Small Polaron Tunneling Research Articles

The complex impedance, dielectric relaxation, complex modulus and photoluminescence studies of pure and L-Methionine doped ammonium dihydrogen phosphate

The ammonium dihydrogen phosphate (ADP) is one of the pioneer nonlinear optical materials utilized for frequency doubling experiment of Nd:YAG laser. The amino acids are bifunctional material that contains the weak hydrogen bonded side chain. In the present study the authors have studied influence of L-Methionine on structural, photoluminescence and a.c.electrical properties of ADP. The powder XRD study suggested tetragonal structure symmetry and single phasic nature of all the samples. The photoluminescence is confirmed existence of the Self Trapped Excitons. The Correlation Barrier Hopping and Nonoverllaping Small Polaron Tunnelling kind of conduction mechanism is observed in present system. The Complex Impedance and Complex Modulus spectroscopy have suggested presence of grain and grain boundary within the samples.

Study of the magnetic, electrical and magneto-dielectric properties and dielectric relaxation in 0.8BiFeO3-0.2Ba0·8Sr0·2TiO3 solid solution

The structural, magnetic, electrical, magneto-dielectric and relaxor characteristics of 0.8BiFeO3-0.2Ba0·8Sr0·2TiO3 sample have been investigated. The XRD analysis explains that the sample possesses a rhombohedral crystal structure with an R3c space group. The sample has been observed to exhibit mixed magnetic ordering (comprising antiferromagnetic and ferromagnetic domains) with a magnetic transition temperature (TM) ≈ 556 K. The dielectric relaxation has been observed to be of non-Debye nature for all measuring temperatures (543 K–813 K). An anomalous dielectric, as well as relaxor behavior with dielectric constant (ε׳) peaking between 586 K–609 K for frequency varying from 2999 Hz to 99999 Hz, has also been observed. The sample exhibits positive magnetoelectric coupling with the highest magnetocapacitance (MC) value of 6.12% at 5 kHz. The ac conductivity obeys Universal Jonscher's Power law. The frequency dependence of ac conductivity at two different temperature regions follows the small polaron tunneling (NSPT) model and correlated barrier hopping model (CBH) model.

Sintering temperature effects on the impendence spectroscopy properties of Nd0.67Pb0.33Mn0.9Al0.1O3 perovskites

ABSTRACTThe impendence spectroscopy properties of polycrystalline Nd0.67Pb0.33Mn0.9Al0.1O3 perovskites prepared by sol–gel method at two different sintering temperatures (900°C and 1100°C), have been investigated by performing the measurements as a function of temperature and frequency of electrical conductance and impedance. In the whole explored temperature range, conductance measurements show that both samples present semiconductor-like behaviors. The non-overlapping small polaron tunneling (NSPT) model dominates the conduction process for the studied samples. An electrical relaxation phenomenon was observed from the variations of imaginary parts of impedance. Electrical parameters deduced from the Nyquist plots analysis using an equivalent circuit show that the conduction process for the samples is due to the grain boundaries contribution.

A New Anionic Structure Type Of Chlorobismuthate Salt: X-ray Characterization, DFT, Optical and Dielectric Properties of (C4H10N)8[Bi2Cl11][BiCl6]·2H2O

In recent years, attributed to their excellent anionic structure diversity, halogenobismuthate(III) materials have been a popular research direction. Apart from their interesting structural motifs, such hybrids often demonstrate potentially valuable properties such as luminescence and optoelectronics. Herein, we report the study of the first chlorobismuthate(III) hybrid compound involving discrete binuclear Bi2Cl11 and mononuclear BiCl6 anions coexisting together in the same structure. The anionic framework is connected to water molecules by O–H···Cl hydrogen bonds to build a corrugate chain spreading in the (111) direction. The intermolecular interactions were examined using Hirshfeld surfaces. The vibrational properties were investigated using the IR absorption spectroscopy. The optical study revealed the absorption and photoluminescence properties of the compound. Theoretical investigations were undertaken by means of DFT and TDDFT calculations on a suitably chosen cluster. A good agreement was found between calculations and experimental results leading to consistent vibrational and optical features assignments. The thermal behaviour was discussed. The variation of the dielectric loss log(ɛ″) with log(ω) was found to follow the empirical law, ɛ″ = Bωm(T). The dependence of the exponent m(T) on temperature, suggested that the correlated barrier hopping “CBH” and the non-overlapping small polaron tunneling “NSPT” are the appropriate models for the conduction mechanism.

Conduction mechanisms study in CaCu2.8Ni0.2Ti4O12 ceramics sintered at different temperatures

CaCu2.8Ni0.2Ti4O12 ceramics were prepared via the solid-state reaction. The pellets were sintering at 950 °C, 1000 °C, 1050 °C and 1100 °C for 24h. The X-ray diffraction (XRD) exhibits the crystallization of all the samples in the cubic structure with Im3‾ space group. The scanning electronic microscope (SEM) analysis show the growth of grain size by increasing sintering temperature from 0.65 to 18.58 μm. The conductivity analysis present a notable increase with the heating treatment. Both ac and dc electrical conductivity are fully examined as function of frequency and temperature. The ac conductivity discloses that correlated barrier hopping and non-overlapping small polaron tunneling models are the dominated conduction mechanisms existing in our compounds. By rising the sintering temperature the barrier height, above which jump the polarons or the electrons, tends to lower. The dc conductivity reveals that the conduction is achieved by the jump of polaron to the neighboring site and at variable distances.

Octahedral distortion and correlated electrical conduction of corundum type Co4Ta2O9 and Co4Nb2O9

The structural, elemental and electrical conductivity properties along with the relaxor phenomena are studied for corundum type and ceramics. The refined x-ray diffraction results confirm that the unit cells of both the ceramics are isostructural to A comparative study of polyhedral distortion shows that octahedra are more distorted than octahedra. X-ray photoelectron spectra of and compositions reveal that the majority of ions possess high spin 2+ oxidation state, whereas both and ions exhibit reduction in their oxidation state to 4+ suboxide state on the surface, because of the generation of oxygen vacancies along with trapped electrons in species. The effects of defect chemistry, ionic vacancies and crystal distortion on the charge transport and relaxor phenomenon are analyzed using impedance measurement over a wide range of temperature (30 °C–275 °C) and frequency (100 Hz–1 MHz). The frequency exponent versus plot of proposes that both correlated barriers hopping and small polaron tunneling are the principal charge transport mechanisms at high and low-temperature regimes respectively. In this composition, at high-temperature regime vacancy trapped electrons are the major charge carriers, whereas at low-temperature regime the charge carriers are small polarons. For a single conduction process is observed above 5 kHz frequency where overlapping large polarons tunneling is the predominant conduction mechanism and below this frequency, electrode polarization prevails.

Growth and characterization of pure and picric acid doped ADP single crystals

Ammonium dihydrogen phosphate (ADP) is a pioneer kind of nonlinear optical crystal. It can be used in various electro-optical, nonlinear optical applications. The pure and different weight percentage of Picric acid doped ADP single crystals are grown using the slow solvent evaporation technique at room temperature. The powder XRD study shows single phasic nature of all the grown crystals. The photoluminescence study suggests that the dopant causes defect in structure of ADP by virtue of vibration relaxation and self trapping exciton. The dielectric constant and the dielectric loss of all the grown crystals exhibited universal behaviour over frequency range considered. The Correlation Barrier Hopping and the Non Overlapping Small Polaron Tunnelling kind of conductivity mechanism are observed. The grain and grain boundary is indentified using the complex Impedance and the complex modulus spectroscopy. The third order nonlinear optical susceptibility is increased on doping the Picric acid in ADP crystals. The results are discussed here.

Influence of the chemical functionalization of carbon nanotubes on low temperature ac conductivity with polyaniline composites

We report the ac conductivity measurement of polyaniline (PANI) composites with carbon nanotubes (CNT) and functionalized carbon nanotubes (fCNT) with different degrees of functionalization from 300 K–4.2 K in the frequency range 40 Hz–5 MHz. The ac conductivity of PANI follows the universal power law and the charge transport is dominated by small polaron tunneling. In the CNT/PANI composite, metallic behaviour emerges, and ac conductivity does not obey the power law which is explained by the Drude model. Due to the chemical functionalization of CNT, disordered semiconducting behaviour appears in the transport below 50 K and follows the Johnscher’s power law. The enhancement of polaron formation due to the polar interaction between fCNT and PANI is reflected in the ac conductivity of the composites. The charge transport is manifested by single electron tunneling for a lower degree of fCNT/PANI (6 h) composite which changes to small polaron tunneling transport for a higher degree of fCNT/PANI (48 h) composite. The dc conductivity behaviour is explained by Mott’s VRH model and the characteristic Mott temperature increases with an increasing degree of chemical functionalization.

Electrical and NO2 Sensing Properties of a Series of Liquid Crystalline Porphyrins

The electrical and NO2 sensing properties of a series of porphyrin derivatives (PPC12, PPC16, ZnPPC12) were studied within wide temperature range from 293 to 423 K under the N2 gas flow and in the frequency range from 1 kHz to 1 MHz for ac electrical measurements. The dc conductivities and the activation energies were found to be in the range of 10−13−10−12 S cm−1 at the room temperature and 0.46–1.03 eV respectively. The increased C number in the alkyl chain and addition of metal ion to the porphyrin causes a decrease in the conductivity. The ac conductivity was well represented by the power law form Aωs. The s parameter depending on temperature was varied 0.79–1.2. The conduction mechanism for all porphyrins could be related to small polaron tunneling (SPT) model. As an application, the NO2 sensing properties of ZnPPC12 were investigated depending on temperature.

Structural, Optical and Electrical Properties of Eu-doped CuS Nanoparticles Synthesized Through the Aqueous Route

Eu-doped CuS nanoparticles stabilized by L-cysteine were synthesized by a low-temperature soft aqueous route. X-Ray Diffraction (XRD) patterns of the synthesized products reveal the formation of the hexagonal structure of covellite CuS. Scanning Electron Microscopy (SEM) images depict that the as-prepared nanoparticles exhibit relatively sphere like shaped morphology. Transmission Electron Microscopy (TEM) analyses show that the average size of the nanoparticles was found to be reduced with increasing the Eu concentration. UV-Visible optical absorption measurements reveal that the optical band gap is increased with increasing the Eu concentration, showing the presence of a blue shift due to quantum size effects. Impedance spectra were well modelled by introducing an electrical equivalent circuit. The electrical conductivity was found to be increased with increasing the Eu concentration. The temperature dependence of the DC conductivity confirmed the semiconducting nature of the as-prepared nanoparticles and was found to obey the Arrhenius law with two activation energies. The frequency dependence of the AC conductivity has been analyzed by Jonscher’s power law suggesting the non-overlapping small polaron tunneling (NSPT) type of conduction. The polaron hopping energy was found to be increased with increasing the Eu concentration. The dielectric constant of the as-synthesized nanoparticles was found to be decreased with the increase in Eu concentration. The dielectric loss tangent was found to be decreased and then increased at higher frequencies with increasing the Eu concentration.

Transport phenomena of Cu-S-Te chalcogenide nanocomposites: frequency response and AC conductivity.

In this work, the development and electrical characterization of several chalcogenide nanocomposites have been reported. X-ray diffraction (XRD) has been used to reveal their microstructures. Mott's variable range hopping model has been used to interpret the DC conductivity data of the nanocomposites at lower temperatures. The DC conductivity data at higher temperatures has been explained well using Greave's model. To explain the AC conductivity data, the Meyer-Neldel (MN) conduction rule has been employed. The AC conductivity spectra at different temperatures have been analyzed using Almond-West formalism. Different conduction models, namely, correlated barrier hopping (CBH) and modified non-overlapping small polaron tunneling (NSPT), have been used to interpret the conduction mechanism of the nanocomposites. Scaling of the AC conductivity spectra reveals that the electrical relaxation process is independent of temperature, but depends on the nanocomposite composition. The conductivity mechanism is explained using a schematic structural model.

Investigation of Optical Properties and Electrical Conductivity Mechanism of Fe <sub>2</sub>O <sub>3</sub>–Sm <sub>2</sub>O <sub>3</sub>–ZnO–P <sub>2</sub>O <sub>5</sub> Quaternary Glass Nanocomposite Systems

The influence of incorporation of Fe2O3 on the physical, structural, optical, and electrical properties of zinc- samarium- phosphate glasses synthesized via conventional melt quenching technique have been studied. The polycrystalline nature, crystallinity percentage, and average size of the developed nanocrystallites in the prepared sample have been determined from the obtained X-ray diffraction data. Density and molar volume data show inverse relation, which has been explained by the formation of Non-bridging oxygens. The optical bandgap energy (Eopt) values, obtained from spectroscopic data, and the calculated Urbach energy values (EU) has an inverse relationship varying with Fe2O3 concentration (x). The Fe2O3 concentration (x) dependent refractive index, electronic polarizability, molar refraction, oxide ion polarizability, optical basicity, and electronegativity have been determined. Mott's variable range hopping model is used to explain the DC conduction mechanism of the glass nanocomposites system, while the non-overlapping small polaron tunneling model has been used to explain the AC conduction mechanism. Both the AC and DC conductivity are found to increase with the incorporation of Fe2O3. The scaling property of AC conductivity reveals that the conductivity relaxation process depends on the structural composition of the present sample but is independent of temperature.

Influence of vacancy co-doping on the physical features of NdFeO3 nanostructure perovskites

Orthoferrite nanoparticles of the chemical formula Nd1−x□xFe1−y□yO3, (0.0 ≤ y, x ≤ 0.03, step 0.01) were successfully synthesized using citrate combustion method. The samples possess single-phase orthorhombic structure with space group Pbnm. High-resolution transmission electron microscope and selected area electron diffraction were carried out and confirmed that the samples were formed in nanoscale. The molar magnetic susceptibility (χM) indicated the antiferromagnetic behavior of the samples. The magnetic properties of the samples were enhanced by increasing the vacancy concentration in the samples. Both the saturation magnetization (Ms) and remanent magnetization (Mr) of the sample Nd0.97□0.03Fe0.97□0.03O3 were increased by 1.5 times than the parent sample NdFeO3. The small polaron tunneling and the correlated barrier hopping models were used for the interpretation of the conduction mechanism.

Thermally stimulated small polaron promoted conduction mechanism in Fe-doped La0.7Sm0.3CrO3

In this study, nano-sized La0.7Sm0.3Cr1-xFexO3 (0 ≤ x ≤ 0.3) samples were prepared using the sol–gel process. No extra peaks were present in the X-ray diffraction patterns, which indicated their single phase nature. Orthorhombic crystal configurations with the Pnma space group were confirmed for all of the samples. The crystallite size, unit cell volume, bond lengths, and bond angles were found to increase as the Fe content increased. Scanning electron microscopy and energy dispersive X-ray (EDX) spectroscopy were used to analyze the surface morphology and chemical composition, which demonstrated the formation of uniform, homogeneous, and high purity samples with agglomerated particles. Transmission electron microscopy showed that the samples had rough surfaces where agglomerated particle were evident in the images and the particle size increased with the Fe content. Dielectric measurements were conducted for La0.7Sm0.3Cr1-xFexO3 (0 ≤ x ≤ 0.3) to determine the responses of the real and imaginary parts of the dielectric permittivity as well as the tangent loss with respect to temperature. The results demonstrated that the dielectric behavior was frequency dependent and thermally stimulated. Cole–Cole plots for all the samples indicated the existence of a non-Debye type relaxation phenomenon. The ac conductivity was found to obey the Jonscher power law according to plots of the frequency versus ac conductivity. The temperature dependence of the exponent “s” showed that the correlated barrier hopping and non-overlapping small polaron tunneling models of conduction were followed.

Improvement of spin-charge related magnetoelectric coupling and transport response of rare-earth based eco-friendly multiferroic nanocomposites

Eco-friendly, magnetoelectric LaFeO3–HoMnO3 nanocomposites are synthesized with different weight percentage, and highlighted their observed magnetoelectric coupling, electrical transport and dielectric properties. X-ray diffraction pattern and field emission scanning electron microscope images approve the formation of LaFeO3 and HoMnO3 phases without the presence of any extra phase. These composites reveal the strain mediated magnetoelectric effect with longitudinal (αE33) and transverse (αE31) configurations at lower frequency region under an external magnetic field. Maximum values of αE31 and αE33 are found to ~ 0.54 and ~ 0.31 mV/cm-Oe, respectively for 0.3LaFeO3–0.7HoMnO3 nanocomposites. The electronic transport has been examined by impedance spectroscopy technique for understanding the grain boundary and grain effects in resistive and capacitive behavior of the materials considering an idealized simulated circuit. Temperature dependent Nyquist plots show the non-Debye type phenomena as well as semiconductor in nature (i.e., negative temperature coefficient of resistance) of the materials. Electrical susceptibility reveals the domain motion of the system, which is corresponding to the electrostriction property. The electrical conductivity with frequency follows the Joncher’s single power law and it is controlled through large and small polaron tunneling model. The synthesized nanocomposites are a capable candidate to offer some helpful ideas for developing the lead free device applications.

Temperature dependent dielectric properties and ac conductivity of GdFe1−xMnxO3 (0 ≤ x ≤ 0.3) perovskites

We have synthesized the nano-crystalline samples of GdFe1−xMnxO3(0 ≤ x ≤ 0.3) via solid state reaction route and studied their dielectric properties and ac conductivity in detail. X-ray diffraction patterns show monophase structure of the samples in orthorhombic crystal symmetry (space group Pbnm). The enhancement in dielectric constant is registered on Mn doping at room temperature as well as with the rise in temperature at a constant frequency. The fitting of Cole–Cole plots indicate the non-Debye kind relaxation mechanisms in the samples. The real (Z′) and imaginary (Z″) components of impedance reduce with the increase in frequency and Mn ions in GdFeO3 lattice. Nyquist plots are fitted to a single semicircle that hints towards the single relaxation process. The ac conductivity (σac) is also increased with the frequency and follow the power law,σac(ω) = Aωs wherein the exponent ‘s’ reduces with frequency. We have estimated the activation energies with the help of ln(σ) versus 1/T plots that remain almost constant below 350 K and show variation with the Mn doping in GdFeO3 perovskite at temperatures 620–673 K. The polaron hopping energy (WM) in the framework of the correlated barrier hopping (CBH) and non-overlapping small polaron tunneling (NSPT) models have been calculated and its variation with temperature signifies thermally activated conduction mechanism in the studied materials.

Enhancement in dielectric and optical properties of La1-xCexFeO3 nanoparticles

La1-xCexFeO3; 0≤×≤0.15 nanoparticles were prepared in single phase using flash combustion technique. The crystalline structure and morphology of the investigated samples were researched through XRD and HRTEM. As a result, the dielectric constant of LaFeO3 was enhanced by Ce3+ ion substitution due to some the Ce3+ ions changes its valence to Ce4+ ions. The conduction mechanism in La1-xCexFeO3 samples is the small polaron (SP) tunneling. The type of optical transition is direct allowed transition and the band gap values are in the range of (2.8–1.89eV). ε/ values of La0.90Ce0.10FeO3 decreases by increasing the external applied pressure at different frequencies.

Small polaron hopping-assisted electrical conduction and relaxation in BCT and Mn-doped BCT samples

ABSTRACTPolycrystalline samples of Ba0.96Ca0.04TiO3 (BCT) and Ba0.96Ca0.04Ti0.96Mn0.04O3 (BCMT) were synthesized using the solid-state reaction route. XRD analysis confirmed the presence of the tetragonal phase in both as well as an additional hexagonal phase in the BCMT samples. The wt % of the segregated phase was found to be ≈8.96% using Rietveld refinement. SEM micrographs showed an increase in the density and a decrease in the grain size of the BCMT sample. P-E measurements confirmed higher remnant polarization in the BCT samples. A single depressed semicircle was observed in the Nyquist plots indicating non-Debye-type behavior and overlapping contributions of the grains and grain boundaries. The increase in the value of the conductivity exponent “n” (and n < 1) with temperature suggested that the electrical conduction mechanism was non-overlapping small polaron tunneling. The electron acceptor nature of the Mn ions and lower grain size of the BCMT samples resulted in higher activation energy as compared to the BCT samples.

Impurity induced dielectric relaxor behavior in Zn doped LaFeO3

Sol–gel derived nanocrystalline samples of LaFe1−xZnxO3 have been investigated for their dielectric properties in the broad frequency and temperature range. The pristine sample exhibits an anomaly around Neel temperature in the variation of dielectric permittivity against temperature. However, the loss factor shows frequency dependent peak indicating the relaxational behavior in the LaFeO3 system. The incorporation of Zn enhances the relaxational behavior in the pristine system. A.C. conductivity is determined for all the samples in the temperature range 300–600 K. Activation energy determined in the high-temperature range (500–600 K) may correspond to the motion of singly ionized oxygen vacancies, in the vicinity of 450–500 K corresponds to the motion of doubly ionized oxygen vacancies within the single FeO6 octahedra and in the lower temperature range (300–450 K) it may correspond to hopping of electrons between Fe2+ and Fe3+ ions. The hopping conduction mechanism has been investigated from the temperature dependence of the “s” parameter. The conduction mechanism in pristine sample is governed by the correlated barrier hopping (CBH) model. However, in doped samples, the conduction mechanism follows both CBH and non-overlapping small polaron tunneling (NSPT) models.

Grain size effect on the electrical properties of nanocrystalline Gd2Zr2O7 ceramics

A detailed study on the effect of grain size of nanocrystalline Gd2Zr2O7 (GZO) ceramics on its electrical properties, particularly the ionic conductivity, is reported. The nanocrystalline GZO was prepared by chemical co-precipitation method. The GZO ceramic with different grain sizes from 5 to 55 nm was fabricated by sintering them at different temperatures varying from 800 to 1400 °C. Structural studies were performed by X-ray diffraction and Raman spectroscopy and found the cubic pyrochlore phase of GZO in all cases. Their crystallite size has been increased from 5 to 55 nm as well as the structural ordering has been increased with the increase in sintering temperature. The ac electrical impedance analysis showed a dramatic increase in ionic conductivity with increasing grain size. The ionic conductivity measured at 700 °C was 8.7 × 10−6, 12.5 × 10−6, 20.4 × 10−6 and 45.4 × 10−6 S cm−1 for respective grain sizes 5, 12, 25, and 55 nm of GZO ceramics. Increase in conductivity was correlated to the increase in grain size. The activation energy for grain conductivity was found to be 1.42 ± 0.01 eV for all GZO ceramics irrespective of their grain size. However, the activation energy of grain boundary and total conductivity had decreased with increasing grain size. The activation energy for total conductivity was obtained to be 1.5, 1.48, 1.46 and 1.42 eV for 5, 12, 25, and 55 nm GZO ceramics respectively. Further, the ac conductivity was found to obey the universal Jonscher's power law and the power exponent s was found to increase with the increase in temperature. It was also showed that the non-overlapping small polaron tunneling (NSPT) has been the best suited theoretical model to explain the conduction mechanism occurred in GZO ceramics. In addition, increase in co-operative dynamics of mobile ions with increase in grain size was found to favor the enhancement of conductivity.