Ion and electron conduction in SrFe1−xScxO3−δ

The oxygen ion and electron transport in SrFe_1−xSc_xO_3−δ  (x = 0.1–0.3) system at 700–950 °C were studied analyzing the total conductivity dependencies on the oxygen partial pressure, pO₂. The conductivity measurements were performed both under reducing conditions (10^− 19 ≤ pO₂ ≤ 10^− 8 atm) comprising the electron-hole equilibrium point, and in oxidizing atmospheres (10^− 5 ≤ pO₂ ≤ 0.5 atm) which are characterized by extensive variations of the oxygen content studied by coulometric titration technique. The incorporation of 10% Sc³⁺ cations into the iron sublattice suppresses transition of the cubic perovskite phase into vacancy-ordered brownmillerite, thus improving ion conduction at temperatures below 850 °C. When scandium content increases, the ion conductivity becomes considerably lower. The hole mobility is thermally-activated and varies in the range of 0.001 to 0.05 cm² V^− 1 s^− 1, increasing with oxygen concentration and decreasing on Sc doping.     

The effect of cation nonstoichiometry on the electrical conductivity of acceptor-doped CaZrO3

The electrical properties of acceptor-doped Ca_1−xZr_0.99M_0.01O_3−δ (M = Mg²⁺, In³⁺) systems were investigated as a function of cation nonstoichiometry (0 ≤ x ≤ 0.05). The characterization was carried out using the impedance spectroscopy between 550 °C and 1100 °C in dry air. The contributions of the grain and grain boundary conductivity to the total conductivity were obtained from the impedance data. When the Ca deficiency (x) increased, the total conductivity rapidly decreased with the corresponding increase in activation energy. Although the grain conductivity increased slightly with increasing x, the total conductivity is mostly determined by the highly resistive grain boundary. With varying x, the activation energy of total conductivity showed the percolation behavior. The percolation threshold values vary according to the doped species. It may be due to the difference in concentration of oxygen vacancies of the specimens.     

Impedance and Modulus studies of the solid electrolyte system 20CdI2–80[xAg2O–y(0.7V2O5–0.3B2O3)], where 1 ≤x/y ≤ 3

In the present work, an evaluation of the transport properties of super ion conducting quaternary system 20CdI₂–80[xAg₂O–y(0.7V₂O₅–0.3B₂O₃)], where 1 ≤ x/y ≤ 3, in steps of 0.25, to study the effect of changing the modifier to former ratio on the conduction phenomena has been undertaken. Electrical conductivity measurements were made using complex impedance method. The electrical conductivity and conductivity relaxation of the system were studied in the temperature range from 303 K to 333 K and in the frequency range from 100 Hz to 10 MHz. The highest conductivity at room temperature is obtained for the system with modifier to former ratio 1.75. Impedance and modulus analyses had indicated the temperature independent distribution of relaxation times and the non-Debye behavior in these materials. The co-operative motion due to strong coupling between the mobile Ag⁺ ions is assumed to give rise to non-Debye type of relaxation. The silver ionic transport number (t_Ag+) obtained by the emf technique suggested the occurrence of silver ion conduction in the CdI₂-doped Ag₂O–V₂O₅–B₂O₃ system.     

Correlation of defect structure and ionic conductivity in δ-phase solid solutions in the Bi3NbO7–Bi3YO6 system

Defect structure and conductivity behaviour are discussed in the solid solution Bi₃Nb_1−xY_xO_7−x(0.0 ≤ x ≤ 1.0). Investigations were carried out using a combination of ac impedance spectroscopy and powder X-ray and neutron diffraction. Low temperature conductivity and activation energy both increase as a function of x. The former is attributed to an increase in oxide ion vacancy concentration, whereas the latter is due a redistribution of oxide ion vacancies as determined by neutron diffraction measurements. The defect structures at room temperature and 800 °C are presented. Curvature in Arrhenius plots of conductivity throughout the composition range is associated with a temperature dependent redistribution of oxide ions.     

X-ray photoelectron spectroscopy of CeO2–Na2O–SiO2 glasses

A series of (CeO₂)_x–(Na₂O)_0.3–(SiO₂)_(0.7−x) glasses, where 0.025 ≤ x ≤ 0.075, have been synthesized and investigated by mean of X-ray photoelectron spectroscopy (XPS). The Ce 3d spin-orbit doublet was curve fitted in order to quantify the proportions of each cerium oxidation state in these glasses. It was found that Ce ions are predominantly in the Ce(III) state in glasses with compositions x ≤ 0.075, while mixed Ce valences were found in the glass with composition x = 0.10. The O 1s spectra have also been curve fitted with two components, one from bridging oxygen (BO) and the other from non-bridging oxygen atoms (NBO). The measured number of NBO, based on the fact that only oxygen atoms in the site Si–O–Na⁺ contribute to the NBO peak, was found to be constant at ∼35% for all samples, in good agreement with the value calculated from the glass composition and inductively coupled plasma (ICP) suggesting that Ce ions enter the network as a glass intermediate. The thermal measurements done on these glasses agree well with the XPS findings.     

Effect of nanocrystallization on the electronic conductivity of vanadate–phosphate glasses

Electronically conducting glasses of the composition xV₂O₅·(100 − x)P₂O₅ for 60 < x < 90 were prepared. The glasses of the composition corresponding to x = 90 exhibited the highest electrical conductivity and they were studied in more detail. The effects of the annealing of the samples on their electrical conductivity, structure and other characteristics were studied by impedance spectroscopy, X-ray diffractometry, DSC and SEM microscopy. It was shown that, at temperatures close to the crystallization temperature T_c (determined from DSC), these glasses turned into nanomaterials consisting of crystalline grains of V₂O₅ (average size 25–35 nm) embedded in the glassy matrix. Their electrical conductivity was higher and the temperature stability was better than those of the starting glasses. It is postulated that the major role in this conductivity enhancement is played by the interfacial regions between crystalline and amorphous phases. The annealing at temperatures exceeding T_c led to massive crystallization and to a conductivity drop. The XRD and SEM observations have shown that the material under study undergoes structural changes: from amorphous at the beginning, to partly crystalline after the annealing at 340 °C and to polycrystalline after the annealing at 530 °C.The obtained results are in agreement with those of our earlier studies on mixed electronic–ionic conducting glasses of the ternary Li₂O–V₂O₅–P₂O₅ system.     

Structural and electrical properties of tellurovanadate glasses containing Li2O

Glassy materials are promising intercalation compounds, due to their open network structure and absence of grain boundaries. Some glasses containing alkali ions and a high concentration of transition metal ions can present mixed ionic-electronic conductivity and are therefore potential candidates for application as cathode material in Li-ion batteries. The present work is devoted to the ternary system xLi₂O–(1 − x)[0.3V₂O₅–0.7TeO₂] with 0 ≤ x ≤ 0.4. These compounds were prepared by heat treatment in air at 800 °C followed by traditional quenching. Raman spectroscopy and ⁵¹V nuclear magnetic resonance measurements were performed in order to highlight the structural short range order modifications induced by the introduction of the Li₂O network modifier. These structural effects can be related to the electrical behaviour, as studied by complex impedance spectroscopy measurements.     

Electrical conductivity of (Ba0.3Sr0.2La0.5)(In1−xFex)O3−δ

We have synthesized the perovskite oxides of the (Ba_0.3Sr_0.2La_0.5)(In_1−xFe_x)O_3−δ system and measured the total electrical conductivity as a function of temperature and oxygen partial pressure. It was found that the single-phase composition region extended from x = 0.0 to x = 1.0, and that the Fe valence increased from 3.06 to 3.50 in that region. The electrical conductivity was semiconducting from x = 0.0 to x = 0.40 and metallic from x = 0.50 to x = 1.0. The total electrical conductivity at 800 °C also increased with the Fe content and achieved a maximum value of 140 (S/cm) at x = 1.0. From the dependence of the electrical conductivity on the oxygen partial pressure, we conclude that above x = 0.50, the majority carriers are holes. The estimated hole conductivity increased exponentially with the amount of Fe⁴⁺ cation present. The oxide ion conductivity was dependent on the oxygen vacancy content.     

Synthesis and characterization of La4Ni3−xCoxO10±δ (0.0 ≤ x ≤ 3.0, Δx = 0.2) for solid oxide fuel cell cathodes

The cobalt-doped lanthanum–nickel oxide system, La₄Ni_(3−x)Co_xO_10±δ (0.0 ≤ x ≤ 3.0, Δx = 0.2), was investigated as possible cathode materials for intermediate-temperature solid-oxide fuel cells. X-ray diffraction shows the presence of two structural phases in the series belonging to Bmab for 0.0 ≤ x ≤ 0.2, 0.8 ≤ x ≤ 2.0 and 2.6 ≤ x ≤ 3.0 and Fmmm for 0.4 ≤ x ≤ 0.6 and 2.2 ≤ x ≤ 2.4. All compositions are oxygen-deficient (δ < 0). Electrical conductivity measurements show a systematic decrease in the conductivity as cobalt content increases from x = 0.0 to 2.0, and reverses for x > 2.0. AC impedance measurements of the x = 0.4 composition in symmetrical cells with LSGM as an electrolyte show improved electrode performance over the parent nickelate La₄Ni₃O_9.78. Long-term thermal stability studies show the x = 0.4 composition to be more stable than the x = 3.0 phase after annealing at 1173 K in air for 1 week making this material a viable candidate for cathodes in solid oxide fuel cells.     

Structure and electrochemical properties of Sm0.5Sr0.5Co1 − xFexO3 − δ cathodes for solid oxide fuel cells

Crystal structure, thermal expansion coefficient, electrical conductivity and cathodic polarization of compositions in the system Sm_0.5Sr_0.5Co_1 − xFe_xO_3 − δ with 0 ≤ x ≤ 0.9 were studied as function of Co / Fe ratio and temperature, in air. Two phases, including an Orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9, were observed in samples of Sm_0.5Sr_0.5Co_1 − xFe_xO_3 − δ at room temperature. The adjustment of thermal expansion coefficient (TEC) to electrolyte, which is one of the main problems of SSC, could be achieved to lower TEC values with more Fe substitution. High electrical conductivity above 100 S/cm at 800 °C was obtained for all specimens, so they could be good conductors as cathodes of IT-SOFC. The polarization behavior of SSCF as a function of Fe content was evaluated by means of AC impedance using LSGM electrolyte. It was discovered that the Area Specific Resistance (ASR) of SSCF increased as the amount of substitution of Fe for Co increased. When the amount of Fe reached to 0.4, the highest ASR was obtained and then the resistance started decreasing above that. The electrode with a composition of Sm_0.5Sr_0.5Co_0.2Fe_0.8O_3 − δ showed high catalytic activity for oxygen reduction operating at temperature ranging from 700 to 800 °C.     

Ionic conductivity of Ce1−xNdxO2−x / 2

The electrical conductivity of sintered samples of Ce_1−xNd_xO_2−x / 2 (0.01 ≤ x ≤ 0.2) was investigated in air as a function of temperature between 150 and 600 °C using AC impedance spectroscopy. The individual contribution of the bulk and grain boundary conductivities has been discussed in detail. In the low temperature range (< 350 °C), the activation enthalpy for bulk conductivity exhibited a shallow minimum at 3 mol% Nd, with a value of 0.68 eV. The activation enthalpy also produced a shallow minimum at 5 mol% Nd in the high temperature range (> 350 °C), with a value of 0.56 eV. It was shown that Ce_1−xNd_xO_2−x / 2 is an electrolyte that obeys the Meyer Neldel rule. The bulk conductivity data measured by others for the same system has also been recalculated and re-evaluated to facilitate easier comparison with our own data.     

High temperature properties of the n = 2 Ruddlesden–Popper phases (La,Sr)3(Fe,Ni)2O7−δ

The crystal chemistry and mixed conductor properties of the n = 2 member of the Ruddlesden–Popper (R–P) phases Sr_3−xLa_xFe_2−yNi_yO_7−δ with 0 ≤ x ≤ 0.3 and 0 ≤ y ≤ 1.0 have been studied at high temperature. High-temperature X-ray diffraction and thermogravimetric measurements of the equilibrium pO₂ (10^− 5 ≤ pO₂ ≤ 1 atm) in the temperature range 400 ≤ T ≤ 1000 °C indicate that the Sr₃FeNiO_7−δ phase is able to accommodate a large oxygen non-stoichiometry (δ ∼ 1.5) without structural transformations. The electrical conductivity and oxygen permeability increase with the substitution of Ni for Fe in the range 550 ≤ T ≤ 1000 °C. The electrical transport of the Sr₃FeNiO_7−δ phase is thermally activated and the activation energy decreases with the substitution of Ni for Fe for a given oxygen content. The increase in the oxygen permeation flux with increasing Ni content is due to an increasing oxygen non-stoichiometry and a lower activation energy for permeation.     

Mixed conducting perovskite-like ceramics on the base of lanthanum gallate

Ceramic perovskite solid solutions (La_0.9Sr_0.1)[(Ga_1−xM_x)_0.8Mg_0.2]O_3−y, 0 ≤ x ≤ 0.5, M = Fe, Ni, Cr (systems I–III) and brownmillerite solid solutions (La_0.2Sr_1.8)[Ga(Fe_1−xMg_x)]O_5−z, 0 ≤ x ≤ 0.5, (system IV) have been prepared. The samples have been studied by X-ray diffraction and electron microscopy methods, dielectric spectroscopy and permeability measurements. The correlation between the composition, unit cell parameter changes, electrical transport and oxygen permeation properties has been revealed. Introduction of transition metals (Fe, Ni, or Cr), substituting for gallium, ensures the enhancement of the electronic constituent of the conductivity in the perovskite systems I–III. Stabilization of the transition metal high valence states 4+ or 5+ has been suggested for compositions I and III. This leads to a unit cell volume contraction and provides a decrease in the concentration of oxygen vacancies. The oxygen permeability reaches its maximum values in compositions I–III with x ∼ 0.3. On the contrary, increasing concentration of the doping element with lower valence state (magnesium), substituting for iron, determines the expansion of the brownmillerite unit cell volume and provides an increase of the oxygen vacancy concentration, which in turn, favors the enhancement of oxygen permeability of composition IV.     

Preparation,characterization and electrical property of Mn-doped ceria-based oxides

Manganese-doped ceria-based oxides, Ce_1−xMn_xO_2−δ (0.05 ≤ x ≤ 0.3) and Ce_1−x−yGd_xMn_yO_2−δ˙ (0.05 ≤ x≤ 0.2, 0.05 ≤ y ≤ 0.25) were synthesized, and crystal phase analysis by XRD and measurements of electrical properties were performed. Solubility limit of Mn in Ce_1−xMn_xO_2−δ˙ seemed to be between 5 mol% and 10 mol% and Mn₃O₄ was the main by-product above the solubility limit in the case of heat treatment at 1300 °C. Judging from the oxygen partial pressure dependence of total conductivity and emf measurements, Ce_1−xMn_xO_2−δ˙ is a single-phase mixed conductor within the composition below the solubility limit, and when the composition of Mn exceeds the solubility limit, it becomes the dual-phase mixed conductor of Ce_1−xMn_xO_2−δ˙ and Mn₃O₄. The doing of Mn in gadlia-doped ceria, Ce_1−x−yGd_xMn_yO_2−δ˙ (0.05 ≤ x ≤ 0.2, 0.05 ≤ y ≤ 0.25), was more difficult than that in CeO₂ presumably due to the preferential reaction between Gd and Mn to give GdMnO₃ to the GDC solid solution formation, and the Mn doping seems not to be so effective in preparing the mixed ionic–electronic conductor based on GDC.     

Ionic conductivity and nano-crystallization of CuI–Cu2MoO4 glasses

The correlation between the ionic conductivity and the nano-crystallization processes of (CuI)_x–(Cu₂MoO₄)_1−x superionic conducting glasses has been systematically investigated by X-ray diffraction and high-resolution transmission electron microscopy (HREM). In x = 0.52, CuI crystal precipitates as a primary phase. On the other hand, in x = 0.40, crystallization from homogeneous glass into CuI crystal and other phase has been observed. Nano-crystallization processes depend on CuI content. The difference of the nano-crystallization processes influences the electrical conductivity.     

Influence of Sr substitution on thermoelectric properties of La1−xSrxFeO3 ceramics

Perovskite La_1−xSr_xFeO₃ (0.10 ⩽ x ⩽ 0.20) ceramics have been synthesized by the conventional solid-state reaction technique. Their electrical resistivity, Seebeck coefficient and thermal conductivity have been measured. It has been found that the increase of Sr content reduces significantly both the electrical resistivity and the Seebeck coefficient, but slightly increases the high-temperature thermal conductivity. An adiabatic hopping conduction mechanism of small polaron is suggested from the analysis of the temperature dependence of the electrical resistivity. Seebeck coefficients decrease with increasing temperature, and saturate at temperature above 573 K. The saturated value of Seebeck coefficient decreases with increasing of Sr contents, from 200 μV/K for x = 0.10 to 100 μV/K for x = 0.20. All samples exhibit lower thermal conductivity with values around 2.6 W/m K. The highest dimensionless figure of merit is 0.031 at temperature 973 K in La_0.88Sr_0.12FeO₃.     

Electric properties and structure of Agx(As0.33S0.335Se0.335)100−x bulk glasses

In this paper, results of investigation of the bulk glasses with composition of Ag_x(As_0.33S_0.335Se_0.335)_100−x (x=0–28 at%) are revealed. The amorphous structure of samples was confirmed by the X-ray diffraction analysis. The structure was deduced from the Raman spectra measured for all silver contents in As–S–Se matrix. From the point of their electrical properties, all glasses behave as ionic conductors. Their ac conductivity increases with increasing content of silver. As determined from the comparison of ac and dc conductivities, the contribution of electronic conductivity to the overall conductivity is very low and decreases from about 3% for the glass with 12 at% of Ag to about 1% for the glass with 22 at% of Ag.     

Structure,stability and electrical properties of the La(2−x)SrxMnO4±δ solid solution series

Single phase materials of the La_(2−x)Sr_xMnO_4±δ (0.6 ≤ x ≤ 2.0) solid solution series were prepared via solid state reaction. The structure of each material was examined at room temperature and determined to be tetragonal for all phases examined. An expansion in lattice volume was observed on increasing lanthanum content. The stability and thermal expansion of each member of the solid solution series was determined via the use of in situ high temperature X-ray diffraction. It was found that all materials remained stable up to a temperature of 800 °C. Thermal expansion coefficients were found to be in the region of 15 × 10^− 6 K^− 1 for La_(2−x)Sr_xMnO_4±δ compounds where x > 1.4. The electrical conductivity of each phase was also determined over a similar temperature range with a maximum value of ∼6 Scm^− 1 at 900 °C for the x = 1.8 phase.     

Electrical transport properties of doubly Gd- and Al-doped LaGa0.9Mg0.1O2.95 perovskite compounds

Oxide ion conduction properties of La_1−xGd_xGa_0.8Al_0.1Mg_0.1O_2.95 were discussed in terms of tolerance factor and free volume. The oxide ion conductivities increased with Gd content and attained the maximum at x = 0.10. Enhancement in oxide ion conduction up to x = 0.10 was attributable to the approach to an optimum tolerance factor, which is reportedly around 0.96, whereas degradation above x = 0.10 was considered to result from the reduction of free volume caused by replacement of smaller Gd for La.     

LiMn2−xTixO4 spinel-type compounds (x ≤ 1): Structural,electrical and magnetic properties

LiMn_2−xTi_xO₄ compounds with 0 ≤ x ≤ 1 were prepared by solid state reaction and Pechini technique. Powder X-ray diffraction showed that all samples crystallize with the spinel crystal structure (S.G. Fd3¯m). The cubic unit-cell parameter increases with the Ti content. The influence of the Ti content and cationic distribution on the magnetic properties of the compounds was studied by measuring the temperature and magnetic field dependences of the magnetization: substitution by non-magnetic d⁰ Ti⁴⁺ ions appeared to weaken the magnetic interactions between the manganese ions. The electrical properties of LiMnTiO₄ were studied by AC impedance spectroscopy and DC polarisation measurements, which revealed the electronic character of the conduction process.