Small Polaron Conduction Research Articles

Y-doped CaZrO3/Co3O4 as novel dense diffusion barrier materials for a limiting current oxygen sensor.

Herein, we illustrate a feasible strategy to strengthen the gas sensing of Y-doped CaZrO3 (YxCa1-xZr0.7O3-δ (x = 0.05, 0.06, and 0.07))/0.1Co3O4 used as sensing materials. This compound was prepared via a solid-state reaction technique. The structural, morphological, electrical, and sensing features such as phase identification, microstructure, ionic conductivity, total conductivity and sensitivity of the fabricated sensors were evaluated via X-ray diffraction, scanning electron microscopy, electron-blocking method, electrochemical impedance spectroscopy and cyclic voltammetry. In addition, the influence of the Y-dopant on the properties of YxCa1-xZr0.7O3-δ/Co3O4 was thoroughly studied. XRD results revealed the formation of the orthorhombic perovskite phase of YxCa1-xZr0.7O3-δ. Moreover, the obtained results from the electrical properties elucidated high electronic and low ionic conductivities, and small polaron conduction of YxCa1-xZr0.7O3-δ/Co3O4. Furthermore, the results confirmed an excellent limiting current plateau for the fabricated oxygen sensor based on YxCa1-xZr0.7O3-δ/Co3O4. In particular, experimental observation indicates that Y-doping at the Ca site and/or Zr site might be difficult.

Defects and polaronic electron transport in Fe2WO6.

We report the synthesis of phase pure Fe2WO6 and its structural characterization by high quality synchrotron X-ray powder diffraction, followed by studies of electric and thermoelectric properties as a function of temperature (200-950 °C) and pO2 (1-10-3 bar). The results are shown to be in accordance with a defect chemical model comprising formation of oxygen vacancies and charge compensating electrons at high temperatures. The standard enthalpy and entropy of formation of an oxygen vacancy and two electrons in Fe2WO6 are found to be 113(5) kJ mol-1 and 41(5) J mol-1 K-1, respectively. Electrons residing as Fe2+ in the Fe3+ host structure act as charge carriers in a small polaron conducting manner. A freezing-in of oxygen vacancies below approximately 650 °C results in a region of constant charge carrier concentration, corresponding to an iron site fraction of XFe2+≅ 0.03. By decoupling of mobility from conductivity, we find a polaron hopping activation energy of 0.34(1) eV and a charge mobility pre-exponential u0 = 400(50) cm2 kV-1 s-1. We report thermal conductivity for the first time for Fe2WO6. The relatively high conductivity, large negative Seebeck coefficient and low thermal conductivity make Fe2WO6 an interesting candidate as an n-type thermoelectric in air, for which we report a maximum zT of 0.027 at 900 °C.

Correlation between B value deviation and sintering temperature of perovskite solid solution materials

AbstractThe effects of different sintering temperatures on the system of Ba0.5Sr0.5TiO3–La0.8Ca0.2MnO3 (BST‐LCM) material were investigated by means of scanning electron microscopy, X‐ray, resistance‐temperature measurements, and X‐ray photoelectron spectroscopy. X‐ray diffraction and X‐ray photoelectron spectroscopy confirm that BST‐LCM is completely miscible and BST is the main phase, which under the premise of following the theory of small polaron conduction, titanium ions interact with manganese ions, which affects the electrical properties of the system of BST‐LCM material. For the electrical properties of related, the LCM content is inversely proportional to the activation energy. Through the linear fitting of the experimental data, the empirical formula between the sintering temperature B value and the content of solid solution is obtained, and the approximate B value within a certain range can be preliminarily calculated.

Doping-Induced Modifications in the Magnetoelectronic Properties in LaFeO3 Nanoparticles

Simultaneous effects of Na and Mn co-dopants on either site of LaFeO3 on the structural, magnetic, and low-temperature dielectric properties are studied in detail. The nanoparticles are prepared using a modified wet chemical route. The X-ray diffraction pattern is consistent with the formation of monophase with the distorted orthorhombic crystal structure. The average particle size is measured to be around 34 nm. The temperature and field dependent magnetization data reveals a drastic magnetic phase change of the modified co-doping system than LaFeO3 which is otherwise canted antiferromagnetic. This phase change is attributed to the mixed valence state of Mn ions present in the system due to substitutions. Several orders of change in the dielectric order parameter are found in the doped system. The low-temperature impedance spectra of the doped samples show dominant grain boundary relaxation as evidenced by modulus spectra. The conduction mechanism has been identified by calculating activation energies of relaxation from grain boundary and grain resistances as well as from ac conductivity analysis which are in good agreement. The typical calculated values are evidence of small polaronic hopping conduction mechanism in the sample.

Spin glassy behavior and large exchange bias effect in cubic perovskite Ba0.8Sr0.2FeO3-δ**Project supported by the National Key Research and Development Program of China (Grant Nos. 2018YFA0305700 and 2018YFGH000095), the National Natural Science Foundation of China (Grant Nos. 51772324 and 11574378), and the Fund from the Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH013, GJHZ1773).

A single-phase iron oxide Ba0.8Sr0.2FeO3-δ with a simple cubic perovskite structure in Pm-3m symmetry is successfully synthesized by a solid-state reaction method in O2 flow. The oxygen content is determined to be about 2.81, indicating the formation of mixed Fe3+ and Fe4+ charge states with a disorder fashion. As a result, the compound shows small-polaron conductivity behavior, as well as spin glassy features arising from the competition between the ferromagnetic interaction and the antiferromagnetic interaction. Moreover, the competing interactions also give rise to a remarkable exchange bias effect in Ba0.8Sr0.2FeO2.81, providing an opportunity to use it in spin devices.

Effects of morphotropic phase boundary on the electric behavior of Er/Ti co-doped BiFeO3 ceramics

Morphotropic phase boundaries (MPB) consisting of rhombohedral and orthorhombic phases formed within the range of 0.1 ≤ x ≤ 0.15 in Bi1-xErxFe0.97Ti0.03O3 ceramics. Orthorhombic phase transition was verified by the appearance of a new mode at 310 cm−1 in Raman spectra. Bridging phases as intermedia phases of MPB were rare in Bi1-xErxFe0.97Ti0.03O3. Polarization and permittivity improved at MPB. Moreover, impedance spectroscopy revealed additional arcs with a distinct small polaron conduction mechanism. Er3+ ions enhanced magnetic properties and induced a permittivity change at Neel temperature. The magnetodielectric effect resulted in a permittivity change rate of ~1% at 10000 Oe.

Effect of dehydrogenation on the electrical conductivity of Fe-bearing amphibole: Implications for high conductivity anomalies in subduction zones and continental crust

Magnetotelluric measurements reveal the presence of high conductivity anomalies (up to ∼1 S/m) in both the forearc and backarc regions of subduction zones as well as the continental middle–lower crust. Such anomalies are commonly interpreted as a consequence of aqueous fluid released from the dehydration of hydrous minerals. Amphibole is an important constituent of the continental mid-crust and a major hydrous phase in subduction zones, such that its dehydration at high temperature has been suggested to provide a significant source of aqueous fluid. We performed electrical conductivity measurements of a natural Fe-bearing amphibole at 623–1173 K and 0.5–2.0 GPa using a multi-anvil apparatus and an impedance spectroscopy. Our results show that pressure has a very weak effect on conductivity compared with temperature. An abrupt variation of the impedance semicircular arc followed by a remarkable increase of electrical conductivity is observed at temperature of 843±20 K. However, the enhancement in conductivity is not attributed to conductive aqueous fluid but rather to amphibole oxidation–dehydrogenation, as confirmed by infrared spectroscopy and optical microscopy observations. A slight decrease in activation enthalpy from ∼0.80 eV to ∼0.70 eV suggests that the conduction mechanism does not change before and after dehydrogenation, and small polaron conduction (electron holes hopping between Fe2+ and Fe3+) is considered to dominate the conductivity of amphibole over the entire temperature range. Although amphibole dehydrogenation at high temperature cannot serve as a principal source of aqueous fluid, the enhanced electrical conductivity of amphibole after dehydrogenation is sufficient to account for the high conductivity anomalies observed in slab–mantle wedge interfaces and the continental lowermost mid-crust, particularly in local regions with high heat flow.

Enhancement in conductivity and dielectric properties of rare-earth (Gd3+) substituted nano-sized CoFe2O4

Cobalt ferrites nanoparticles doped with gadolinium CoFe2−xGdxO4, referred to as CFGO (x = 0.0, 0.1, 0.3, 0.5 and 0.7) was achieved by glycine nitrate process. The phase confirmation and crystallite size were obtained from X-ray diffraction spectra and their variation with dopants content was determined. The Williamson–Hall plot was used to investigate the individual contributions of crystallite sizes and lattice strain on the peak broadening of the CFGO nanoparticles. Further confirmation of the spinel structure was done by Fourier transform infrared spectra. Dielectric properties such as dielectric constant (e′) and dielectric loss (e″) have been investigated in the frequency range 100 Hz–1 MHz. The dielectric constant (e′) dispersion for CFGO nanoferrites is fitted in accordance with the modified Debye’s function. The complex impedances and complex modulus analysis confirm a grain interior mechanism contributing to the dielectric properties. The electrical behaviour of the CFGO nanoferrites exhibited small polaron conduction mechanism. From the temperature dependence curve of dielectric relaxation, activation energies for CFGO samples have been calculated. The low loss dielectric makes these samples promising materials to be used at high frequencies.

Investigation of charge transport mechanism in semiconducting La0.5Ca0.5Mn0.5Fe0.5O3 manganite prepared by sol–gel method

Here in, we report the charge transport mechanism in semiconducting La0.5Ca0.5Mn0.5Fe0.5O3 (LCMFO) polycrystalline material synthesized via sol–gel auto combustion route. X-ray diffraction (XRD) analysis confirmed the orthorhombic phase of the prepared material. Temperature dependent resistivity and impedance spectroscopy measurements have been carried out to probe the dielectric and electrical conduction mechanism which revealed a change of Mott variable range to the small polaronic hopping conduction mechanism around 303 K. The complex impedance and modulus spectra undoubtedly showed the contribution of both grain and grain boundary effect on the conduction properties of LCMFO. An equivalent circuit [(RgbQgb) (RgQg)] model has been used to address the electrical parameters associated with the different phases (grains and grain boundaries) having different relaxation times. The values of resistances of two phases obtained after fitting the equivalent circuit in the nyquist plot have been analyzed which confirmed the change of conduction mechanism around 303 K. The resultant change in conduction mechanism is also supported by the conductivity plots.

Ba doped Fe3O4 nanocrystals: Magnetic field and temperature tuning dielectric and electrical transport

Nanocrystalline BaFe2O4 has been prepared through low temperature pyrophoric reaction method. The structural, dielectric and electrical transport properties of BaFe2O4 are investigated in detail. AC electrical properties have been studied over the wide range of frequencies with applied dc magnetic fields and temperatures. The value of impedance is found to increase with increase in magnetic field attributing the magnetostriction property of the sample. The observed value of magneto-impedance and magnetodielectric is found to ∼32% and ∼33% at room temperature. Nyquist plots have been fitted using resistance-capacitor circuits at different magnetic fields and temperatures showing the dominant role of grain and grain boundaries of the sample. Metal-semiconductor transition ∼403 K has been discussed in terms of delocalized and localized charge carrier.We have estimated activation energy using Arrhenius relation indicating temperature dependent electrical relaxation process in the system. Ac conductivity follow a Jonscher’s single power law indicating the large and small polaronic hopping conduction mechanism in the system.

Effect of calcium and copper/iron co-doping on defect-induced properties of La2NiO4-based materials

The La2-xCaxNi1-y(Fe/Cu)yO4+δ solid solutions with х = 0.2, 0.4, 0.5 and y = 0, 0.1, 0.2, 0.3 were synthesized by decomposition of citrate–nitrate precursors. The single-phase La2-xCaxNi1-y(Fe/Cu)yO4+δ materials crystallized in the K2NiF4-type structure, sp. gr. I4/mmm. The crystal structure parameters of La2-xCaxNi1-y(Fe/Cu)yO4+δ were refined by the Rietveld method. The unit cell volume of La2-xCaxNi1-y(Fe/Cu)yO4+δ decreased with x and increased with y. All studied samples exhibited oxygen excess compared to the stoichiometric composition (δ > 0) in the temperature range 30–1100 °C except for La1.6Ca0.4Ni1-yCuyO4+δ (y = 0.2 and 0.3). Calcium and copper doping decreased the total oxygen content while iron doping increased it in La2-xCaxNi1-y(Fe/Cu)yO4+δ. Thermal expansion coefficients (TECs) of La2-xCaxNi1-y(Fe/Cu)yO4+δ slightly increased with dopant concentration and varied in the range (14.5–15.1) × 10−6 K−1. The contribution of chemical expansion was shown to be negligible. The total conductivity of La2-xCaxNi1-y(Fe/Cu)yO4+δ was thermally activated within the whole studied temperature range 30–1000 °C and could be described in terms of small polaron conduction mechanism. The values of total conductivity increased with x and decreased with y in La2-xCaxNi1-y(Fe/Cu)yO4+δ. The analysis of the Seebeck coefficient vs temperature for La2-xCaxNi1-y(Fe/Cu)yO4+δ showed that the main charge carriers are electron holes localized on Ni2+ cations represented by low-spin Ni3+ cations which had a tendency to change the spin configuration gradually to high-spin when the temperature and dopant content increased.

Thermopower and magnetocaloric properties in NdSrMnO/CrO3 composites

The thermoelectric power (TEP) and magnetocaloric effect (MCE) for (Nd0.6Sr0.4MnO3)1-x/(CrO3)x composites have been measured. The TEP measurements show a negative sign value of the Seebeck coefficient (S), in microvolts. TEP data construe in the low range of temperature by the magnon and phonon drag model, whereas at high temperature by small polaron conduction mechanism. Magnetic measurements exhibit that all composites show a paramagnetic–ferromagnetic transition with decreasing temperature. The Arrott plots of composites reveal the occurrence of a second order phase transition. The maximum value of magnetic entropy change (ΔS) is 2.37 J kg−1 K−1, achieved fore the composite with x = 0.015. Moreover, the maximum value of relative cooling power (RCP) is 122.1 J kg−1, achieved for the composite with x = 0.020. These composites may be appropriate for magnetic application near room temperature.

Transition from mobility-activated small polaron to carrier density-activated conduction of sol-gel-derived highly-oriented CuAlO2 thin film and enhanced thermoelectric properties

CuAlO2 is a technologically important material having diverse applications, including superior thermoelectric properties. Its unique crystallographic structure manifests an anisotropic environment for the charge carriers and phonons, which is considered to be the reason for the enhancement of the thermopower. To exploit this novel property, a controlled sol-gel deposition technique is adopted to realize highly c-axis-oriented growth of CuAlO2 thin film on Si and glass substrates. Thermoelectric measurements are performed in such a way that the carriers are confined along the a-b plane of the nanocrystal, which is parallel to the substrate. This allows a two-dimensional confinement of the charge carriers, leading to enhanced thermoelectric properties. Additionally, the temperature-dependent electrical characterizations depict two different charge-transport regimes with a cross-over at 360K. The low-temperature region corresponds to the mobility-activated small-polaron conduction and the high-temperature region belongs to the semiconductor-type carrier-density-activated conduction. Calculation of polaron activation energy from low-temperature regime indicates considerable influence of band carriers (hole) on the polaronic levels, due to which the above-mentioned transition is manifested. Calculations of activation and Fermi energy from high-temperature regime reveal a deep acceptor level and shallow Fermi level, which is typical of a non-degenerate semiconductor with acceptors not fully ionized at room temperature.

Evaluation of electrical conductivity and oxygen diffusivity of the typical Ruddlesden-Popper oxide Sr3Fe2O7-δ

The electrical conductivity (σ) and oxygen diffusivity of the typical Ruddlesden-Popper oxide Sr3Fe2O7-δ were investigated with the variation of oxygen partial pressure, P(O2), and temperatures, and thus the results were discussed based on its defect structure. The σ increases with the increase of P(O2) and a positive slope of log σ depend on P(O2) is close to 1/4 with the small polaron conduction, where the mobility μP are between 0.01 and 0.02cm2V−1s−1 regardless of temperature and P(O2). Oxygen diffusivity derived from the electrical conductivity relaxation (ECR) after an abrupt change of P(O2) increased with the increase P(O2) and temperature. A new pulse isotope 18O-16O exchange (PIE) at 623–773K was carried in order to rapidly determine the tracer oxygen surface reaction coefficient. The numerical relationship of oxygen diffusivity measured by ECR and PIE measurements was successfully established by the ambipolar diffusion theory and defect chemical analysis.

Interrelation of transport properties, defect structure and spin state of Ni3+ in La1.2Sr0.8Ni0.9Fe0.1O4+δ

The total conductivity, Seebeck coefficient and oxygen non-stoichiometry for La1.2Sr0.8Ni0.9Fe0.1O4+δ have been measured vs temperature and oxygen partial pressure P(O2). The measurements were carried out at 800, 850, 900 and 950 °C within the P(O2) range of 10−5–0.21 atm. La1.2Sr0.8Ni0.9Fe0.1O4+δ was shown to be oxygen deficient in all temperature and P(O2) ranges studied. The calculated values of the partial molar enthalpy of oxygen depend very slightly on oxygen content (δ), indicating that La1.2Sr0.8Ni0.9Fe0.1O4+δ with the oxygen deficiency can be considered an ideal solution. The model of point defect equilibria in La1.2Sr0.8Ni0.9Fe0.1O4+δ has been proposed and fitted to experimental dependencies. Subsequent joint analysis of the defect structure and transport properties revealed that electron holes can coexist in both localized and quasi-delocalized states in the oxide: the former corresponded to high-spin state Ni3+ and the latter – to low-spin state Ni3+. The mobilities of localized electron holes were shown to be significantly lower in comparison to quasi-delocalized ones. The behavior of localized electron holes was explained in terms of a small polaron conduction mechanism; in contrast, quasi-delocalized electron holes were described in terms of a band conduction approach. The small polaron conduction mechanism was shown to be predominant in the Sr- and Fe-co-doped lanthanum nickelate.

Thermoelectric properties of Nb-doped TiO2- ceramics reduced at elevated temperature

Abstract A series of Nb-doped TiO2-δ (Ti1-xNbxO2-δ) ceramics was fabricated using a traditional solid state reaction method with subsequent annealing in a reducing H2 atmosphere at 1573 K. From X-ray diffraction patterns it could be seen that, when x ≤ 0.04, samples were composed of mixed anorthic phases; tetragonal phase diffraction peaks for rutile TiO2 occurred in the samples with x = 0.08 and 0.20; for x = 0.40 and 0.60, only the rutile structure was seen, which suggests that the rutile structure could be maintained up to a temperature of 1573 K with a higher Nb doping level. The thermoelectric properties of the Ti1-xNbxO2-δ samples were measured in the 100–380 K temperature range. The resistivity values indicated that these samples exhibited semiconductor behavior, while their electrical transport behavior in the temperature range from room temperature to 380 K was consistent with small polaron conduction. The thermal conductivity for this series of samples gradually increased with an increase in temperature and was dominated by lattice thermal conductivity. The largest observed figure of merit ZT value, 0.016, was obtained for the sample with x = 0.20 at the highest test temperature of 380 K.

Investigation of the thermoelectric properties of Nb and oxygen vacancy co-doped SrTiO3 ceramics

High quality Nb doped SrTi1−xNbxO3 polycrystalline ceramics were fabricated using a conventional solid state reaction method. By annealing in a reducing atmosphere at an elevated temperature, a series of Nb and oxygen vacancy co-doped SrTi1−xNbxO3−δ (0 ⩽ x ⩽ 0.2) samples was obtained. The thermoelectric properties of the samples were measured in the temperature range from 15 K to 380 K. These measurements showed that the transport behavior of these samples is consistent with the small polaron conduction mechanism for the temperature range from room temperature to 380 K. Furthermore, after annealing, samples with a lower Nb doping were found to give a relative higher ZT value, while excess Nb led to a reduced ZT value. The x = 0.02 sample gave the optimal thermoelectric properties, with a ZT value of 0.023 at 300 K, and 0.028 at 380 K.

Phase diagram, ac susceptibility and thermoelectric power properties of the La0.90Li0.10−xNaxMnO3 manganites

Ac susceptibility and thermoelectric power properties of bulk La0.90Li0.10-xNaxMnO3 (0 ≤ x ≤ 0.10 at. %, step 0.02 at. %) compounds have been studied. The temperature dependence of the ac susceptibility measurements showed a cusp and shoulder below Curie temperature (TC) related to a frequency-independent and dependent peak in χ″ (T) curves respectively. Slower spin dynamics and the cluster sizes augment with increases the Na content. In terms of magneto-transport a properties, a peak is observed in vicinity of Tms transition, which it has industrial applications. The thermoelectric power data analyzed at low temperature by the magnon and phonon drag concept, while at high temperature by small polaron conduction mechanism. We found an unexpected correlation of structures in MR (T) and S(T) curves.

Investigation of the thermoelectric properties of reduced Nb-doped TiO2- ceramics

Abstract A series of Nb-doped TiO2-δ (Ti1-xNbxO2-δ) ceramics was fabricated using a conventional solid state reaction method with subsequent annealing in a reducing atmosphere. X-ray diffraction revealed that there are different phases in the ceramics with different Nb doping levels, x. The thermoelectric properties of the Ti1-xNbxO2-δ samples were measured in the temperature range from 100 K to 380 K, and the resistivities and Seebeck coefficients for these samples were found to show semiconductor behavior. It has been shown that the transport behavior of these samples is consistent with the small polaron conduction mechanism for the temperature range from room temperature to 380 K. The largest ZT value observed, 0.023, was obtained for the x = 0.01 sample at the highest test temperature, 380 K.

Pressure dependence of electrical conductivity in forsterite

AbstractElectrical conductivity of dry forsterite has been measured in muli‐anvil apparatus to investigate the pressure dependence of ionic conduction in forsterite. The starting materials for the conductivity experiments were a synthetic forsterite single crystal and a sintered forsterite aggregate synthesized from oxide mixture. Electrical conductivities were measured at 3.5, 6.7, 9.6, 12.1, and 14.9 GPa between 1300 and 2100 K. In the measured temperature range, the conductivity of single crystal forsterite decreases in the order of [001], [010], and [100]. In all cases, the conductivity decreases with increasing pressure and then becomes nearly constant for [100] and [001] and slightly increases above 7 GPa for [010] orientations and a polycrystalline forsterite sample. Pressure dependence of forsterite conductivity was considered as a change of the dominant conduction mechanism composed of migration of both magnesium and oxygen vacancies in forsterite. The activation energy (ΔE) and activation volume (ΔV) for ionic conduction due to migration of Mg vacancy were 1.8–2.7 eV and 5–19 cm3/mol, respectively, and for that due to O vacancy were 2.2–3.1 eV and −1.1 to 0.3 cm3/mol, respectively. The olivine conductivity model combined with small polaron conduction suggests that the most part of the upper mantle is controlled by ionic conduction rather than small polaron conduction. The previously observed negative pressure dependence of the conductivity of olivine with low iron content (Fo90) can be explained by ionic conduction due to migration of Mg vacancies, which has a large positive activation volume.