Small Polaron Tunneling Research Articles

Investigation of optical properties and electrical conductivity mechanism of Fe2O3–Sm2O3–ZnO–P2O5 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 as-prepared samples have been determined from the obtained X-ray diffraction data. TEM micrographs and SAED patterns confirm the formation of nanocrystallites over the amorphous glassy network. EDX spectra reveal the weight and atomic percentage of each constituent element. 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 UV-Vis spectroscopic data, and the calculated Urbach energy (EU) values reduce with increasing 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, whereas the non-overlapping small polaron tunneling model is 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.

VRH and NSPT conduction mechanisms in CuIn0.7Ga0.3Se2 near the liquid nitrogen temperature

Variable range hopping conduction process of Shklovskii-Efros type is identified, for the first time, in CuIn0.7Ga0.3Se2 bulk material in the temperature range [-190 °C, −10 °C]. Two different methods are considered to confirm this mechanism. The corresponding characteristic temperature and the localization length of mobile carriers are estimated. The AC electrical conductivity analysis suggests the predominance of Non-overlapping Small Polaron Tunneling mechanism. In the frequency range [20 Hz, 1 MHz], the equivalent circuit model of this material is established and the microscopic contributions to the total electrical conduction are discussed.

Usefulness of theoretical approaches and experiential conductivity measurements for understanding manganite-transport mechanisms

Experimental measurements and numerous theoretical models have been employed to comprehend the dynamics of charge carriers in manganite system. The present work highlights the electrical transport of La0.5Ca0.4Ag0.1MnO3 compound in the temperature range [80 K–700 K]. It is found that the conductivity spectra of the studied sample present a double Jonscher variation, universal Jonscher evolution and Drude variation respectively for low, intermediate and high temperature ranges. The temperature dependence of the frequency exponent confirms the efficiency of the quantum mechanical tunneling, the non-overlapping small polaron tunneling and the correlated barrier hopping mechanisms at separated dispersive regions. DC conductivity analysis confirms the semiconductor behavior of the studied sample. Then, the transport properties have been explained in term of a thermally activated small polaron hopping and Mott-variable range hopping processes respectively at high and low temperature sides. Also, the disorder energy is predicted to decrease with rising frequency which could be due to variation in the separation path between the hop centers. Moreover, Greaves variable range hopping model is the suitable one that describes the transport properties in the intermediate temperature range. The scaling approach indicates that the time temperature superposition principle is valid in the temperature side [180 K–360 K].

Mg-substitution effect on microstructure, dielectric relaxation and conduction phenomenon of Fe based perovskite nanomaterials

In this work, we have investigated the electrical and dielectric properties of the Mg doped perovskite (La0.8Ca0.1Pb0.1Fe1-xMgxO3 (x = 0.0, x = 0.1 and x = 0.2)) using the electrical impedance spectroscopy in the temperature range of (200–320 K). The crystal structure evaluated by X-Ray diffraction proved an orthorhombic structure of all the studied compounds with a Pbnm space group. Impedance fitting based on an equivalent circuit and the M″ fitting confirmed the existence of two relaxation phenomena attributed to grains and grain boundaries contributions. The study of the dc electrical conductivity proved that all compounds followed the variable range hopping (VRH) model at low temperature and the Arrhenius model at high temperature range. From conductivity analysis and the temperature dependence of the Jonscher’s power low exponent, we noted an alternation of correlated barrier hopping (CBH) and non-overlap small polaron tunneling model (NSPT) conduction processes at the studied temperature range. The relaxation phenomenon for both contributions and the deduced activation energies were found to be sensitive to the Mg composition; for the grain contribution, a minimum activation energy value was recorded for x = 0.1 compound and increased for suplementary Mg content. While, for grain contribution the activation decreases with the increase in Mg ion concentration. Concerning the evolution of the magnetization versus temperature, it was measured to investigate the effect of the Mg amount on the magnetic behavior of the prepared compounds under the studied temperature range.

Crossover from overlap large polaron to small polaron tunneling: A study of conduction mechanisms and phase transitions in a new long chain organic inorganic hybrid

AC-conductivity study of the new long chain hybrid [NH3 (CH2)8 NH3]Cl2 in the temperature range 210 K–420 K showed the following consecutive phase transitions:(PhaseVIII)T7=232.5K(PhaseVII)T6=253K(PhaseVI)T5=287.9K(PhaseV)T4=316.5K(PhaseIV)T3=330.7K(PhaseIII) T2=378.8 K(PhaseII)T1=399 K(PhaseI).Transitions are confirmed by differential scanning calorimetry (DSC). Room temperature X-ray powder diffraction indicated a monoclinic P21/c with crystallographic parameters a = 18.179 Å, b = 15.380 Å, c = 4.881 Å and β = 109.54(0), V = 1286 (Å)3, Z = 4. Frequency dependent ac-conductivity at different temperatures is interpreted in terms of the jump relaxation model and is fitted to super-linear power law σtot = σdc + A1ωs1+ A2ωs2. The fit parameters lnA1/s1 and lnA2/s2 are sensitive to the type of conduction mechanism. Temperature and frequency dependence of ac-conductivity indicated that in the low (T < 231 K) and intermediate (290 K–316 K) temperature ranges, conduction takes place via quantum mechanical tunneling (phases (VIII) and (V)). Crossover from large polaron to non-overlapping small polaron tunneling takes place associated with transition from phase (VII) to phase (VI). Correlated barrier hopping (CBH) prevails at high temperature (phases (I, II, III and IV)). Activation energy and its frequency dependence were calculated for the different phases. Minimum tunneling and hopping distances, effective barrier height WM as well as the density of states at the Fermi level (N(Ef)) were evaluated from ac-conductivity results.

Crystal structure and electrical conduction mechanism of the new bi-tetrabutylphosphonium hexachlorostannate compound

The bi-tetrabutylphosphonium hexachlorostannate [P(C4H9)4]2SnCl6 compound is a new member of the family of organic-inorganic hybrid like perovskite system. This compound crystallizes in the monoclinic space group C2/c with unit cell parameters a = 14.4008 Å, b = 18.7256 Å, c = 17.5135 Å and β=113.1780° The (SnCl6)2− anions are surrounded by six [(C4H9)P]+ cations, forming an octahedral configuration. Regarding the differential scanning calorimetry, three order- disorder reversible phase transitions were observed at T1 = 388/363 K, T2 = 403/395 K and T3 = 446/410 K. Analysis of frequency and temperature dependence of impedance and electrical modulus spectroscopic data and Nyquist plots has shown the contributions of grains and grain boundaries to the electrical properties of the prepared material. The variation of the dielectric loss log (ε ‘’) with log (ω) is found to follow the empirical law, ε’’= Bωm(T). Moreover, the temperature dependence study of frequency exponent m (T) is investigated toexplain the conduction mechanism in the different parts, which is attributed to the non overlapping small polaron tunneling (NSPT) in the region I, II and III and by the correlated barrier hopping (CBH) model in phase IV.

Frequency dependence of the hopping and disorder energies and conduction mechanisms in Cr-(Pr/Ca) MnO3

Transport properties of Pr0.7Ca0·3Mn0·85Cr0·15O3(Cr-PCMO) manganite have been studied in a large frequency and temperature ranges by mean of impedance spectroscopy. The conductance spectra are discussed in the outline of a double and single Jonscher laws for the temperature ranges [80 K–110 K] and [120–170 K] respectively. The correlated barrier hopping and the non small polaron tunneling (NSPT) conduction models are used to explain the dynamic of charge carriers in the ac regime. The lower temperature range is characterized by the coexistence of such two conduction processes. Below the frequency ν = 105 Hz, the compound exhibits a semiconductor behavior with no detectable electrical transition. A metal - semiconductor transition at around T = 190 K was observed at high frequency range (ν = 2.106 Hz). Beyond the Debye temperature, θD/2 = 130 K, the small polaron hopping (SPH) conduction process is thermally activated. Over the investigated frequency interval ν ≤ 105, the activation of the SPH conduction process needs the same activation energy Ea1 = 112 meV. At lower temperature range, the interaction between electron and acoustic phonon becomes important, the variable range hopping (VRH) mechanism was activated via a disorder energy WD. The temperature dependence of the local energy proves that type of the VRH process is sensitive to the frequency parameter and the investigated temperature range. The hopping and the disorder energies are strongly sensitive to frequency.

Frequency and temperature impact on the electrical properties of LaCr0.99Pd0.01O3-ẟ compound

The LaCr0.99Pd0.01O3 compound has been synthesized with solid-state reaction method. The microstructure, crystalline structure, chemical composition, and oxidation states of elements were investigated via SEM, Raman and XPS analysis techniques. SEM has revealed that particles have various sizes. XPS investigation has unveiled the oxidation states of La (3+), Pd (4+), and Cr (both 3+ and 6+). Electrical characterization has been performed in nitrogen ambient using Novocontrol Dielectric Spectrometer. Real and imaginary parts of the dielectric function, impedance, modulus, and conductivity of studied compound were investigated in wide temperature and frequency range. The activation energy from the log fmax vs. (kT)−1 and log σdc vs. (kT)−1 plots has been obtained as 0.240 eV and 0.231 eV, respectively. It has been demonstrated that the correlated barrier hopping (CBH) and non-overlapping small polaron tunneling (NSPT) conductions take place in the LaCr0.99Pd0.01O3 compound.

Developing the magnetic, dielectric and anticandidal characteristics of SrFe12O19/(Mg0.5Cd0.5Dy0.03Fe1.97O4)x hard/soft ferrite nanocomposites

Hard/soft ferrite nanocomposites (NCs) (SrHF)/xMCD (1.0 ≤ x ≤ 3.0) consisting of SrFe12O19 HF (SrHF) and Mg0.5Cd0.5Dy0.03Fe1.97O4 (MCD) were obtained via one-pot sol-gel auto-combustion approach. The measurements of impedance spectroscopy are presented for new fractional (SrHF)/xMCD (1.0 ≤ x ≤ 3.0) hard/soft ferrite NCs, including reference soft and hard ferrites. The electrical conductivity and activation energies of fractional composite ferrites are observed to be regulated depending on the fractional ratios. It has been established that the temperature dependence of the DC conductivity obeys the Arrhenius law, especially for the fractional composite ferrites. To control the conduction mechanism, the conductivity and its frequency power exponent are evaluated based on a small polaron tunnelling model. Magnetic properties were verified by applying a DC magnetic field up to ±70 kOe at 300 and 10 K. Magnetization data significantly decreased with increasing fraction of MCD in NCs. These results showed that exchange-coupling was not achieved in these NCs by one-pot approach. Additionally, anticandidal activity of the synthesized nanomaterials was studied by evaluating the colony forming unit (CFU). The obtained results are significant for potential applications in the field of biomedicine.

Effects of sintering temperature on structural, infrared, magnetic and electrical properties of Cd0.5Zn0.5FeCrO4 ferrites prepared by sol–gel route

In this work, an attempt is made to study the effects of sintering temperature on the structural, infrared, magnetic and electrical properties of Cd0.5Zn0.5FeCrO4 ferrites. Samples were prepared by sol–gel route and sintered at 900 and 1100 °C. The structural characterization of the synthesized powders by X-ray powder diffraction showed that only single phase with cubic spinel structure existed. Estimated values of unit cell parameter and average grains size show an increase with sintering temperature. The intensities of absorption bands, spontaneous magnetization, electrical conductivity, and dielectric constants were found to increase with increasing sintering temperature. The non-overlapping small polaron tunneling (NSPT) model dominates the conduction process for the studied samples which present a relaxation phenomenon with non-Debye nature. Close activation energies values were found from analyzes of relaxation time and dc-conductivity indicating that the relaxation and the conduction processes may be attributed to the same type of charge carriers. The equivalent electrical circuit (Rg+ Rgb//CPEgb) was used for modeling the Nyquist data.

Effect of doping in the physico-chemical properties of BaTiO3 ceramics

Barium titanate is a very attractive perovskite given its ferroelectric, piezoelectric and dielectric properties. These can be further improved by the substitution of barium and titanium by different elements. Introducing both rare earths and/or metals in this compound enhances its optical and electrical properties, thus making it a multifunctional material suitable for many different applications. In the present work, the compound Ba0·91Er0·1/3Ca0·04Ti0·92Sn0·08O3 is prepared by solid state reaction and its structural, morphological, thermal optical and electrical properties are investigated in detail by X ray diffraction, scanning electron microscopy, differential scanning calorimetric, absorption spectroscopy and impedance spectroscopy respectively. Scanning electron microscopy shows that the material is composed by a major polycrystalline phase with micrometric grain size. Two-phase transitions at 448 K and 572 K are deduced from thermal and electrical measurements. The optical measurements indicate that both erbium and tin are well introduced in the structure, which is confirmed by energy dispersive spectroscopy, and affects the vibrational modes. a. c. and d. c. electrical measurements show that, depending on the measurement temperature range, conduction mechanism is either a non-overlapping small polaron tunneling or a correlated barrier hopping model. . Moreover, dielectric measurements prove a high dielectric constant and identify a phase transition from the ferroelectric phase to paraelectric one which is in agreement with literature. Such substitution improves the compound properties and makes it suitable for different devices.

Studies of electric, dielectric properties, and conduction mechanism of {(C2H10N2)(MnCl (NCS)2)2}n polymer

AbstractDielectric and electrical properties correlated with the structure analysis have been studied for new crystalline polymer {(C2H10N2)(MnCl (NCS)2)2}n. The frequency and temperature dependence of dielectric constant (ε′), dielectric loss (ε″) and loss tangent (tanδ) in the temperature range 308‐348 K and frequency range 20 to 2 MHz have been made for {(C2H10N2)(MnCl (NCS)2)2}n. The Z′ and Z″ versus frequency plots are well fitted to an equivalent circuit model. The analysis of Nyquist plots is well fitted to an equivalent circuit consisting of series of combination of grains and grain boundary elements. The spectra follow the Arrhenius law with two activation energy the first in the region of temperatures before 332 K (Region I) and the second in the region after 332 K (Region II). An agreement between the experimental and theoretical results is discussed, and it is suggested that the ac conductivity can be explained by the correlated barrier hopping (CBH) and nonoverlapping small polaron tunneling model (NSPT) in regions I and II, respectively. The AC conduction mechanism is governed by the correlated barrier‐hopping (CBH) with WM (Region I) = 270 meV and a tunneling distance of 4.5 Å, whereas it becomes a small polaron tunneling mechanism (SPTM) mechanism with a WH (Region II) = 153 meV. The imaginary part of the electrical modulus formalism obeys the Kohlrausch‐Williams‐Watt (KWW) model. The theoretical Kohlrausch exponent (βKWW) confirms the existence of the transition phase on the new crystalline polymer in the vicinity of the 332 K as deduced by the DC and the AC conduction parameters. Thus, the near values of activation energies obtained from the impedance and modulus spectra confirm that the transport is through an ion hopping mechanism.

Summerfield scaling model and conduction processes defining the transport properties of silver substituted half doped (La–Ca) MnO3 ceramic

Conductivity spectra of La0.5Ca0.3Ag0.2MnO3 (LCAM) oxide have been investigated in the temperature range [80 K–680 K]. The studied system displays a semiconductor behavior over the explored temperature and frequency ranges. From direct current study, the transport of charge carries is assisted via disorder energy below θD/4. For the temperature range [θD/4- θD/2], the transport properties are governed by activation of Mott-variable range hopping (Mott-VRH) process. Beyond θD/2, the conduction phenomena are assisted through the thermally activation of the small polaron hopping (SPH) mechanism. At high frequencies, the conductivity spectra reveal a dispersive region. They are explained by the double Jonscher response. The latter side was characterized by the appearance of the overlapping-large-polaron-tunneling (OLPT), the correlated barrier hopping (CBH) and the non small polaron tunneling (NSPT) conduction processes. It is found that the calculated disorder energy is frequency independent. At elevated frequencies, the hopping energy is predicted to decrease with increasing frequency which can be related to the change in the Polarons radius. In the temperature range [80 K–220 K], the conductivity spectra follow double Jonscher law. Using the scaling model, the spectra merge into a single master curve, which confirmed the validity of the time temperature superposition principle (TTSP). A confident divergence from the Summerfield is noticed at high frequencies. It proves that the charge mobility is temperature dependant. The Summerfield correction scaling is effectuated by introducing a scaling positive parameter (α = 1.2). An excellent coalescence of conductivity spectra is observed.

Investigation of the conduction mechanism, high dielectric constant, and non-Debye-type relaxor in La0.67Ba0.25Ca0.08MnO3 manganite

The conduction mechanism and electric properties of La0.67Ba0.25Ca0.08MnO3 (LBCM) were analyzed depending on temperature and frequency. The frequency dependence of electrical conduction plot was explained by the jump relaxation model. The electrical conduction process was based on both theoretical conduction models assigned to the Correlated Barrier Hopping and Non-overlapping Small Polaron Tunneling mechanism. The impedance plot studies reveal that the relaxation is a non-Debye poly-dispersive pattern in the LBCM. This compound has excellent dielectric properties such as the dielectric permittivity exceeding 105 at room temperature, i.e., colossal permittivity, and is one of the highest values recorded for ceramic capacitors. Thus, evolution of dielectric permittivity, described in terms of spatial charge polarization based on Koop's phenomenological theory and Maxwell–Wagner's (M– W) model and high dielectric response, has been driven by barrier layers into the grain boundaries and blended structures of Mn3+/Mn4+. The relaxation processes were examined with modulus, impedance formalism, and electrical conductivity. In the thermal analysis, the relaxations mechanism are related oxygen vacancies.

Temperature-dependent transport properties of micro and nano-sized zinc cobalt oxide (ZnCo2O4) and zinc manganese oxide (ZnMn2O4) particles synthesized by a hydrothermal route

In the present work, a hydrothermal method has been adopted to synthesize micro (~150 nm), and nano (~20 nm) particles of Zinc based, cobalt, and manganese oxide structures, i.e., ZnCo2O4 (ZCO) and ZnMn2O4 (ZMO). These structures have been examined by X-ray diffraction, Field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. The effect of different reducing agents and solvents have been ascertained on the morphological and the crystalline nature of the samples. The highly crystalline and micron-sized particle formation has been observed with the usage of NaBH4 as the reductant in the water solution, and a polycrystalline structure with nanosized particle formation has been found with NaOH as the reductant in the water and DMF solution. The electrical properties have been investigated between 100 Hz and 1 MHz by varying the temperature from 303 K to 513 K. The micro and nano sized particles of ZCO and ZMO have shown frequency dependence, thermally activated and relaxation-type dielectric behavior confirming the semiconducting nature. The variation of ac conductivity with frequency at different temperatures is according to Jonscher's power law for micro and nano sized particles of the ZCO and ZMO but supporting different conduction mechanisms. The conduction mechanism in microparticles and nano-particles is according to an overlapping-large polaron tunneling (OLPT) model, and the Non-overlapping small polaron tunneling (NSPT) model, respectively.

The sodium-ion battery: Study of alternative current conduction mechanisms on the Na3PO4 - Based solid electrolyte

Abstract Sodium-ion batteries have been dominating as a power source in digital cameras, laptop computers, mobile phones and electric/hybrid electric vehicles. This can be explained by their high life cycle and power density which differentiate them from other battery types. Sodium phosphate is considered among the favored solid electrolyte materials for the sodium-ion battery because of its high ionic conductivity. In this study, Na3PO4 was obtained using the classic ceramic method and characterized by X-ray powder diffraction patterns, infrared spectroscopy, differential scanning calorimetry (DSC) and electrical impedance. The Na3PO4 compound crystallized at room temperature in the tetragonal system with a P 4 ‾ 2 1 c space group. The morphology and composition of Na3PO4 were studied by a scanning transmission electron microscope coupled with the energy dispersive X-ray spectroscopy (STEM-EDS). The phase transition at T1 ≈ 603/605 K was confirmed by the differential scanning calorimetry (DSC). Infrared spectroscopy confirmed the presence of the (PO4)3− group and its vibrations. The electrical technique was measured in the 10–106 Hz frequency range and 540–700 K temperature intervals. The frequency dependence of alternative current conductivity was explained using Jonscher law. The alternative current electrical conduction in Na3PO4 was interpreted through several processes, which could be associated with two different models: the overlapping large polaron tunneling (OLPT) model in phase I and the non-overlapping small polaron tunneling (NSPT) model in phase II. The conduction mechanisms of Na3PO4 were explained by Elliott's theory and consequently the Elliott's parameters were calculated.

Thermal transport and electrical properties of Se95-xSn5Bix (x = 0, 4, 8) chalcogenide alloys

This study reports the structural, thermal-transport and electrical properties of the Se95-xSn5Bix (x = 0, 4, 8) chalcogenide alloys. The x-ray diffraction analysis (XRD) reveals the polycrystalline nature of samples. Transmission electron microscopy (TEM) demonstrates the formation of nano-rods and clusters of nano-particles. Differential scanning calorimetry (DSC) of the samples, x = 4&8 indicates, there may be two co-existed phases in these samples. Thermal conductivity (λe) shows a decreasing trend with the inclusion of Bi in Se–Sn system. Thermal diffusivity (χe) and volumetric heat capacity (QCp) show the optimum value for x = 4. Thermal transport parameters remain almost temperature independent. The dielectric constant/loss decreases with frequency and increases with temperature. Maximum dielectric constant/loss is obtained for the un-doped sample, while for doped samples, the values have decreased with Bi content. The strong dependence of conductivity over temperature is explained in terms of non-overlapping small polaron tunneling (NSPT) for undoped and CBH models for Bi doped samples.

Electrical and thermal transport properties of Ni1-xCexO nanostructures

In this report, we discussed the structural, electric conduction (DC and AC) mechanisms and thermal properties such as specific heat (CP) and change in enthalpy (ΔH) of Ni1-xCexO (x = 0.00, 0.01, 0.03 and 0.05) nanostructures synthesized by the sol-gel method which have been explored through LCR and differential thermal analysis (DTA) measurements. The dielectric relaxation in this system has also been discussed as a function of both frequency (75 kHz–1 MHz) and temperature (30 °C to 300 °C). In the light of several theoretical models, the AC conductivity has been analyzed using a power law σac∝ωs (s < 1), where frequency exponent ‘s’ examined as a function of temperature. Our experimental investigations based on theoretical models suggest that the AC conduction mechanism in all samples successfully described by non-overlapping small polaron tunnelling model (NSPT) and correlated hopping model (bipolaron hopping over single polaron hopping) (CBH) which provide reasonable physical parameters such as polaron hopping energies (WH and WM), characteristic relaxation time (τo=10−13 s), tunnelling distance (Rw) and density of states at Fermi level N(EF) as well as the concentration of a pair of defects sites (N), respectively. Also, specific heat and change in enthalpy (ΔH) have been discussed and it is observed that value of change in enthalpy (ΔH) decreases with increasing Ce concentration in NiO lattice.

Influence of Mn doping on dielectric properties, conduction mechanism and photocatalytic nature of gadolinium-based orthochromites

The samples of manganese (Mn)-doped gadolinium chromite [GdCr1−xMnxO3 (x = 0, 0.1, 0.2 and 0.3)] are prepared through sol–gel auto combustion route. The influence of Mn-doping in GdCrO3 is studied in respect of structural, dielectric, electrical and photocatalytic studies. Raman spectra assured the formation of single-phase orthorhombic structure with Pbnm space group and Raman vibration modes for GdCrO3 shift on Mn doping which may be attributed to the influence of the lattice strain, defects, and crystallite size. Dielectric constant (e′), dissipation factor (tanδ) and AC conductivity (σac) are investigated as a function of frequency as well as temperature. Dielectric constant, energy dissipation factor and AC conductivity of pristine sample GdCrO3 show relatively higher values but these values decrease with the increase in Mn doping. The dielectric constant as a function of temperature for all the investigated samples peaks around 250 °C which may be attributed to the high temperature phase transformation. These peaks are observed to shift toward lower temperature with the increase in Mn-doping. The enhancement in AC conductivity is noticed for all the samples with the rise in temperature for the probing frequencies. The non-overlapping small polaron tunneling (NSPT) and correlated barrier hopping (CBH) models dominate the conduction mechanism in these samples. An increase in the experimental values of the hopping energy is observed with the increase Mn concentration for CBH as well as NSPT models. A systematic reduction in activation energy with frequency is noticed for all the samples except for GdCr0.9Mn0.1O3 sample. Real (Z′) and imaginary (Z″) values of impedance are found to reduce with the increase in frequency up to a certain limiting range and then become almost frequency independent at higher frequencies. Nyquist plots of the samples show single semicircular behaviour that indicates the dominance of grain boundary resistance as compared to grain resistance in this system. The grain and grain boundary resistances are determined, with the help of equivalent circuit designed using Zview software, to study the contribution of the grains and grain boundaries to the conductivity. Photo-degradation of Congo red dye under visible light irradiation was used to investigate the photocatalytic activity of the GdCrO3 and GdCr0.9Mn0.1O3 samples. The results confirmed that the catalytic function of the GdCrO3 enhances on Mn doping.

Structural properties and electrical conductivity mechanisms of semiconducting quaternary nanocomposites: Effect of two transition metal oxides

Several quaternary nanocomposites, doped with two transition metal oxides, of the generalised composition formula 0.4ZnO–0.1P2O5–0.5[xV2O5–(1-x) MoO3] were prepared via the melt quenching process. We investigated the microstructural and conductivity properties of two ternary systems and four quaternary systems to determine the better semiconductor nanocomposite system for wider application purposes. The X-ray diffraction results showed the presence of superposed nanocrystallites within amorphous networks or matrices. Analysis of ultraviolet–visible absorption spectra showed that optical bandgap energy (Eopt) values varied with V2O5 concentration, and there was an inverse relationship between Eopt and average nanocrystallite size. The responsible mechanisms of ac conductivity were examined using the model of Jonscher's universal power-law and Almond–West formalism, and it was found that ac conductivity increased as temperature rose, exhibiting semiconducting features. All the ternary (two examples) and quaternary (four examples) nanocomposite systems established non-linearity in dc conductivity, caused by the small polaron hopping process with dissimilar activation energies at different temperature regions. The power-law exponent (s) of Jonscher's universal power-law revealed that the ac conductivity mechanism could be described by correlated barrier-hopping (CBH) or non-overlapping small polaron tunnelling (NSPT) models for different samples due to major structural alterations. We applied modified CBH and NSPT models (as s > 0.8) to get realistic values of different fitting parameters, and in this process, we also obtained the values of ideal or hypothetical glass transition temperature from the theoretical models. The different activation energies associated with ac conductivity and for the small polaron migration process diminished with an increment in conductivity. Scaled ac conductivity spectra demonstrated that the conductivity relaxation process relied upon the composite structural features and did not depend on temperature. These materials can be used for such applications as gas sensors, even at higher temperatures due to their semiconducting nature and the different valence states of transition metal ions.