Mobility Of Electron Holes Research Articles

Hole polarons in LaFeO3 and La1−xSrxFeO3−δ : Stability, trapping, mobility, effect of Sr concentration, and oxygen vacancies

The stability, trapping, and mobility of electron holes are investigated in lanthanum ferrite ${\mathrm{LaFeO}}_{3}$, and in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{FeO}}_{3\ensuremath{-}\ensuremath{\delta}}$ ($x\ensuremath{\approx}0.1$, 0.4, and 0.6) by hybrid-density-functional and density-functional-$\mathrm{theory}+U$ calculations. In pure ${\mathrm{LaFeO}}_{3}$, the electron hole is more stable under a localized (polaronic) form than under a delocalized form, the energy difference (self-trapping energy) lies between $\ensuremath{\approx}\ensuremath{-}0.3$ and $\ensuremath{-}0.4$ eV. This self-trapped hole polaron is not strictly localized on a single Fe atom: instead, it occupies a quantum state made of a $3d$ orbital of a Fe atom, strongly hybridized with $2p$ orbitals of four neighboring oxygens. The hole polaron is thus localized on five atoms (among which one single Fe), which can be described as the ${\mathrm{Fe}}^{3+}$ oxidation into ${\mathrm{Fe}}^{4+}$. Electron hole transport results from the combination of onsite reorientations and hoppings, with energy barriers estimated at $\ensuremath{\approx}0.01--0.20$ eV and $0.3--0.4$ eV, respectively. The aliovalent substitution of lanthanum by strontium in ${\mathrm{LaFeO}}_{3}$ induces the presence of localized electron holes, preserving the insulating character of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{FeO}}_{3}$, regardless of the studied Sr concentration. The formation energy of the oxygen vacancy in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{FeO}}_{3}$ ($x\ensuremath{\approx}0.1$ and 0.4) is estimated at $\ensuremath{\approx}+0.8$ eV. This value is here successfully used to quantify the evolution of defect concentration as a function of the oxygen partial pressure.

Synthesis of mesoporous Ag/α-Fe2O3/TiO2 heterostructures with enhanced and accelerated photo/-catalytic reduction of 4-nitrophenol.

4-Nitrophenol (4-NP) is reported to originate disadvantageous effects on the human body collected from industrial pollutants; therefore, the detoxification of 4-NP in aqueous contamination is strongly recommended. In this study, the heterojunction mesoporous α-Fe2O3/TiO2 modulated with diverse Ag percentages has been constructed via a sol-gel route in the occurrence of a soft template P123. The formation of biphasic crystalline TiO2 anatase and brookite phases has been successfully achieved with the average 10nm particle sizes. The photo/-catalytic reduction of 4-NP has been performed utilizing NaBH4 as a reducing agent with and without visible illumination. All Ag/Fe2O3/TiO2 nanocomposites exhibited significantly higher photo/-catalytic reduction efficiency than pure Fe2O3, TiO2 NPs, and Fe2O3/TiO2 nanocomposite. 2.5% Ag/Fe2O3/TiO2 nanocomposite was considered the highest and superior photocatalytic reduction efficiency, and it almost achieved 98% after 9min. Interestingly, the photocatalytic reduction of 4-NP was accelerated 9 times higher than the catalytic reduction over 2.5% Ag/Fe2O3/TiO2; its rate constant value was 709 and 706 times larger than pure TiO2 and Fe2O3 NPs, respectively. The enhanced photocatalytic reduction ability of Ag/Fe2O3/TiO2 nanocomposite might be referred to as significantly providing visible light absorption and a large surface area, and it can upgrade the effective separation and mobility of electron holes. The stability of the synthesized catalysts exhibited that the obtained catalysts can undergo a slight decrease in reduction efficiency after five successive cycles. This approach highlights a novel route for constructing ternary nanocomposite systems with high photo/-catalytic ability.

Electrotransport in the La2NiO4-based solid solutions

Abstract This work combines new and earlier obtained results on electron hole and oxygen-ion transport in the La2NiO4-based solid solutions. The effect of lanthanum substitution with Ca/Sr and nickel with Fe, Mn, Co or Cu on transport properties of La2− x A x Ni1− y Me y O4+δ was analyzed and discussed at different substitution levels. Besides the changes in concentration and mobility of electron holes induced by the doping with cations of different nature, the partial transformation of Ni3+ from low-spin to high-spin state was shown to have a profound effect on transport properties of these materials leading to a notable decrease in mobility of electron holes, especially in the strontium-rich oxides. The obtained results suggested that the size factor was the main driving force behind the observed transformation of Ni3+. The oxygen-ion transport in La2− x A x Ni1− y Me y O4+δ was characterized by significant surface exchange limitations, which can be reduced only at relatively high concentrations of strontium and iron, and should be taken into account while evaluating the ionic conductivity by means of oxygen permeation or the modified Hebb-Wagner polarization method.

The effect of Sr2 + substitution on the electrical conductivity and chemical stability for cubic perovskite Ba1 − xSrxFe0.9In0.1O3 − δ

The effect of Sr2+ substitution on the electrical conductivity and chemical stability for Ba1−xSrxFe0.9In0.1O3−δ prepared by the Pechini method was investigated. The specimens could be indexed as cubic perovskite structure with disordered oxygen vacancies (0≤x≤0.8). To evaluate the properties of Ba1−xSrxFe0.9In0.1O3−δ, thermogravimetry–differential thermal analysis and electrical conductivity measurements were performed under N2 flow. These experiments revealed that the major charge carrier shifted from oxide ions to electron holes by the partial substitution of Sr2+ into the Ba2+-site. The substitution resulted in an enhanced electrical conductivity; this may be caused by an increase in the mobility of electron holes.

Point defects diagrams for pure and doped manganese oxide Mn1−δO in the temperature range of 1173–1830 K

This work presents point defects diagrams for the manganese oxide Mn1−δO, both pure and doped with M3+ and M+ metal ions, and with all the types of defects in the cation sublattice considered. The diagrams were calculated using a novel method. The calculations were based on the data from the deviation from stoichiometry studies by many authors, in the temperature range of 1173–1830 K.The values of ΔGo of the formation of Frenkel-type defects and of manganese vacancies, were determined and their temperature dependence. The obtained results of the calculations indicate that the manganese oxide MnO reaches the stoichiometric composition near the decomposition pressure of the oxide (Mn/MnO). It was shown that character of the dependence of the sum of concentrations of electronic defects ([h•]+b[e′]) on oxygen pressure is fully consistent with its dependence of the electrical conductivity. The values of parameter b, which is the ratio of the mobility of electrons and the mobility of electron holes (μe/μh = b), vary from 900 to 100. It was shown that even a small amount of the dope, M+, in (Mn1−yMy)1−δO causes the shift of pO2 towards higher oxygen pressures, where the oxide reaches the stoichiometric composition. In the range of the negative values of the deviation from the stoichiometry there is a significant concentration of interstitial manganese ions. As a result of the increase of the concentration of M3+ ions, the concentration of manganese vacancies increases and the concentration of electron holes decreases.

Oxygen Nonstoichiometry and Electrochemical Properties of GdBaCo2−xFexO6−δ Double Perovskite Cathodes

Mixed ionic- and electronic-conducting perovskite-type oxides are the state-of-the-art materials for high-temperature solid-state electrochemical devices such as solid oxide fuel cells (SOFCs), oxygen membranes, and sensors. Many of such materials are cobaltite-based oxides. Recently, double perovskites REBaCo2O5.5±δ, where RE is a trivalent rare earth and the oxygen content δ varies in wide range, have received a great attention as attractive materials for such application. Many interesting phenomena, such as giant magnetoresistance, charge ordering, and metal-insulator transition, have been observed in these compounds. Powder samples of GdBaCo2−xFexO6−δ(x=0;0.2) were synthesized by glycerol-nitrate method. Oxygen nonstoichiomentry of oxides GdBaCo2−xFexO6−δ(x=0;0.2) was measured by the thermogravimetric (TG) method as a function of temperature in the range of 25–1100°C in air. Total conductivity of aforementioned oxides was studied by the four-probe dc-method as a function of temperature in the range of 25–1100°C in air. Polarization resistance of double perovskite cathodes was investigated by impedance spectroscopy in symmetrical cell of the type electrode|electrolyte|electrode. “Metal-insulator” transition was found at 80°C in GdBaCo2O6−δ, whereas it was not observed in iron-doped sample GdBaCo1.8Fe0.2O6−δ due to the increase in oxygen content upon Fe-doping. At high temperatures, both double perovskites have almost the same total conductivity. Chemical interaction was found to decrease the performance of GdBaCo2−xFexO6−δ cathodes in YSZ-based SOFCs due to the chemical interaction between electrolyte and cathode materials, which significantly increases their polarization resistance. Behavior of total conductivity of oxides GdBaCo2−xFexO6−δ(x=0;0.2) with temperature was explained by assuming small polaron charge transfer. The particularity of the latter is larger mobility of electron holes as compared with that of electrons. Increase in cathode performance was shown in the case of YSZ covered by the Ce0.8Sm0.2O2 layer in comparison with pure Zr0.9Y0.1O2 electrolyte.

Structural features, nonstoichiometry and high-temperature transport in SrFe 1−xMo xO 3−δ

The oxide solid solutions SrFe 1− x Mo x O 3− δ , where x=0.05, 0.1 and 0.2, are studied in this work. It is shown that substitution of iron for molybdenum results in stabilization of a cubic quasi-perovskite locally inhomogeneous structure, which is evidenced by HREM and Mössbauer spectroscopy. The coulometric titration is used in order to determine changes of oxygen nonstoichiometry in the obtained solutions with temperature and ambient oxygen partial pressure. Partial molar thermodynamic functions of the labile oxygen are calculated from the measured data. The variations of partial molar entropy with oxygen content follow the ideal gas model reasonably well thus demonstrating approximately random distribution of oxygen vacancies in the doped ferrite at high temperatures. The partial molar enthalpy is found to increase with doping, which is indicative of a progressive decrease in average values of the bonding energy of labile oxygen ions. The measurements of total conductivity are used in order to determine partial contributions of charge carriers. The oxygen ion component is shown to increase at small level of doping, x=0.05 while further increase in molybdenum content is accompanied with the decline in the ion conductivity. The electron contribution in reducing conditions tends to increase with molybdenum content, which is interpreted as a manifestation of involvement of Mo 5+ cations in electron transport. Concentration and mobility of electron carriers are calculated. Some increase in mobility of electron holes at small doping is explained as related to the filling of oxygen vacancies.

Effect of Prolonged Oxidation on Semiconducting Properties of Titanium Dioxide

The present work reports the effects of prolonged oxidation of titanium dioxide on its semiconducting properties at elevated temperatures. The studies were performed by using simultaneous measurements of both electrical conductivity and thermoelectric power during oxidation at 1123 and 1323 K in the gas phase of controlled oxygen activity, p(O2) = 75 kPa. The effects of oxidation on these electrical properties are considered in terms of a change in the concentration of titanium vacancies, which are formed during oxidation. It is shown that the increase in the concentration of titanium vacancies results in a change of semiconducting properties, including the following: (1) The shift of the n−p transition point toward lower values of oxygen activity. This shift results in an increase of the oxygen activity regime in which TiO2 exhibits p-type properties. (2) Increase of the ionic electrical conductivity component. (3) Increase of the mobility terms for electrons and decrease of the mobility of electron holes. It is shown that the band gap of TiO2 is 3.1 eV and remains independent of the concentration of titanium vacancies. The data reported in this work may be used for processing TiO2 with controlled semiconducting properties. These data also pave the way for the processing of undoped TiO2 that exhibits p-type properties without the need to incorporate acceptor-type foreign ions.

Mobility of Electronic Charge Carriers in Titanium Dioxide

The present work reports the mobility of electronic charge carriers for well-defined TiO2. The mobility terms were determined by using the electrical conductivity for high-purity TiO2, including both single-crystal and polycrystalline specimens. The concentrations of electronic charge carriers were derived from defect disorder diagrams. The determined data indicate that the transport of electrons and holes occurs according to the band model and the hopping model, respectively. The differences in the electrical conductivity data between the single-crystal and polycrystalline specimens are considered within two scenarios: (1) The difference is due to the mobility terms. This scenario indicates that (i) the mobility of electrons for polycrystalline specimens is larger than that for the single crystal, and (ii) the mobility of electron holes for polycrystalline specimens is smaller than that for the single crystal. (2) The effect is due to the concentration terms. The analysis of data according to this scenar...

P-Type electron transport in La 1−xSr xFeO 3−δ at high temperatures

Electrical conductivity, thermopower and oxygen content were measured for La 1− x Sr x FeO 3− δ ( x=0.2, 0.5, 0.9) within the oxygen partial pressure range 10 −4–0.5 atm and at temperatures 750–950 °C. The dominating charge carriers under these experimental conditions are electron holes. The results of oxygen nonstoichiometry measurements are used to estimate the concentration of holes and to analyze data on conductivity and thermopower. The changes in thermopower are described by the model assuming that the number of sites accessible for migration of holes is independent on oxygen content. The mobility of electron holes is calculated, and it is found to be virtually independent on temperature in the compositions with x<0.5 while compositions with x>0.5 exhibit thermally activated mobility and mobility values about 0.1 cm 2 V −1 s −1 or smaller. It is suggested that the main contribution to the composition dependent variations in electron hole mobility are associated with Coulomb interactions at small x's, whereas at high degrees of doping the mobility of holes is most strongly affected by the increasing amount of oxygen vacancies.

Oxygen Nonstoichiometry, Conductivity, and Seebeck Coefficient of La 0.3Sr 0.7Fe 1− xGa xO 2.65+ δ Perovskites

The total electrical conductivity and the Seebeck coefficient of perovskite phases La 0.3Sr 0.7Fe 1− x Ga x O 2.65+ δ ( x=0–0.4) were determined as functions of oxygen nonstoichiometry in the temperature range 650–950°C at oxygen partial pressures varying from 10 −4 to 0.5 atm. Doping with gallium was found to decrease oxygen content, p-type electronic conduction and mobility of electron holes. The results on the oxygen nonstoichiometry and electrical properties clearly show that the role of gallium cations in the lattice is not passive, as it could be expected from the constant oxidation state of Ga 3+. The nonstoichiometry dependencies of the partial molar enthalpy and entropy of oxygen in La 0.3Sr 0.7(Fe,Ga)O 2.65+ δ are indicative of local inhomogeneities, such as local lattice distortions or defect clusters, induced by gallium incorporation. Due to B-site cation disorder, this effect may be responsible for suppressing long-range ordering of oxygen vacancies and for enhanced stability of the perovskite phases at low oxygen pressures, confirmed by high-temperature X-ray diffraction and Seebeck coefficient data. The values of the electron–hole mobility in La 0.3Sr 0.7(Fe,Ga)O 2.65+ δ , which increases with temperature, suggest a small-polaron conduction mechanism.

Hydrogen in diopside; diffusion, kinetics of extraction-incorporation, and solubility

The kinetic of H extraction-incorporation in diopside single-crystals (Ca 0.97 Na 0.02 Cr 0.01 Mg 0.97 Fe 0.036 Si 1.99 O 6 ) deduced by monitoring OH infrared absorption bands for samples heated from 973 to 1273 K at 0.1 atm and 1 atm of pH 2 , is independent of crystallographic orientation, P O₂ , and pH 2 . The diffusion law is D = D 0 exp[ -(126 ± 24) kJ/mol/RT], with log D 0 (in m 2 /s) = -6.7 ±1.1. Hydrogen self-diffusion obtained from H-D exchange in the same diopside samples over 873-1173 K, and along directions [001] and [100]* at 1 atm total pressure is two orders of magnitude faster than H uptake and follows the diffusion law D H = D 0 exp[ -(149+16) kJ/mol/RT], with log D 0 (in m 2 /s) = - 3.4 ± 0.8. Self-diffusion along [010] follows the diffusion law D H = D 0 exp[ -(143 + 33) kJ/mol/RT], with log D 0 (in m 2 /s) = -5.0 + 1.7 and is one order of magnitude faster than H uptake. The kinetics of extraction incorporation of H in this diopside follows the reaction Fe 3+ + O 2- + 1/2H 2 (g) = Fe 2+ + OH - and are not rate limited by the mobility of protons but more probably by the mobility of electron holes connected with the Fe oxidation-reduction process. The results suggest that the kinetics of H uptake in clinopyroxenes will increase with increasing Fe content until it is rate controlled by the kinetics of H self-diffusion. We predict a rate for H exchange in diopside appropriate to the upper mantle almost as fast as H exchange in olivine. The insensitivity of H solubility on temperature and P O₂ for samples recovered from low-temperature conditions (below 1273 K) and/or rapidly quenched samples let us suggest the use of OH concentration measurements in diopside as a potential pH 2 sensor.

Manganese olivine II: point defect relaxation

To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn2SiO4, the point defect relaxation time (τ) characterizing the rate of re-equilibration of electrical conductivity following a change in oxygen fugacity was measured for single crystals oriented for electrical conduction along the [010] direction. The experiments were carried out at temperatures T = 1173–1473 K and oxygen fugacities \(\) with the MnSiO3 activity controlled at unity. The value of τ, which ranges from 130 to 1463 s, increases with decreasing temperature. At 1273 K, the value of τ in the \(\) regime is a factor of ∼1.8 smaller than that in the \(\) regime. The point defect relaxation time was used to calculate the chemical diffusivity (\(\)). Values of \(\) lie in the range 2.2× 10−10 − 2.5×10−9 m2/s. For the \(\) regime, a semi-log plot of \(\) vs 1/T yields a concave downward curve. Based on these results combined with those from Part I of this work for the point defect structure and electrical conductivity in Mn-olivine, it is proposed that the relaxation rate of electrical conduction is determined by the coupled diffusion of manganese vacancies \(\) and electron holes \(\) which rate-control the relaxation process at low and high temperatures, respectively. Deconvolution of the \(\) curve into two straight-line segments yields values for the mobilities and the diffusivities for Mn vacancies and electron holes. These results, combined with the measured electrical conductivity data, were used to calculate the concentrations of Mn vacancies and electron holes. These results in conjunction with those published for other transition-metal silicate olivines reveal the following: The mobility of electron holes in Mn-olivine is about two orders of magnitude smaller than that in Fe-olivine and is somewhat larger than that in (Mg0.9Fe0.1)-olivine. The mobilities of metal vacancies in these three olivines are similar both in magnitude and in temperature dependence. The concentrations of the majority point defects are the highest in Fe-olivine crystals and the lowest in Co-olivine crystals, while those in Mn-olivine and (Mg0.9Fe0.1)-olivine crystals lie in between. The deviation from stoichiometry in the cation sublattice for a transition-metal silicate olivine is about two orders of magnitude lower than that for the corresponding transition-metal oxide.

Thermopower and electrical conductivity of single crystal and polycrystalline CoO

Comparative studies of both electrical conductivity and thermopower (Seebeck effect) were carried out for undoped Co1z.sbnd;yO involving both single crystal and polycrystalline specimens within the temperature range 1223–1473 K and under oxygen partial pressure range 10–105 Pa. It was observed that the reciprocal of oxygen pressure exponent determined by thermopower (nα) and electrical conductivity (nσ) for both single crystal and polycrystalline CoO increases with temperature. The parameter nα assumes much higher values than nσ for both specimens. The mobility of electron holes increases with non-stoichiometry (y) for both the single crystal and the polycrystalline specimen. Based on the electrical conductivity data on temperatures it was concluded that conduction in CoO occurs according to the hopping model rather than the band model. The effect of p(O2) on the exponent of the oxygen pressure dependence have been considered in terms of interactions between the defects in this region. Die elektrische Leitfähigkeit und die Thermokraft (Seebeck-Effekt) von undotierten Co1z.sbnd;yOEin- und Vielkristallproben wurden für den Temperaturbereich 1223–1473 K bei einem Sauerstoffpartialdruck zwischen 10–105 Pa untersucht. Dabei zeigte sich, daß der Kehrwert des Sauerstoffdruckexponenten, bestimmt aus der Thermokraft (nα) und der elektrischen Leitfähigkeit (nσ), sowohl für CoOEin- als auch für CoO-Vielkristallproben mit der Temperatur zunimmt. Für beide Probenarten erreicht der Parameter nα wesentlich höhere Werte als nσ. Die Mobilität der Elektronenfehlstellen nimmt mit der Nichtstöchiometrie (y) sowohl für Ein- als auch für Vielkristallproben zu. Augrund der Variation der elektrischen Leitfähigkeit mit der Temperatur wurde geschlossen, daß die Leitfähigkeit in CoO entsprechend dem ‘hopping’ Modell und nicht dem ‘band’ Modell erfolgt. Der Effekt von p(O2) auf den Exponenten des Sauerstoffdruckes wurde bezüglich der Wechselwirkung zwischen den Defekten diskutiert. Nous avons procédé à une étude comparative d'échantillons mono et polycristallins de Co1z.sbnd;yO, non dopés, du point de vue de la conductivitéélectrique et du comportement thermoélectrique (effect Seebeck), dans la gamme de températures 1223–1473 K et sous une pression partielle d'oxygène variant de 10 à 105 Pa. Nous avons observé que l'inverse de l'exposant de la pression d'oxygène, déterminé par la constante thermoélectrique (nα) et la conductivité électrique (nσ) pour les échantillons monocristallins ou polycristallins augmente avec la température. Le paramètre nα suppose des valeurs plus élevées que le paramètre nσ ceci pour les deux types d'échantillons. La mobilité des trous d'électrons augmente avec l'écart à la stoechiométrie (y) pour les deux catégories. La variation de la conductivité avec la température nous conduit à penser que la conduction dans CoO est une conduction par sauts plutôt qu'une conduction de bande. L'effet de p(O2) sur la mobilité des trous, aussi bien que sur l'exposant de la variation de la pression d'oxygène est examiné en termes des interactions entre les défauts.

Electrical Conductivity of Y2O3 as a Function of Oxygen Partial Pressure in Wet and Dry Atmospheres

The electrical conductivity of polycrystalline Y2O3 has been studied as a function of the partial pressure of oxygen (10–14 to 105 Pa) at 900° to 1500°C in atmospheres saturated with water vapor at 12°C or dried with P2O5. Yttria is a p‐conductor at high oxygen activities. The p‐conductivity increases with increasing PO2 and decreases with increasing PH2O. At low oxygen activities the oxide is a mixed ionic/electronic conductor. The ionic conductivity is approximately independent of PO2 and increases with increasing PH2O. In the Y2O3 samples, excesses of lower‐valent cation impurities (in the 10 to 100 mol‐ppm range) are the dominating negatively charged defects, and in the presence of water vapor they are compensated by interstitial protons. At high PH2O levels additional protons are probably compensated by interstitial oxygen ions. At high temperatures (±1100°C) and for high PO2 and low PH2O, the protons are no longer dominant, and the lower‐valent cations are mainly compensated by electron holes. The electrical conductivity exhibits hysteresis‐like effects which are interpreted in terms of segregation/desegregation of impurities at grain boundaries. The mobility of electron holes in yttria at 1500°C is estimated to be of the order of magnitude of 0.05 cm2. s–1. V–1

Investigations on Cuprous Sulfide

The activity of copper in cuprous sulfide as a function of the copper-to-sulfur ratio has been determined with the help of coulometric titrations. The homogeneity range of cuprous sulfide at 400°C extends from a copper-to-sulfur ratio of 1.9996±0.0002 for samples coexisting with metallic copper to a ratio of less then 1.93 for samples coexisting with digenite (Cu1.8S). The ratio of the effective mass of electron holes to that of the free electron has been found to be 7±2 at 435°C. Preliminary conductivity measurements suggest a considerable increase of the mobility of electron holes with an increasing copper deficit of the samples presumably due to fewer collisions of electron holes with excess electrons.