A comparison of the transport properties of La2−xSrxNi1−yFeyO4+δ where 0<x<0.2 and 0<y<0.2

Recently there has been substantial interest in optimising perovskite type ceramics as mixed ionic-electronic conductors (MIECs) for use in ceramic oxygen generators and solid oxide fuel cells. However these materials suffer from thermomechanical deficiencies and therefore there is a need to develop alternative materials. Using the IEDP/SIMS technique the La_2−xSr_xNi_1−yFe_yO_4+δ series of compounds has been investigated and the oxygen tracer diffusion and surface exchange coefficients determined. It has been found that the oxide ion diffusivity of the x and y=0 material is very close to that of the fast oxide ion conducting perovskites such as La_1−xSr_xCoO₃ (LSC) but on acceptor doping with Sr a considerable decrease in the oxygen tracer diffusion coefficient was observed. Further studies on the effects of B site doping, where y>0, indicate that this has very little effect on the transport properties. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Ionic and electronic transport in perovskite-type La(Ga,M)O3−δ (M=Mg, Cr, Fe, Co, Ni, Nb)

Oxygen ion conduction in La_0.9Sr_0.1Ga_1−xM_xO_3−δ (M=Cr, Fe; x=0 – 0.20), LaGa_1−xM_xO_3−δ (M=Co, Ni; x=0.20 – 0.60), LaGa_1−x−yCo_xMg_yO_3−δ (x=0.35 – 0.60; y=0.10 – 0.25) and LaGa_0.85−xMg_0.15(Nb_0.33Mg_0.66)_xO_3−δ (x=0 – 0.20) is reported. At temperatures below 1200 K the ionic conductivity of La(Ga,M)O_3−δ (M=Co, Ni) increases with increasing oxygen nonstoichiometry, but is lower than for La(Ga,Mg)O_3−δ and (La,Sr)GaO_3−δ. Co-doping with Nb and Mg was found to result in decreasing ionic transport in La(Ga,Nb,Mg)O_3−δ due to blocking of oxygen sites by Nb⁵⁺. Small additions of Fe to the B-site of La_0.9Sr_0.1GaO_3−δ increase the ionic conductivity, whereas substitution of Cr for Ga has the opposite effect. Incorporation of transition metal cations into the Ga site leads to a higher p-type electronic conductivity in all studied perovskites. Paper presented at the 6th Euroconference on Solid Sate Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Electronic phase separation in a low-temperature tetragonal phase of lanthanum-strontium cuprates according to 139La NQR data

A nonuniform electron density distribution is observed in La_{1−x− y} Nd_ySr_xCuO₄ and La_1−x−y Eu_ySr_xCuO₄, and long-lived magnetic fluctuations in these compounds are studied. The dynamics of the magnetic fluctuations depends strongly on the magnetic properties of the rare-earth ions, which stabilize the low-temperature tetragonal phase. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 344–349 (10 March 1998)     

Cd x Hg 1−x−y Mn y Se solid solutions are promising materials for filters operating in the near- and middle-IR regions of the spectrum

Transmission spectra of new fourfold semimagnetic semiconductors Cd_xHg_1−x−yMn_ySe (x≤0.6; y≤0.01–0.1) at 77 and 300 K and the effect of annealing of them in vapors of the components on their optical properties were investigated. It is shown that Cd_xHg_1−x−yMn_ySe crystals possess high light transmission (≥60%) and can be used for devices operating in the near- and middle-IR regions of the spectrum. Chernovtsy State University, 2, Kotsyubinskii Str., Chernovtsy, 274012, Ukraina. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 282–286, March–April, 1999.     

Anomalous electrical behavior in ferromagnetic La1−xBaxCoO3−δ materials (0<x≤0.50)

We prepared in this work La_1−xBa_xCoO_3−δ (0<x≤0.50) compounds using the nitrate decomposition method, and focus in the study of the transport properties of the ferromagnetic compounds (x>0.15, T_c≈200 K) in the temperature range 77 K≤T≤300 K. We find that while for x<0.20 these materials show semiconducting behavior, their electrical conduction being dominated by small-polaron holes, for x>0.20 they show metallic behavior for T>100 K, with a transition to semiconducting behavior being observed for T ≤ 100 K. For x=0.20, small changes in the oxygen stoichiometry of the samples - due to slight variations in the thermal treatments - greatly affect the transport properties of the materials that can show either two metal-insulator transitions as the temperature increases or a semiconducting behavior. Very interestingly all these ferromagnetic samples are very sensitive to the polarity of the applied electrical current, and display peculiar “diodic” behaviors. All these observations are explained on the basis of an inhomogeneous electronic structure in the Ba-doped cobalt perovskites. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Investigation of impurity effects in La2?xSrxCu1?yMyO4 (M=Ga,Zn) by electronic specific heat

The electronic specific heat C_el was studied at T≤10K on Ga- and Zn-doped La_2−xSr_xCuO₄ (0.16≤x≤0.22). In pure La_2−xSr_xCuO₄ (0.16≤x≤0.22), the C_el at T<T_c contains only a T² component but no T-linear one, which is peculiar to a clean d-wave superconductor. Partial substitution of Ga or Zn for Cu changes the T²-term of C_el into a quite different one described by the sum of a T-linear and a nearly T³ term. The coefficient of the T-linear term, γ, markedly increases with Zn-or Ga-content. The λ/λ_N vs. T_c/T_co relation for Zn-doped samples with x≥0.2 is in good agreement with the theoretical curve for resonant impurity scattering in a d-wave superconductor, while those for Ga-doped samples and for Zn-doped samples with x<0.2 deviate slightly from the theoretical curve. Such a deviation will be discussed in relation to the change in the magnetic properties of the present system caused by impurity-doping.     

Defect chemistry and electrical properties of La1−xSrxCoO3−δ III. Oxygen nonstoichiometry

This paper concerns verification of the defect chemistry models of La_1−xSr_xCoO_3−δ (LSC), involving the random defect model and the cluster defect model, against empirical data of oxygen nonstoichiometry. It appears that the experimental data may well be explained within the random defect model.     

Defect chemistry and electrical properties of La1−xSrxCoO3−δ IV. Electrical properties

This paper considers electrical properties of La_1−xSr_xCoO_3−δ in terms of defect models, such as random defect model and the cluster model. It is shown that the experimental data of the electrical conductivity may be explained in terms of the random defect model rather than the cluster model.     

Synthesis and conductivities of the apatite-type systems, La9.33+xSi6−yMyO26+z (M=Co, Fe, Mn) and La8Mn2Si6O26

Apatite-type oxides of formula (La/Sr)_10−xSi₆O_26+y have been attracting significant interest recently, because of their high oxide ion conductivity. In this paper we report the synthesis and conductivities of phases based on doping La_9.33Si₆O₂₆ with Co, Fe, Mn on the Si site, according to the formula La_9.33+x/3Si_6−xM_xO₂₆ (M=Co, Fe, Mn). Substitution limits observed were x≤1.5 (Co), x≤1.25 (Fe), x≤0.5 (Mn). Higher Mn levels could be achieved by substituting onto the La site, with it being possible to prepare the phase La₈Mn₂Si₆O₂₆. The highest conductivities were observed for the Co doped samples, although investigations into the dependence of conductivity on p(O₂) (0.2–10⁻⁵ atm.) indicated that the conductivity was dominated by the electronic component in these cases. In contrast, the conductivities for the Fe and Mn doped samples were mainly ionic in the same p(O₂) range. Experiments into varying the oxygen content of these doped phases indicated that increasing the oxygen content above the nominally stoichiometric O₂₆ appears to increase the oxide ion conductivity. Preliminary studies of the reactivity of the electrolyte La_9.33Si₆O₂₆ with potential SOFC cathode materials (La_1−xSr_xMO₃; M=Co, Fe, Mn) suggests that reaction can occur at high temperatures leading to the incorporation of the transition metal into the apatite electrolyte. However, the fact that these doped phases exhibit high conductivities suggests that this may limit any problems caused by such a reaction at the electrolyte-electrode interface. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

The study of lithium fast ion conductors of Li3xLa0.67−xScyTi1−2yNbyO3 system

Perovskite-type lithium fast ion conductors of Li_3xLa_0.67−xSc_yTi_1−2yNb_yO₃ system were prepared by solid state reaction. X-Ray powder diffraction shows that perovskite solid solution form in the ranges of x=0.10, y≤0.10. AC impedance measurements indicate that the bulk conductivities and the total conductivities are of the order of 10⁻⁴ S·cm⁻¹ and 10⁻⁵ S·cm⁻¹ at 25 °C respectively. The compositions have low bulk activation energies of about 17 kJ/mol in the temperature ranges of 298 – 523 K and total activation energies of about 37 kJ/mol in the temperature ranges of 298 – 523 K.     

Possibilities of NOx and CHx determination using galvanic cells with perovskite-electrodes on YSZ

Perovskite-type mixed oxides with the formula La_1−xA_xMe_1−yB_yO_3±δ (x=0.01...0.2; y=0.01...0.5; A=Ca, Sr; Me=Cr, Mn, Fe, Co; B=Mg, Ga) were investigated with the aim to use the oxides as electrode materials for galvanic cells with Y₂O₃-stabilized ZrO₂ solid electrolytes (YSZ). The catalytic activity of the oxides for the oxidation, reduction and decomposition of gas components was varied by changing the transition metal and by partial substitution of lanthanum and the metal. The behaviour of different substituted lanthanum chromite/YSZ and manganite/YSZ electrodes in the presence of nitric oxides (NO_x) and combustible components (CH_x) in gases containing oxygen was investigated. Various electrode combinations were tested in gas-symmetrical cells. For the NO determination cells with a catalytically active Pt electrode and a La_0.8Sr_0.2MnO₃ electrode with a low catalytic activity for the decomposition of NO were used. The results show the possible quantitative measurement at low and constant p(O₂) on the basis of a calibration. For the determination of C₃H₆ a CH_x-sensitive Au and an O₂-sensitive La_0.99Sr_0.01CrO₃ electrode were combined in a potentiometric thick film sensor. In gases containing oxygen with small amounts of C₃H₆ the sensor provided a utilizable voltage in dependence of the C₃H₆ concentration. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Effect of the spin and valence states of cobalt ions on the kinetic properties of Ho1 ? xSrxCoO3 ? δ and Er1 ? xSrxCoO3 ? δ compounds

The electrical conductivity and Seebeck effect in ceramics based on cobaltites Ho_{1 − x} Sr _x CoO_{3 − δ} (x = 0.65, 0.75, 0.85, 0.95) and Er_{1 − x} Sr _x CoO_{3 − δ} (x = 0.75, 0.85, 0.95) with a perovskite-like structure have been investigated in the temperature range T > 77 K. All the compounds under study are characterized by the variable-range-hopping conductivity with the temperature dependence of the electrical resistivity corresponding to the Mott law. It has been found that, in the Ho_0.35Sr_0.65CoO_{3 − δ} compound, thermally excited Co³⁺ ions contribute to the electrical conductivity with an increase in temperature to 250 K. The Seebeck coefficient of the systems studied decreases as the strontium concentration and temperature increase. It has been shown that, for an adequate explanation of this behavior, proper allowance must be made for the splitting of the 3d levels, as well as for the charge disproportionation of the cobalt ions.     

Defect chemistry and electrical properties of La1-xSrxCoO3-δ

This paper considers defect chemistry models of La_1−xSr_xCoO_3−δ, such as random defect model and the cluster defect model. These models are considered in terms of defect equilibria with respect to (i) the formation of oxygen vacancies, (ii) intrinsic electronic ionisation and (iii) the formation of defect clusters. The defect models are derived using the available data of nonstoichiometry for the LSC materials.     

Defect chemistry and electrical properties of La1-xSrxCoO3-δ

This paper reports defect diagrams of Sr-doped lanthanum cobaltate, La_1−xSr_xCoO_3−δ (LSC), involving both random defect model and the cluster defect model in the temperature range between 873 K and 1173 K. The diagrams are derived in the form of defects concentrations as a function of oxygen partial pressure using the nonstoichiometry data reported in the literature. The effect of the Sr content on defect structure of LSC is discussed in terms of both defect models.     

Synthesis and electrical characterisation of the perovskite niobate-titanates,Sr1−x/2Ti1−xNbxO3−δ

The synthesis and electrical characterisation over a range of oxygen partial pressures (10⁻²⁰ to 1 atm) are reported for the cubic perovskite niobate-titanates Sr_1−x/2Ti_1−xNb_xO_3−δ, which are proposed as potential anode materials for solid oxide fuel cells. Single phase samples were observed for 0≤x≤0.4, and phase purity was retained on annealing at both high and low oxygen partial pressures. Good electrical conductivity was observed on reduction in low oxygen partial pressures, with a maximum for the sample with 25% Nb (x=0.25), σ=5.6 Scm⁻¹ at 930°C (P (O₂)=10⁻¹⁸ atm). For dense samples the higher the Nb content the more resistant the reduced sample was to reoxidation as the oxygen partial pressure was increased. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Size of the interpolated cation and hole carrier density: two key parameters for the optimisation of colossal magnetoresistive properties of Pr-based manganites

The exploration of the magnetic and transport properties of four series of manganese perovskites, Pr_0.7Ca_0.34−xA_xMnO_3−δ (A=Sr, Ba), Pr_0.7−xLa_xCa_0.3 MnO_3−δ and Pr_0.66Ca_0.34−x Sr_xMnO_3−δ has allowed four phases with colossal magnetoresistive (CMR) properties to be isolated: Pr_0.7Ca_0.25Sr_0.025MnO_3−δ and Pr_0.66Ca_0.26Sr_0.08MnO_3−δ that exhibit a variation of resistance of 2.5. 10⁷% and 10⁹% at μ₀ H=5 T for T=88 K and 50 K respectively, Pr_0.58La_0.12Ca_0.3 MnO_3−δ that exhibits a variation of 6.10⁶% for μ₀ H=5 T at T=80 K and Pr_0.7Ba_0.025Ca_0.275MnO_3−δ for which a resistance variation of 5.10⁹%, at T=50 K, for μ₀ H=5 T is evidenced. for each compound of this series except the barium phase, one observes that the temperature T_max, which corresponds to the resistance maximum on the R(T) curves in zero magnetic field, increases dramatically as the mean size of the interpolated cations increases, and that the CMR effect correlatively decreases dramatically. The comparison of the two series Pr_0.7Ca_0.3−xSr_xMnO_3−δ and Pr_0.66Ca_0.34−xSr_xMnO_3−δ shows also the crucial role of the hole carrier density: for a same mean ionic radius of the interpolated cation T_max is decreased of about 50 K by introducing 0.034 hole per Mn mole.     

Relation between highT c superconductivity and structure: A study of Ba3+x Cu6O14+δ and YBa2Cu3O7−δ systems

Superconductivity is found in tetragonal La_3−x Ba_3+x Cu₆O_14+δ and La, Ba)_6−x Sr _x Cu₆O_14+δ even though they do not possess Cu-O chains or the K₂NiF₄ structure. Resistivity measurements confirm the occurrence of a transformation from chain-superconductivity to sheet-superconductivity in YBa₂Cu₃O_7−δ as δ is varied in the range 0.0–0.5. Contribution No. 481 from the Solid State and Structural Chemistry Unit     

Influence of thermal treatment in oxidizing and reducing atmospheres on the composition and properties of solid solutions based on bismuth vanadate: (Bi,La)4(V,Me)2O11?y (Me = Ga, Zr, Cu)

Ceramic solid solutions (Bi_{1 − y} La _y )₄(V_{1 − x} Me _x )₂O_{11 − z} (x, y < 0.2: Me = Zr, Ga, Fe, Cu) were prepared by solid-state reaction. It was shown that the annealing (973 K, reducing atmosphere H₂/Ar (20/80)) of the samples whose compositions belong to the stability domains of α, β, and γ′ polymorphs increases their electronic conductivity by six orders of magnitude. The samples with low concentrations of dopant cations exhibited good compositional stability and a reversible change in their structure parameters. At the same time, the solid solutions with a high concentration of lanthanum cations and/or copper cations (y, x ∼ 0.1) underwent partial decomposition.     

Oxygen thermodynamics and ion conductivity in the solid solution La1−xSrxCoO3−δ at large strontium content

The equilibrium oxygen content was measured in the model system and important oxygen permeable material La_1−xSr_xCoO_3−δ, where x=0.6, in the temperature range 650–900 °C and oxygen partial pressure range between 10⁻⁵ and 1 atm. The data were utilized to obtain changes in the partial entropy and enthalpy of oxygen in the solid as a function of the oxygen content. It is shown that the initially cubic perovskite undergoes to a phase transition to a tetragonal structure at δ >0.3. The oxygen permeation of L_0.4Sr_0.6CoO_3−δ at 700–900 °C is found to be controlled by bulk solid state processes. The activation energy equals about 0.8 eV at high oxygen pressure and small oxygen nonstoichiometry. Increasing oxygen deficiency results in a rapid increase in the activation energy. In combination with thermodynamic data, these changes can be explained as resulting from the intrinsic spatial inhomogeneouty in oxygen vacancy distribution which varies both with temperature and oxygen nonstoichiometry. It is shown that, when the oxygen deficiency increases at constant temperature, the oxygen vacancies form locally ordered microdomains (clusters), which eventually results in a transition of the cubic perovskite structure to the tetragonal structure. The oxygen ion conductivity depends strongly on the development of the ordering. Paper presented at the 6th Euroconference on Solid state Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Structural aspects and ionic conductivity in perovskite-like doped niobates and titanates

Aliovalent substitution in the cation sublattices of perovskite-like niobates of the type (Pb_1−xLa_2/3x−yLi_3x)Nb₂O₆ were studied. Their crystallographic peculiarities and electrophysical properties were investigated. It has been found that at x=1 lanthanum-lithium niobates with the structure of the defect La_2/3−yLi_3yNb₂O₃ perovskite are formed, which possess a high lithium-ion conductivity. The symmetry of the perovskite crystal lattice is shown to become higher and to change from rhombic to tetragonal and cubic with increasing lithium content. The La_2/3-xLiTiO₃ compounds have a perovskite structure and a high lithium ion conductivity. The Lines model for superionic conductor gives a satisfactory fit to the frequency-temperature nature of the electrophysical properties.