Non-adiabatic Small Polaron Hopping Research Articles

Elaboration of La (Sr/Na) Mn (Ti) O3 ceramic, structural, and morphological investigations, and contribution of direct and indirect interactions on transport properties

The charge carrier's dynamics of La0.7(Sr5/6Na1/6)0.3Mn0.7Ti0.3O3 (LSNMTO) ceramic is conducted using the temperature and frequency dependences of the electrical conductivity and their scaling formalisms. The structural study indicates that LSNMTO crystallizes in a rhombohedral R3‾c perovskite structure with no detectible secondary phases. A phase transition from the high-temperature metallic behavior to the low-temperature semiconductor nature is observed from the electrical conductivity curve at TS-M = 415 K. Further, it is observed that the cation-anion-cation and the cation-cation interactions are present in the material, and play important role in governing the electrical behaviors of LSNMTO. From 200 K to 415 K, the Non-adiabatic Small Polaron Hopping is the predominant conduction process, although the Mott-Variable Range Hopping mechanism governs the conductivity at low temperatures. In the intermediate temperature range, the dynamic of the charge carriers is explained using the Shklovskii Efros model. The frequency-activated conductivity obeyed the universal laws (double-Jonscher and Jonscher laws). At high frequencies, the dispersion of the conductivity spectra is attributed to the presence of hopping and tunneling conduction processes. The Time-Temperature Superposition Principle (TTSP) is obeyed over a large temperature range from 160 K to 350 K. Below 160 K, the deviation from the TTSP is attributed to the coexistence of hopping and tunneling conduction mechanisms. Deviations from the Summerfield scaling and shifts of the isotherms to higher values on decreasing the temperature are due to the local structural disorder in LSNMTO. Therefore, the structural particularities of LSNMTO can result in diverse conduction pathways giving an increase to the deviations from the Summerfield scaling. The studied ceramic exhibits a very motivating maximum negative temperature coefficient of resistance NTCR = −13.36% which is significantly elevated as compared to previously investigated compounds.

Nature of polaronic hopping conduction mechanism in polycrystalline and nanocrystalline Gd0.5Sr0.5MnO3 compounds

In the present study, we have investigated the structural, electrical and magneto-transport properties of polycrystalline and nanocrystalline Gd0.5Sr0.5MnO3 compounds. The variation of transport properties is modified by tuning the grain size of the material. In the high temperature semiconducting region, temperature dependent resistivity data can be well explained by non-adiabatic small polaron hopping (SPH) mechanism. In addition, the resistivity data for all compounds in the low-temperature paramagnetic region can also be well explained by the variable range hopping (VRH) model. The parameters obtained from SPH and VRH mechanisms are found to be reasonable. In the case of nanocrystalline compounds, there is an overlapping temperature range where both SPH and VRH models are valid simultaneously, and a new conduction mechanism - variable range hopping of small polaron s(VR-SPH) is satisfactorily valid for the whole temperature range of these compounds. However, for the polycrystalline compound, the overlapping temperature region between VRH and SPH models does not exist and the VR-SPH mechanism is not valid here. Thus, polarons play a leading role in selecting different conduction mechanisms in different temperature ranges.

Non-overlapping small polaron and overlapping large polaron assisted conduction relaxation in polycrystalline YFe0·9Cr0·1O3 – An impedance spectroscopic and AC conductivity studies

Detailed ac electrical investigations within the frequency range 100 Hz-1 MHz have been conducted on the polycrystalline YFe0·9Cr0·1O3 samples prepared via solid state reaction route, in a broad thermal interval of 298–673 K. A systematic development of grain, grain boundary and electrode polarization contribution have been observed with increasing temperature. The thermal variation of the grain resistance (R g) and grain boundary resistances (R gb) followed the non-adiabatic small polaron hopping (SPH) model and is responsible for the long-range conduction with different values of the activation energies at different thermal regimes. The ac conductivity of the material exhibited series of dispersive regions with broad region, where the slope changes occurred up to 473 K. Above 473 K, appreciable dc conductivity in the material can be seen. Different dispersive regions in the conductivity spectrum obeyed single power law and power law variation (Aωn).

The Effect of Alkaline Earth (Ba, Sr and Ca) Doped Iron Bismuth Glasses on the Structural, Thermoelectric and Electrical Properties

Glasses with nominal composition 70Bi2O3–30Fe2O3 and 10X–60Bi2O3–30Fe2O3 (mol%); (X = Ba, Sr and Ca) were prepared by the conventional melt quenching technique. X-ray diffraction and differential scanning calorimeter confirm the amorphous nature of the glass samples. The iron-bismuth glass shows good solubility of alkaline earth elements ions. In temperatures range of 310–450 K, the dc conductivity of the glass samples containing alkaline earth elements enhanced. Glass sample containing Sr shows interesting electrical properties. All glass samples showed a transition from negative to positive Seebeck coefficient, this means that the conduction is mixed of electrons and holes charge carriers. The conduction mechanism of all samples obeys non-adiabatic small polaron hopping model of electron between iron ions. The calculated small polaron coupling constant, (γp) was found to be in the range of 10.25–17.28. Also, the calculated hopping mobility (μ) and carrier density (Nc) of glasses were in the range of 4.65 \(\times {10}^{- 7}\) to 4.11 \(\times {10}^{-3},\left({\mathrm{cm}}^{2}\,{\mathrm{V}}^{-1}\,{\mathrm{s}}^{-1}\right)\) and 0.029–10 \(({\times {10}^{17}\,\mathrm{cm}}^{-3})\) at 333 K, respectively. The conductivity of the present glasses was primarily determined by hopping carrier mobility.

Tuned Transport Behavior of the IPA-Treated PEDOT:PSS Flexible Temperature Sensor via Screen Printing

A flexible temperature sensor based on poly-(3, 4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) with isopropyl alcohol (IPA) treatment is successfully fabricated by screen printing. The distribution of PEDOT is investigated by the SEM, AFM, and Raman techniques. It can be found that the microstructure is manipulated by the ratio of the PEDOT:PSS to IPA content. Raman spectra demonstrate that the transition between quinoid and benzoid structures of PEDOT are tuned by IPA treatment as well. The temperature dependence of resistance for IPA-treated PEDOT:PSS film is found to be well fitted by non-adiabatic small polaron hopping transport. Moreover, a constant temperature coefficient of resistance (TCR) value can be obtained by the optimized ratio of IPA to PEDOT:PSS. Therefore, this paper suggests that the tuned microstructure of PEDOT:PSS film can be realized by IPA treatment, which plays an important role in the physical properties of the temperature sensor based on PEDOT:PSS.

Non-adiabatic small polaron hopping transport above metal-like to insulator transition in the vacant 3d-orbital Tb2Ti2O7 pyrochlore oxide

We report the validity of Mott variable range hopping (VRH) conduction mechanism and small polaron hopping in Tb2Ti2O7 in the temperature range 603–803 K. The temperature-dependent resistivity data are in good agreement with Mott 3d VRH. The other parameters estimated such as hopping range, hopping energy and density of states near Fermi level are in good agreement with those of many semiconductor oxides. The Holstein’s condition for non-adiabatic conduction mechanism is also satisfied. The ac conductivity is governed by Jonscher’s power law as $$ {\sigma}_{a.c}^{\prime}\left(\upsilon, T\right)={\sigma}_{d.c}(T)+\alpha (T){\upsilon}^n $$ and the exponent n values in the range of 1.02–1.23 suggest that conduction takes place due to the small polaron hopping mechanism. The possible polaron formation and their mechanism are discussed systematically.

High-temperature transport properties of lithium manganese spinels substituted with Ni and Ti

Spinel Li1.05Mn2O4−δ substituted with Ni and Ti were fabricated via a solid-state reaction technique. The Rietveld refinement of synchrotron X-ray powder diffraction data confirmed an Fd3‾m space group of spinel structure for all synthesized materials. Variation in the lattice parameter and M-O distances (M: transition metal at 16d site) due to the dopant content indicated that Ni substitution enhanced the overlap in Mn 3d wave functions and Ti substitution induced structural distortion. Electrical conductivity measurements based on a four-probe method were conducted at high temperatures, demonstrating that both the Ni- and Ti-substituted samples exhibited the same non-adiabatic small-polaron hopping mechanism as that of the non-substituted LiMn2O4. Thermopower measurements were performed at high temperatures via a steady-state technique. All samples with and without substitution were characterized as n-type semiconductors and their Seebeck coefficients were thermally activated. The electrical conductivities and the Seebeck coefficients decreased with increasing Ni content, whereas they were less affected by Ti substitution. The activation energies of electrical conductivity Eσ and thermopower ES were derived from the experimental results and were used to calculate the hopping energies, WH. Ni substitution initially reduced WH but substantially increased WH when Ni ​= ​0.5. In contrast, Ti substitution had a major effect on only ES. Variation in carrier density and structural distortion following the substitution for Mn exerted a large influence on the electrical conductivity and Seebeck coefficients of the doped materials.

Electrical conduction in 70V2O5·30TeO2 (mol%) pelletized glasses annealed in an H2 gas atmosphere

The direct-current electrical conductivities (σ) of 70V2O5∙30TeO2 (mol%) pelletized glasses annealed in an H2 gas atmosphere at 473 K were determined in the temperature (T) range of 322–473 K. Based on the log(σT)–T−1 relation in the considered temperature range, the electrical conduction is discussed in detail. The conduction of each glass sample at high temperatures (T > ~460 K) was attributed to nonadiabatic small polaron hopping. At T < ~353 K, the slopes of the curves were the smallest. The smallest slopes at temperatures below ~353 K are discussed based on the Mott's and Greaves' variable-range hopping (VRH) conduction models. The tunneling factors α and densities of states at the Fermi level N(EF) estimated using the Mott's VRH model confirmed that this model was not suitable. On the other hand, the α and N(EF) values estimated by the Greaves' VRH model indicated that the conduction was attributed to the Greaves VRH. The values of WD (0.0765 ± 0.0006 to 0.108 ± 0.0006 eV) calculated using the Greaves' model were on the same order as those of the reported vanadate binary glasses. These parameters demonstrated that the Greaves' VRH model was applicable for the considered glasses and could be used to explain the conductivity data at temperatures below ~354 K.

Enhanced low-field magnetoresistance in LSMO/AZO composites prepared by plasma activated sintering

To improve the low-field magnetoresistance of La0.7Sr0.3MnO3 (LSMO) ceramics, Al (3 wt%)-doped ZnO (AZO) was added to LSMO as the second phase, and (1 − x) LSMO/xAZO (x = 0, 0.1, 0.2, 0.3 and 0.4) composites were first prepared by plasma activated sintering. The effect of the AZO content on the structural, electrical and magnetic properties of the composites was studied. The results of XRD, SEM, EDS and TEM analyses indicated that the LSMO phase coexisted with the AZO phase without any reaction, and that all of the samples had a high relative density (> 96.9%). The magnetization of the (1 − x) LSMO/xAZO composites decreased with the increase of the AZO content, while the Curie temperature remained at approximately 360 K. With an increase of the AZO content, both the resistivity and metal-insulator transition temperatures decreased. The resistivity (ρ) – temperature (T) data were well fitted to the equation of ρ=ρ0+ρ2T2+ρ4.5T4.5+ρ5T5 in the low-temperature region, and to the non-adiabatic small polaron hopping model in the high-temperature region. The maximum value of the low-field magnetoresistance at room temperature was obtained for the 0.8LSMO/0.2AZO composite, which reached 16.2%, indicating the addition of AZO effectively enhanced the low-field magnetoresistance of LSMO ceramics.

Polaronic transport in antimony-vanadium tellurate glasses at high temperatures

The ternary bulk glasses of the form (60-x)V2O5-40TeO2-xSb with 0 ≤ x≤15 (in mol%) were prepared by using the standard melt quenching technique. The dc electrical conductivity of the mentioned samples was investigated within the temperature range of 296–408 K. The dc conductivities at 370 K, with values in the range of 4.80 × 10−6–1.08 × 10−4 S cm−1, were found to decrease with increase of Sb content. The experimental temperature-dependency of conductivity and the variation of the pre-exponential factor were interpreted in terms of polaron hopping theory. It founds that within the studied temperature range, the non-adiabatic small polaron hopping (NASPH) mechanism for electrons between different valent states of vanadium ions is applicable. The ratio of wave function decay (tunneling factor α) was found to be 2.26 × 108 cm−1 for samples with x ≥ 8 mol%.

Investigation of DC Conductivity and Non-Adiabatic Small Polaron Hopping in V2o5-Seo2-Zno Glass Nanocomposites

Investigation of DC Conductivity and Non-Adiabatic Small Polaron Hopping in V2o5-Seo2-Zno Glass Nanocomposites

Effect of sulfur addition and nanocrystallization on the transport properties of lithium–vanadium–phosphate glasses

The glasses defined by the formula 37.5Li2O–25V2O5–37.5P2O5 mol% containing different sulfur (0, 10, 50 and 100 mol%) content were studied before and after nanocrystallization. X-ray diffraction and transmission electron micrograph of the heat treated samples indicated nanocrystals embedded in the glass matrix. The average crystallite size was found between 18 and 37 nm. Sulfur (S) behaved as a reducing agent for redox reaction during preparation of glass and affected the conductivity, i.e., the V4+–V5+ or V3+–V4+ion pairs increased with increasing S content and led to increasing conductivity of glasses. After creation of the nanocrystalline phase, S-free glass–ceramic nanocomposite exhibited improvement in electrical conductivity around three orders of magnitude than initial glass. This great improvement of electrical conductivity is related to increase in a concentration of V4+–V5+or V3+–V4+ ion pairs and also, forming of defective and well-conducting regions along the crystallite/glass interfaces. The decrease in electrical conductivity in the 50S glass–ceramic nanocomposite, which possessed the highest crystallite size, could be related to the increase of grain boundaries scattering because of the increasing crystallite size. The conduction was attributed to non-adiabatic small polaron hopping and mostly determined by hopping carrier mobility.

Negative Thermal Sensitivity of La&lt;sub&gt;1-x&lt;/sub&gt;Sr&lt;sub&gt;x&lt;/sub&gt;FeO&lt;sub&gt;3&lt;/sub&gt; Films Prepared by Screen-Printing Method

The impacts of different Sr content on the phase structure, negative temperature coefficient (NTC) characteristic and conduction mechanism at high temperature of lanthanum strontium ferrite (La1-xSrxFeO3, x=0.1~0.6) (LSFO) films were systematically discussed. The LSFO films were prepared on the alumina substrate by the screen printing method. The results showed that the crystal structure transformed from orthorhombic (x=0.1~0.3) to rhombohedral (x=0.4~0.6). All the samples presented NTC performance. With increasing the Sr content, B values increased to the maximum 3885 K (x=0.4) and then decreased. Non-adiabatic small polaron hopping mechanism was dominant for their electric transport in the temperature range from 450 K to 873 K. The activation energy was calculated between 0.37 eV and 0.57 eV, and the sample La0.7Sr0.3FeO3 showed the minimum value of the activation energy. Therefore the La1-xSrxFeO3 (x=0.3, 0.4, 0.5) films have the potential to be developed into high-performance NTC resistors.

Correlation between grain size and transport properties of lead titanate based-glass–ceramic nano-composites

Glass of composition 10PbTiO3–60V2O5–30B2O3 in mol% was prepared by conventional melt quenching technique. Annealing process was applied to the glass samples at temperature close to crystallization temperature Tc for different time intervals 1, 2, 4, 8 and 24 h, respectively to obtain glass ceramic nano-composites. The samples densities were measured by Archimedes method. Differential scanning calorimeter data confirms the amorphous nature and shows high homogeneity of the glass sample. Transmission electron microscopy and X-ray diffraction were also used to confirm the amorphous nature and the existence of nano crystallites embedded in the glass matrix. Glass ceramic nano-composites show giant enhancement in electrical conductivity (about 6 orders of magnitude) as a function of heat treatment times. This enhancement of conductivity is attributed to the increase in crystal size which leads to the increase in the concentration of V ions pairs and the reduction of grain boundary scattering effect. The dc electrical conductivity in the present glass and corresponding glass ceramic nano-composites at high temperature T > (θD/2), (θD: Deby temperature) was due to non-adiabatic small polaron hopping of electron between vanadium ions.

Investigation of magnetic interactions and transport mechanism in Al−substituted La0.4Bi0.6MnO3 manganites

In this paper, the effect of Al substitution on structural, magnetic, electrical and magnetotransport properties of the polycrystalline La0.4Bi0.6Mn1−xAlxO3 (where, x = 0.01, 0.05 and 0.1) have been investigated. Powder X-ray diffraction measurement and Rietveld refinement technique revealed that all the samples crystallize into orthorhombic structure having Pnma space group. The X-Ray photoemission spectra (XPS) measurements have been carried out on the samples to understand the core level states of Mn, Al and O in the samples. The lattice parameters and grain size are found to decrease with increasing Al concentration which is also evident from scanning electron micrographs. The magnetic measurements suggest the ferromagnetic nature along with trivial charge ordered phase present in the samples. Interestingly the suppression of charge ordering is observed with increasing Al substitution at Mn site. Frequency dependent ac susceptibility measurements reveal dynamic fluctuations due to cooperative interactions between antiferromagnetic domain pinning effects giving rise to spin-glass property. Electrical resistivity measurements reveal the semiconducting behavior of the sample in the entire temperature range studied. Analysis of resistivity data with temperature suggests that for T>(θD/2, the Debye temperature) the electrical transport is governed by non-adiabatic small polaron hopping mechanism. Whereas for T≤θD/2 the conduction process follows Shklovskii-Efros variable range hopping mechanism. Further the MR studies explain the grain contribution on the observed electrical transport properties.

Grain size effects on the transport properties of Li3V2(PO4)3 glass–ceramic nanocomposites for lithium cathode batteries

The effect of grain size on the structural and transport properties of glass–ceramic nanocomposites consisting of Li3V2(PO4)3 crystals were investigated by differential scanning calorimeter, X-ray diffraction (XRD), transmission electron micrograph (TEM), density (d) and dc conductivity. XRD patterns exhibit the formation of Li3V2(PO4)3 with monoclinic structure. It was shown by XRD and TEM studies that by appropriate heat-treatment glasses can be turned into glass–ceramic nanocomposites consisting of crystallites smaller than 56 nm embedded in the glassy matrix. Density was found to increase with increasing grain size while the molar volume decreases. The dc conductivity measured in high temperature range increased with decreasing of the grain size while the activation energy decreased. The calculated dc conductivity at 400 K for the present glass was 5.77 × 10−7 S/cm, while in its glass–ceramic nanocomposites was in the range of 1.712 × 10−5–2.10 × 10−6 S/cm. On the other hand, the calculated activation energy for the present glass was 0.542 eV, while in its glass–ceramic nanocomposites was in the range of 0.165–0.468 eV. The increase in electrical conductivity in the glass–ceramic nanocomposite annealed at 450 °C for 4 h compared to that annealed at 450 °C for 2, 8, 24 h can be attributed to the decrease in grain boundary scattering due to the reduction in grain size. The conduction was confirmed to obey non-adiabatic small polaron hopping. The electron–phonon interaction coefficient (γ p) was calculated and found to be in the range of 5.09–16.64.

Transport properties of Ba-doped BiFeO3 multiferroic nanoparticles

Multiferroic nanoparticles of Bi1-xBaxFeO3 (BiBaFeO3); (x = 0.10, 0.15, 0.20 and 0.25 mol%) sam- ples were prepared using conventional solid-state method. The nanostructural and transport properties of the prepared samples were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and electrical con- ductivity. XRD patterns show the formation of BiBaFeO3 with single-phase rhombohedral-hexagonal structure. The particles sizes were found to be in the range 20-33 nm. SEM micrograph revealed the nanostructure consisting of small, randomly oriented and non-uniform grains. Dc conductivity shows that all samples are semiconductor and the maximum was found at x = 0.15 mol%. This increase of the conductivity can be attributed to the decrease in grain boundary scattering due to the reduction in crystallite size. The calculated activation energy for the multiferroic nanoparticles was found to be 0.413-0.929 eV. The con- duction was confirmed to obey non-adiabatic small polaron hopping (SPH). The electron-phonon interaction coeffi- cient (cp) was calculated and found to be in the range of (12.79-27.21). The hopping carrier mobility varied from 1.85 9 10 -7 cm 2 V -1 s -1 to 8.01 9 10 -13 cm 2 V -1 s -1 at 418 K. The conductivity was primarily determined by hopping carrier mobility.

Non-adiabatic small polaron hopping and the effect of spin-polarized tunneling in electron-doped Ca0.85Nd0.15MnO3

The electron-doped manganite Ca0.85Nd0.15MnO3 has been investigated through magnetic and electrical transport studies. It shows insulating behavior down to the lowest temperature (5 K) measurement even on application of high magnetic field (up to 15 T) indicating a robust insulating behavior. Small polaron hopping and variable range hopping conduction have been observed at high- and low-temperature region, respectively. High-T conductivity of the sample follows non-adiabatic hopping conduction mechanism. Magnetic field-dependent resistivity has been thoroughly examined in terms of a phenomenological model taking into account the spin-polarized tunneling (SPT) of the conduction electrons. In order to explore the nature of the temperature-dependent magnetoresistance (MR), we have separated the contribution related to SPT and spin fluctuation basing on the model. Scaled isothermal MR data collapse into two curves separating the charge order temperature (TCO), which indicates the existence of general formalism of MR and also suggests temperature-independent behavior of the reduced MR.

Nature of electrical hopping conduction and magnetotransport studies in the electron doped manganite Ca0.85Gd0.15MnO3

The magnetotransport properties of Ca0.85Gd0.15MnO3 have been studied analyzing the temperature and magnetic field dependent resistivity. High temperature (T>TCO: charge ordering temperature) conductivity has been found to be described by the non-adiabatic small polaron hopping (SPH) model. The manganite shows a parallel combination of SPH and variable range hopping (VRH) model below TCO. VRH model fits the corresponding low temperature data. The field depended magnetoresistance (MR) has been investigated with a phenomenological model considering the spin-polarized tunneling. The field dependent MR in scaled coordinate collapses into single curve demonstrating the temperature-independent behavior.

Electrical Conductivity of V2O5–TeO2–Sb Glasses at Low Temperatures

Semiconducting glasses of the type 40TeO2–(60 − x) V2O5–xSb were prepared by rapid melt quenching and their dc electrical conductivity was measured in the temperature range 180–296 K. For these glassy samples, the dc electrical conductivity ranged from 2.26 × 10−7 S cm−1 to 1.11 × 10−5 S cm−1 at 296 K, indicating the conductivity is enhanced by increasing the V2O5 content. These experimental results could be explained on the basis of different mechanisms (based on polaron-hopping theory) in the different temperature regions. At temperatures above ΘD/2 (where ΘD is the Debye temperature), the non-adiabatic small polaron hopping (NASPH) model is consistent with the data, whereas at temperatures below ΘD/2, a T−1/4 dependence of the conductivity indicative of the variable range hopping (VRH) mechanism is dominant. For all these glasses crossover from SPH to VRH conduction was observed at a characteristic temperature TR ≤ ΘD/2. In this study, the hopping carrier density and carrier mobility were determined at different temperatures. N (EF), the density of states at (or near) the Fermi level, was also determined from the Mott variables; the results were dependent on V2O5 content.