercive field (Ec) depend on both the electric field (E0) and frequency (f). The loop area (A′) and dielectric breakdown field (EBD) depend on E0 as A′∝E02 and EBD∝E0, respectively. The PMax, Pr and Ec depend on f as PMax∝f−0.13, Pr∝f−0.48 and Ec∝f−0.48, respectively. The energy storage efficiency (η) varies with frequency as: η%=20.44f0.21.

Insight into OLPT model conduction mechanism and dielectric relaxation of lead borate glass containing Cr and Ge ions

Lead borate glass with compositions 25B2O3–73.8PbO–xGeO2-(1.2 − x) Cr2O3 (x ≤ 1.2 mol. %) is fabricated by the melt quenching procedure. The electrical conductivities (σdc, σac) of fresh samples are measured in the frequency range 100 Hz–1.0 MHz and temperature range 303–393 K. The anomalous behavior observed in the dc activation energy at x = 0.6 mol% is argued to the formation of bridging and non-bridging oxygen bonds. These bonds are formed due to substitution of GeO2 to the expense of Cr2O3 ions in the PbO–B2O3 network. The dependence of ac conductivity on frequency is analyzed through the power law: σacαωs where s ≤ 1.0. The experimental results of s for the considered samples have shown that overlapping large polaron tunneling (OLPT) is the possible conduction model in the considered frequency and temperature ranges. The strength of the OLPT model established on the normal Mayer–Neldel (MN) rule is analyzed and discussed. Furthermore, results of the real and imaginary dielectric constant composed with Cole–Cole diagrams for investigated samples are presented and discussed.

Structural and impedance spectroscopic investigations of eco-friendly alkali phosphoborate glass-ceramics containing zirconium ion.

Glass-ceramics with novel composition xZrO2.7P2O5.19CaO.24Na2O.(50-x)B2O3 (x = 0, 2, 4, 6, and 8mol%) have been synthesized using melt quench technique. The synthesized compositions were characterized and analyzed by X-ray diffraction, field emission scanning electron microscopy, infrared absorption, and impedance spectroscopy. X-ray diffraction profiles of prepared samples confirm the existence of phases corresponding to Na3Ca6(PO4)5 crystal (with crystallite size ~ 23nm). Infrared absorbance spectra reveal the presence of phosphate and borate units (PO3, PO4, BO3, BO4) in the glass matrix. Different dielectric parameters such as dielectric loss, electric modulus, and tangent loss were evaluated. Their variations with temperature and frequency confirm the non- Debye relaxation behavior of prepared samples. A phenomenal description of the capacitive behavior was studied by considering the circuit having a parallel combination of constant phase element and bulk resistance. The conduction is found to be governed by overlapping large polaron tunneling (OLPT) and follow OLPT model. The results indicate that ZrO2 substituted alkali phosphoborate glass-ceramics can be used as eco-friendly and safe dielectric materials.

Charge transport mechanism in BaFe12O19 and BaFe11CoO19

The conduction and charge transport mechanism of cobalt substituted barium hexaferrite (BaCoM) were studied by impendence spectroscopy at different temperatures and compared with BaM. Frequency dependent impedance spectra showed two distinct relaxations in BaM whereas a single relaxation peak was present in BaCoM. The relaxation frequencies were found to be temperature dependent. Scaling behaviour confirmed the dynamic conduction process is temperature independent and deviates from Debye type relaxation. Brick layer model has been employed to investigate the charge conduction through grain and grain boundaries. Arrhenius plot of conductivity showed two sharp transitions in three different temperature regime which indicates parallel conduction mechanism in the system. The frequency dependent conductivity was fitted with Jonscher’s power law at different temperatures. The variation in frequency exponent with temperature confirmed the evidence of three types of conduction process. Higher conductivity and different transition temperature was observed for BaCoM as compared to BaM. The conduction process was in accordance with overlapping-large polaron tunnelling model.

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.

Structural and electrical properties of Bi3TiVO9 ferroelectric ceramics

Bi3TiVO9 (BTV), a member of Aurivillius family, has been fabricated by a mixed-oxide route at high temperature. Room temperature structural analysis using X-ray diffraction data confirms the crystallization of ceramic in the orthorhombic crystal structure. Surface morphological analysis using field-emission scanning electron microscopic (FESEM) images and elemental study with energy dispersive X-ray spectroscopy (EDX) reveal the uniform distribution of grains and compositional elements of the prepared material, respectively. Based on the temperature and frequency dependence of capacitive and resistive parameters, the electrical phenomena underlying the sample are discussed. The contribution of grain and grain boundary towards the conduction and relaxation mechanism is described by the complex impedance spectroscopy. The semi-conducting property (negative temperature coefficient of resistance (NTCR)) behavior is observed from the temperature dependent bulk resistance and J∼E characteristics of the sample. The frequency dependent ac conductivity of the system obeys the universal Jonscher's power law with conduction mechanism based on overlapping-large polaron tunneling model and correlated barrier hopping model. Measurement of room temperature hysteresis loop confirms the existence of ferroelectricity in the sample.

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.

Studies of electric, dielectric, and conduction mechanism by OLPT model of Li4P2O7

The polycrystalline samples of Li4P2O7 were prepared by solid-state reaction technique. The formation of the compounds was checked by X-ray diffraction technique (XRD). Detailed dielectric and electrical properties of the compounds were analyzed as a function of frequency (200 Hz–5 MHz) and temperature (611–671 K). The impedance data were well fitted to two equivalent electrical circuits. The results of the modulus study reveal the presence of two distinct relaxation processes suggesting the presence of grains and grain boundaries in the sample. Analysis of the dielectric constants e″ and loss tangent tan (δ) with frequency shows a distribution of relaxation times. The activation energy found from the Arrhenius plot confirms that the conduction process in the material is not due to simple hopping mechanism. The temperature dependence of frequency exponent s was investigated to understand the conduction mechanism in Li4P2O7. The overlapping large polaron tunneling (OLPT) model can explain the temperature dependence of the frequency exponent.

AC conductivity and dielectric properties of SiO2–Na2O–B2O3–Gd2O3 glasses

Abstract The present paper studies the dielectric properties and ac conductivity of sodium borosilicate glasses doped with varying concentrations of Gd2O3, while maintaining the SiO2:Na2O:B2O3 ratios constant for all compositions. The samples were prepared using rapid quenching method. Their dielectric and electrical properties were measured in the frequency range (102–106 Hz) and temperature range (298–523 K). Experimental results showed that all dielectric and electrical properties are dominated by mole% of Gd2O3. The decrease and increase in these properties were found to be due to formation of short range ordered structure and non-bridging oxygen, respectively. Different models were suggested to describe the conduction mechanism of the investigated compositions based on mole% of Gd2O3. Correlated barrier hopping model (CBH) was suggested to describe the conduction mechanism of 0–2.52 mole% Gd2O3 doped – samples. While, overlapping large polaron tunneling (OLPT) model was applied to describe conductivity of 2.83 mole% Gd2O3 doped sample. Interestingly, an inverse – OLPT model was suggested to describe the conduction mechanism of composition doped with 3.13 mole% of Gd2O3.

Frequency dependent conductivity of cadmium vanadate glassy semiconductor

We have studied the frequency dependent conductivity of thesemiconducting cadmium vanadate glasses of compositionsxV2O5–(1−x)CdO where x = 0.6, 0.7, 0.8 and 0.9 in the temperature range 93–423 K and in the frequency range10 Hz–2 MHz. We have observed sub-linear frequency dependence and weak temperaturedependence of the ac conductivity. We have analyzed the experimental results in theframework of various theoretical models based on quantum mechanical tunneling andclassical hopping over barriers. It has been observed that the temperature andfrequency dependence of the ac conductivity is consistent with the overlapping largepolaron tunneling model, while the classical barrier hopping models fail to interpretthe data.

Transport and dielectric properties of V2O5-MnO-TeO2 glasses

The frequency (1-10 kHz) and temperature (80-350 K) dependences of the ac conductivity and dielectric constant of the V2O5-MnO-TeO2 system, containing two transition-metal ions, have been measured. The dc conductivity dc measured in the high-temperature range (200-450 K) decreases with addition of the oxide MnO. This is considered to be due to the formation of bonds such as V--O--Mn and Mn--O--Mn in the glass. The conductivity arises mainly from polaron hopping between V4+M and V5+ ions. It is found that a mechanism of adiabatic small-polaron hopping is the most appropriate conduction model for these glasses. This is in sharp contrast with the behaviour of the Mn-free V2O5-TeO2 glass, in which non-adiabatic hopping takes place. High-temperature conductivity data satisfy Mott's small-polaron hopping model and also a model proposed by Schnakenberg in 1968. A power-law behaviour ( ac = Aωs , with s < 1) is well exhibited by the ac conductivity σac data of these glasses. Analysis of dielectric data indicates a Debye-type relaxation behaviour with a distribution of relaxation times. The MnO-concentration-dependent σac data follow an overlapping large-polaron tunnelling model over the entire range of temperatures studied. The estimated model parameters are reasonable and consistent with changes in composition.

Ac conductivity of glassy Bi4-nPbnSr3Ca3Cu4Ox semiconductors (with n=0.0, 0.1, 0.5, and 1.0): Precursors for high-Tc superconductors.

Experimental results are reported for the temperature- (T= 77--450 K) and frequency- (\ensuremath{\omega}=${10}^{2}$--${10}^{4}$ Hz) dependent (ac) conductivities (${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ac}}$) of Pb-doped ${\mathrm{Bi}}_{4\mathrm{\ensuremath{-}}\mathit{n}}$${\mathrm{Pb}}_{\mathit{n}}$${\mathrm{Sr}}_{3}$${\mathrm{Ca}}_{3}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{\mathit{x}}$ (n=0, 0.1, 0.5, and 1.0) semiconducting glasses. All these glasses show small clusters or grains of different sizes arising due to the phase separation as observed from the scanning electron microscopic study. The ac conductivities of these glasses follow the well-known relation ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ac}}$(\ensuremath{\omega})=A(T)${\mathrm{\ensuremath{\omega}}}^{\mathit{s}}$. In these glasses the electron-phonon interaction is strong enough to form small polarons and electrical conduction occurs by a small-polaron hopping mechanism. Like Li-doped ${\mathrm{Bi}}_{4}$${\mathrm{Sr}}_{3}$${\mathrm{Ca}}_{3}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ glasses, the exponents of all the present Pb-doped glasses are found to be a little higher than unity (1.1--1.2) in the low-temperature regime where ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ac}}$(\ensuremath{\omega}) also shows little nonlinear variation with temperature. Similar values of s (\ensuremath{\ge}1) have also been reported in the literature for glassy nonoxide semiconductors. The experimental ac conductivity data have been analyzed with different theoretical models based on quantum-mechanical tunneling through the barrier and classical hopping over the barrier mechanisms.The correlated barrier hopping (CBH) model is found to be most appropriate for these glasses, explaining the behavior of both ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ac}}$(\ensuremath{\omega},T) and its exponent s (except in the region where s&gt;1). Other models, such as the quantum-mechanical tunneling model and the overlapping large-polaron tunneling model, are found to be inadequate to explain the general behavior of both temperature- and frequency-dependent ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ac}}$ and s. The extended-pair approximation model and the percolation theoretical model are also found to fit the ac-conductivity data of these glasses (with s\ensuremath{\le}1). However, the fitting with the CBH model appears to be better. All the Pb-doped glasses are also found to be good precursors for superconductors with ${\mathit{T}}_{\mathit{c}}$ between 110 and 115 K in their crystalline phases obtained by annealing for optimum time of 24 h at 840 \ifmmode^\circ\else\textdegree\fi{}C. The thermoelectric powers of these superconductors are positive (hole-type superconductors).