Small Polaron Hopping Model Research Articles

Impact of nickel substitution on the structural and conduction behaviour of YBaCo4−xNixO7 (0 ≤ x ≤ 0.3) cobaltites

A systematic series of polycrystalline compounds of YBaCo4−xNixO7 (0 ≤ x ≤ 0.3) have been synthesized by conventional solid-state reaction technique in order to scrutinize comprehensively the effect of Ni substitution on structural, micro-structural and electrical transport properties. X-ray diffraction patterns recorded at room temperature illustrate the structural phase transition with Ni from hexagonal (P6 3 mc) phase symmetry to trigonal (P31c) phase symmetry. The grain morphology observed via scanning electron microscopy indicates the decrease in grain size with increasing Ni percentage. The EDS analysis indicates that Ni is distributed randomly on Co sites also supports the change in crystal structure. The calculated electrical resistivity revealed the semiconducting nature of all compound with and without magnetic field. The resistivity data supported by using Mott’s variable range hopping (VRH) model to calculate the hopping distance (Rh), hopping energy (Eh) and density of states at Fermi level N(EF). Moreover, resistivity data are well fitted with small polaron hopping (SPH) model to investigate the conduction behaviour in the system. Thus, conduction mechanism carried out electronically induced structural phase changes involves in Ni–O–Ni and Ni–O–Co bond angles and bond lengths.

Electron Paramagnetic Resonance study of hopping in CCTO mixed with TiO2

Electron Paramagnetic Resonance (EPR) is used to study the transport properties in CaCu3Ti4O12(CCTO) and its mixtures with TiO2. The temperature dependence of the peak to peak linewidth of the EPR spectra in the temperature range 430 ≤ T ≤ 660 K, is studied for the powders of CCTO, 5% CCTO + 95% TiO2 and 1% CCTO + 99% TiO2. All the samples show a behavior predicted by the Small Polaron Hopping model. This behavior is studied as a function of the TiO2 content, finding an activation energy that decreases with increasing TiO2 concentration. We found that two mechanisms govern the conductivity in these system one, intrinsic to the CCTO, via oxygen vacancies and the other one, which improve the conductivity, related to the polarizability of the Ti-ions at the surface of the grains and mediated by the oxygen vacancies at the surface of the CCTO.

Influence of electron beam irradiation on electrical, structural, magnetic and thermal properties of Pr0.8Sr0.2MnO3 manganites

In this communication, the effect of electron beam (EB) irradiation on the structural, electrical transport and thermal properties of Pr0.8Sr0.2MnO3 manganites has been investigated. Rietveld refinement of XRD data reveals that all samples are single phased with orthorhombic distorted structure (Pbnm). It is observed that the orthorhombic deformation increases with EB dosage. The Mn–O–Mn bond angle is found to increase with increase in EB dosage, presumably due to strain induced by these irradiations. Analysis on the measured electrical resistivity data indicates that the small polaron hopping model is operative in the high temperature region for pristine as well as EB irradiated samples. The electrical resistivity in the entire temperature region has been successfully fitted with the phenomenological percolation model which is based on phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. The Seebeck coefficient (S) of the pristine as well as the irradiated samples exhibit positive values, indicating that holes is the dominant charge carriers. The analysis of Seebeck coefficient data confirms that the small polaron hopping mechanism governs the thermoelectric transport in the high temperature region. In addition, Seebeck coefficient data also is well fitted with the phenomenological percolation model. The behavior in thermal conductivity at the transition is ascribed to the local anharmonic distortions associated with small polarons. Specific heat measurement indicates that electron beam irradiation enhances the magnetic inhomogeneity of the system.

Impact of silver substitution on the magnetotransport and thermal behavior of polycrystalline Sm0.55Sr0.45−xAgxMnO3 (x = 0 & 0.15) manganites

We have synthesized a series of Sm0.55Sr0.45−xAgxMnO3 (x = 0 & 0.15) by conventional solid state reaction method to scrutinize the effect of monovalent Ag+ as a replacement of Sr2+ ion on structural, electrical, magnetotransport and thermoelectric properties. The structural parameters obtained using Rietveld refinement of X-ray diffraction data which showed perovskite structure having orthorhombic (Pnma) symmetry without any detectable impurity phase. It is observed that MZFC and MFC curves become more manifested as Ag+ content is increased which indicates an increase in magnetic frustration arising due to bending of Mn-O-Mn bond. The M-H plots show the narrow hysteresis loops which indicate the soft ferromagnetic nature. Magnetic studies also show that the Curie temperature TC is found to decrease with Ag+ content. The metal–insulator transition temperature (TMI) is observed which decreases with the increase of Ag+ content when magnetic field is applied. The resistivity data in high temperature region are explained within the framework of variable range hopping (VRH) and small polaron hopping (SPH) models. The thermoelectric power (S) of the reported samples exhibits a crossover from positive to negative value and value of S increases in doped samples. The analysis of thermoelectric power data confirms that the small polaron hopping model in high temperature region and at low temperature electron–magnon scattering is the dominating in the FM region. With increasing the Ag+ content the samples showed very high magneto-resistance (MR%) and temperature coefficient of resistance (TCR%) which make the compound more promising for scientific and technological applications.

Electrical transport properties and conduction mechanisms of semiconducting iron bismuth glasses

DCelectrical conductivity measurements were carried out on iron bismuth glasses with the nominal composition (100–x)Bi2O3–x Fe2O3(where x=15, 25, 35and 45mol%) in an attempt to understand the nature of the conduction mechanism governing the DC electrical conductivity and the effect of addition of Fe2O3 on electrical properties in these glasses. The investigated glasses were prepared by a normal melt quenching technique. X-ray diffraction (XRD) patterns confirmed the amorphous nature of the present glasses. DC conductivity measurements showed an enhancement in conductivity by increasing Fe2O3 concentration. The change in activation energy of DC conductivity with temperature reveals the change in conduction mode from small polaron hopping (SPH) at high temperatures (T>θD/2) to variable range hopping (VRH) at low temperatures (T<θD/2). The conductivity results were analyzed in terms of different theoretical models to determine the possible conduction mechanism. The analysis showed that the conductivity data are consistent with Mott's nearest-neighbor SPH model. However, both Mott VRH and Greaves VRH models are suitable to explain the data. Further, Schnakenberg's polaron hopping model is also consistent with the temperature dependence of the activation energy, but the different model parameters, for example, hopping energy and disorder energy determined from the best fits were found not to be in accordance with the prediction of the SPH model.

Electrical transport and specific heat properties of La0.6Pr0.1Sr0.3Mn1−xRuxO3 (x=0.00, 0.05 and 0.15) perovskites

The electrical transport and magnetoresistance properties of La0.6Pr0.1Sr0.3Mn1−xRuxO3(x=0.00,0.05and 0.15) were established. A careful study of the electrical transport properties has been carried out through the measurement of resistivity ρ (T). The dependence of electrical resistivity on temperature showed a metal-semi-conductor (m-SC) transition at TM-SC for all samples. When analyzing the data of electrical resistivity, it was concluded that the metallic (ferromagnetic) part of the resistivity (below TM-SC) can be explained by the following equation ρ(T)=ρ0+ρ2T2+ρ4.5T4.5, signifying the importance of the grain/domain boundary, electron–electron and magnon scattering processes. However, at a high temperature (T>TM-SC), the paramagnetic semiconducting regime, the adiabatic small polaron and Variable Range Hopping models were found to fit well. The electrical resistivity was fitted with the phenomenological percolation model. Hence, it was found that the estimated values of the resistivity were in good agreement with the experimental data. The values of magnetoresistance (CMR) were very high; about 53% even at 325K for x=0.15.

Structural, electrical, magnetic and thermal studies on Eu1-xSrxMnO3 (0.2 ≤ x ≤ 0.5) manganites

In the present work, a series of Eu1-xSrxMnO3 (0.2 ≤ x ≤ 0.5) manganites has been prepared and systematic investigation on structural, electrical, magnetic and thermoelectric properties is reported. The samples were prepared by conventional solid-state reaction. The samples are crystallized in orthorhombic crystal structure with Pbnm space group. Rietveld analysis of XRD results show the unit volume is decreasing with increasing Sr content. The observed grain size is decreasing with increase the concentration of Sr. All the samples are found to exhibit insulating behaviour down to the low temperatures. The magnetization results reveal that the antiferromagnetic (AFM) nature is observed in zero field condition (ZFC), while ferromagnetic (FM) behaviour is observed under field cooled conditions (FC). The samples with x = 0.2 and 0.3 show the spin glass behaviour in ZFC condition. The metamagnetic behaviour is observed at low temperatures for the samples with x = 0.4 and 0.5. The thermoelectric power (S) of the samples with x = 0.4 & 0.5 is negative at high temperature range. The samples with x = 0.4 exhibits a crossover in S from positive to negative values. Samples with x = 0.3 and 0.2 show positive S value for entire temperature range. The analysis of S value indicates the small polaron hopping model is presence in the high temperature region.

Magneto-transport properties of La0.75Ca0.15Sr0.1MnO3 with YBa2Cu3O7–δ addition

We report the structural, magnetic, electrical and magentoresistance properties of (La0.75Ca0.15Sr0.1MnO3)1−x(YBa2Cu3O7−δ)x (with x=0, 0.025, 0.05, 0.075, 0.1, 0.2, and 0.3) composites synthesized through sol–gel method. The powder X-ray diffraction patterns indicate no evidence of reaction between La0.75Ca0.15Sr0.1MnO3 (LCSMO) and YBa2Cu3O7−δ (YBCO). The addition of YBCO induces a reduction of the total magnetization while the Curie temperature remains almost constant (∼312K). The behavior of the electrical resistivity evolves differently depending on the doping level. Above the paramagnetic–insulating transition temperature the resistivity data were best-fitted by using the adiabatic small polaron and variable range hopping models. Ferromagnetic–metallic regime in the composites seems to emanate from the electron–phonon or/and electron–magnon scattering processes. With increasing the YBCO doping content (until x=0.1), the positive magnetoresistance (MR) of YBCO phase dominates the negative MR of LCSMO one, which gives rise to the decreasing of MR of the composites.

Effects of Cr-doping on the electronic transport properties in antiperovskite nitrides Mn3−xCrxZnN (0≤x≤0.5)

Antiperovskite intermetallic Mn3−xCrxZnN (0≤x≤0.5) compounds have been prepared to study the Cr doping effect on the electronic transport properties. The temperature dependent resistivity ρ(T) for x≤0.3 shows notable and broader slope change. Meanwhile, with increasing Cr doping, the slope change shifts to lower temperatures and is gradually suppressed for x>0.3. For x=0.0 and 0.5, the measurements of magnetization from 10K to 350K reveal that Mn3−xCrxZnN compounds go through a transition from the dominant AFM interactions to one where the FM and AFM interactions coexist. For low-temperature region (15–80K), the electron–electron scattering is dominant, and the number of phonons abruptly increases with increasing temperature. Small polaron hopping model (SPH) is used to understand the temperature dependence of electrical resistivity in the high-temperature range of 200–300K and a clear decrease of the polaron activation energy was observed for x≤0.2. The residual resistivity ratio (RRR) decreases while the residual resistivity increases (ρ0) with increasing Cr-doping level. The maximum total entropy change (ΔSmax) decreases by increasing the Cr doping from x=0.0 to 0.2.

GROWTH AND CHARACTERISTIC OF AMORPHOUS NANO-GRANULAR TeO2–V2O5–NiO THIN FILMS

TeO2–V2O5–NiO thin films were deposited using thermal evaporation from 40TeO2–([Formula: see text])V2O5–yNiO ([Formula: see text]–30[Formula: see text]mol%) target. Structural analysis of the films was identified by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The amorphous TeO2–V2O5–NiO films have nanosized clear grain structure and sharp grain boundaries. DC conductivity and current–voltage (I–V) characteristic of TeO2–V2O5–NiO thin films were measured in the temperature range of 300–423[Formula: see text]K. As nickel oxide (NiO) content increases, the DC conductivity decreases up to two orders in value ([Formula: see text]–[Formula: see text][Formula: see text]S[Formula: see text][Formula: see text][Formula: see text]cm[Formula: see text]. Temperature dependence of conductivity is described using the small polaron hopping (SPH) model as well. Poole–Frenkel effect is observed at high external electric field. The optical absorption spectra of the TeO2–V2O5–NiO thin films were recorded in the wavelength range of 380–1100[Formula: see text]nm. The absorption coefficient revealed bandgap shrinkage (3.01–2.3[Formula: see text]eV) and band tail widening, due to an increase in NiO content. Energy dispersive X-ray spectroscopy (EDX) was used to determine elemental composition. In TeO2–V2O5–NiO thin films, the NiO content is around fifth of the initial target.

Effect of bismuth doping on the physical properties of La–Li–Mn–O manganite

The effects of bismuth doping at La site on magnetic, electrical and thermopower properties of LaLiMnO3 manganites have been investigated. The substitution of Bi ion leads to the weakening of ferromagnetic ordering at low temperature, and Curie temperature (T C) decreases with increase in Bi content. Interestingly, a dramatic increase in the magnitude of Seebeck coefficient at low temperature is observed in Bi-doped samples which might find potential application as thermoelectric. The results are attributed to the combined effect of the disorder and antiferromagnetic interaction induced by Bi doping. Both ρ(T) and S(T) data in the high-temperature region are discussed using small polaron hopping model.

Study of polaron transport mechanisms in two transition metal ions doped borophosphate glasses

Borophosphate glasses in the compositions, (B2O3)0.2 · (P2O5)0.3 · (V2O5) x · (CoO)0.5x , where x = 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50 were synthesized at 1500 K by standard melt quenching method. Non-crystalline nature of the samples was confirmed by XRD studies. Room temperature density and dc electrical conductivity in the temperature range from 350 K to 625 K have been measured. Density decreased up to about 0.25 mole fractions of V2O5 and increased thereafter. Conductivity was almost constant up to 0.25 mole fractions of V2O5 and increased for higher amount of V2O5. Temperature variation of conductivity data has been analyzed using Mott’s small polaron hopping (SPH) model and, activation energy and Debye’s temperature were determined. Activation energy decreased with increase of V2O5 content. The data deviated from the Mott’s SPH model has been analyzed in view of variable range hopping models of Mott and Greaves and the density of states at Fermi level was determined. It is for the first time that borophosphate glasses doped with V2O5 and CoO were synthesized to study conductivity variation with temperature. The data analyzed using small polaron hopping and variable range hopping models.

Size-induced modification of magneto-transport properties in nanocrystalline Sm0.5Ca0.5MnO3 compound

We report the electronic and magneto-transport behaviors of the nanocrystalline Sm0.5Ca0.5MnO3 compound and the results are compared with their bulk counterpart. Our experimental findings indicate that the insulating nature of Sm0.5Ca0.5MnO3 compound is found to reduce and the enhancement of magnetoresistance has been observed with the reduction of particle sizes. Resistivity of the nanocrystalline and bulk samples in different temperature regions are analyzed by considering the adiabatic small polaron hopping model. Our analysis of resistivity data suggests that the activation energy markedly reduces and magnetoresistance increases in nanoscale regime due to the pronounced surface effect.

The role of deep acceptor levels in hydration and transport processes in BaZr1 − x Y x O3 – δ and related materials

A statistical theory of hydration and defect formation in acceptor-doped proton-conducting perovskites has been developed taking into account contributions of the electron hole and vibrational subsystems. Using yttrium-doped BaZrO3 as an example, we show that deep acceptor states can significantly affect hydration, oxidation, and transport processes in proton-conducting oxides. The impact of these states on defect thermodynamics strongly depends on their energy and becomes negligible at a depth less than a certain value. We demonstrate that the experimental data on the hole conductivity of barium zirconates can be described both in the band transport model and in the model of small polaron hopping. These scenarios correspond to a somewhat different depth of the acceptor levels, but in both cases, the concentration of bound holes at the acceptor centers can be high enough and these states must be taken into account in the defect formation considerations. The results obtained agree well with experimental data on the hydrogen solubility and the contributions of different carriers (protons, oxygen ions, holes) to the charge transfer in barium zirconates with different doping levels.

Influence of electron beam irradiation on the structural, electrical and thermal properties of Gd0.5Sr0.5MnO3 and Dy0.5Sr0.5MnO3 manganites

We present systematic studies on the effect of electron beam irradiation on structural, electrical and thermal properties of Gd0.5Sr0.5MnO3 and Dy0.5Sr0.5MnO3 manganites. The XRD patterns and Rietveld analysis show that the samples remain single phased even after they undergo electron beam irradiation. Both the series of the samples Gd0.5Sr0.5MnO3 and Dy0.5Sr0.5MnO3 show insulating trends in their temperature dependent electrical resistivity, ρ(T) behavior. The resistivity data for both the series of samples (pristine as well as irradiated) indicate that the small polaron hopping model is valid in high temperature region; on contrary, variable range hopping model governs the low temperature regime. Magnetic studies demonstrate that the Neel temperatures of pristine and irradiated samples of Gd0.5Sr0.5MnO3 and Dy0.5Sr0.5MnO3 do not change appreciably when they are subjected to irradiation. Thermo-electrical power is observed to increase with irradiation in Gd0.5Sr0.5MnO3 samples, whereas for Dy0.5Sr0.5MnO3 samples a decrease in thermo-electric power is seen when the samples are irradiated.

Microstructural properties, electrical behavior and low field magnetoresistance of (1−x)La0.67Sr0.33MnO3 (LSMO)+(x)Ni0.5Zn0.5Fe2O4 (NZFO) composites

Abstract The composites with composition of (1−x)La0.67Sr0.33MnO3 (LSMO)+(x)Ni0.5Zn0.5Fe2O4 (NZFO) with x=0.0 (S1), 0.04 (S2), 0.07 (S3), 0.10 (S4), 0.30 (S5) and 1.0 (S6) were synthesized by solid-state reaction route. Structural study using Rietveld refinement of X-ray diffraction (XRD) pattern indicates a rhombohedrally-distorted structure (space group R3c) for LSMO phase while to that NZFO compound crystallizes in cubic structure (space group Fd3m). XRD patterns and microstructural analysis show that LSMO and NZFO phase exists independently in Ni0.5Zn0.5Fe2O4 doped composites. The transport properties of the compositions x=0.0, x=0.04, and x=0.07 showed that NZFO phase improves the resistivity and shifts the metal–insulator transition temperature TMI towards lower temperature. The magnetoresistance (MR) of composite samples with x=0.04 and x=0.07 decreases monotonously from 200 to 300 K in a magnetic field of 8 T. At lower temperatures (~5 K), a sharp drop of negative MR at low fields (H 1 T) where MR is almost linear with applied magnetic field. Temperature dependence of resistivity for composites samples with x=0.04 and x=0.07 has been best fitted by small Polaron hopping (SPH) and variable range hopping models (VRH).

Effect of B-site Ti doping on the magnetic, low–field magnetocaloric and electrical transport properties of La0.7Sr0.3Mn1−xTixO3 perovskites

We report the effect of Ti doping on the magnetic, magnetocaloric and transport properties of La0.7Sr0.3Mn1−xTixO3 (0 ≤ x ≤ 0.3) polycrystalline compounds. Magnetic studies indicate that Ti doping weakens the double exchange ferromagnetic–paramagnetic transition and lowers the paramagnetic–ferromagnetic transition temperature. Also the results show that by increasing Ti doping level, the metal-insulator transition temperature decreases and the system with x ≥ 0.2 becomes an insulator. The magnetocaloric effect of 0 ≤ x ≤ 0.1 samples was studied using phenomenological method. With the increase in Ti-doping level, the ΔSmax/ΔH×102 value decreases from 16.2 J/kg K kOe (x = 0.0) to 4.2 J/kg K kOe (x = 0.05). However, the peak of the magnetic entropy change versus temperature plot for the sample with x = 0.05 broadens significantly. These characteristics, together with the critical transition temperature being near room temperature, indicate that this sample is promising as an efficient magnetic refrigerant at around room temperature. Magneto-resistance MR% data shows that its peak value increases with Ti-doping and MR reach highest value at x = 0.1. Moreover, the isothermal field dependent magnetoresistance in the temperatures range from 20 K to 150 K has been analyzed using a model based on spin polarized tunneling at the grain boundaries and it shows that there is a good agreement with our experimental data. The samples with 0 ≤ x ≤ 0.1 exhibit metallic behavior, which fits well to the resistivity ρ=ρ0+ρ2T2+ρ4.5T4.5, implying that combination of electron–electron, electron–magnon, and electron–phonon scattering processes contribute to the electrical resistivity in these samples. The electrical resistivity data in the insulating region of other samples (x ≥ 0.1) is well described using small polaron hopping and Mott's variable range hopping models. The non-adiabatic hopping conduction mechanism seems to be the most appropriate model explaining the conduction mechanism in La0.7Sr0.3Mn1−xTixO3 (0.1 ≤ x ≤ 0.3).

Influence of polarization and iron content on the transport properties of praseodymium–barium manganite

Polarization and iron effects on the electrical properties of Pr0.67Ba0.33Mn1−xFexO3 have been studied using impedance measurements. When iron is introduced, the insulator–metal transition (MI), observed in free compound, disappears and destroying such transition needs an iron concentration less than 5%. We also found that electrical conductance decreases when increasing Fe content. Such results are attributed to the decrease of Mn3+/Mn4+ ratio. Also, they are ascribed to the high probability of encountering Fe3+–O–Fe3+ and Mn3+–O–Fe3+ interactions, which greatly weakens the influence of Mn3+–O–Mn4+ interactions. The AC conductivity studies indicate that different types of hopping are involved. The contribution of hopping mechanism is confirmed by the temperature dependence of the frequency exponent ‘s’. Conductivity analysis shows that small polaron hopping (SPH) and variable range hopping (VRH) models are present in the conduction process. For small iron concentrations (x<0.1), we found that activation energy (Ea) does not changes significantly. Such result is in good agreement with the literature. But, for high iron concentrations (x>0.1), we found that Ea depend strongly in Fe content. We also found in this work that DC-bias does not affect the conduction process but proves its thermal activation. The variation of the conductance with polarization is a proof of an electro-resistance effect.

Structural, electrical, magnetic and thermal properties of Gd1–xSrxMnO3 (0.2≤x≤0.5) manganites

A systematic study on structural, electrical, magnetic and thermoelectric properties of bulk samples of Gd1–xSrxMnO3, synthesized by solid state reaction, is carried out in this communication. All the samples are in single phase with no detectable impurities. All the samples exhibit insulating behavior and the results indicate that high temperature electrical resistivity can be explained using small polaron hopping (SPH) model, whereas variable hopping model (VRH) model is valid in the low temperature regime. The magnetization results show that large irreversible magnetization is observed at low temperatures. The thermoelectric power measurement demonstrates that these samples exhibit colossal thermoelectric power.

A systematic study on the effect of electron beam irradiation on structural, electrical, thermo-electric power and magnetic property of LaCoO3

In this communication, the effect of electron beam irradiation on the structural, electrical, thermo-electric power and magnetic properties of LaCoO3 cobaltites have been investigated. Rietveld refinement of XRD data reveals that all samples are single phased with rhombohedral structure. Increase in electrical resistivity data is observed with increase in dosage of electron beam irradiation. Analysis of the measured electrical resistivity data indicates that the small polaron hopping model is operative in the high temperature regime for all samples. The Seebeck coefficient (S) of the pristine and the irradiated samples exhibits a crossover from positive to negative values, and a colossal value of Seebeck coefficient (32.65mV/K) is obtained for pristine sample, however, the value of S decreases with increase in dosage of irradiation. The analysis of Seebeck coefficient data confirms that the small polaron hopping model is operative in the high temperature region. The magnetization results give clear evidence of increase in effective magnetic moment due to increase in dosage of electron beam irradiation.