Synthesis and crystallographic studies of garnet-related lithium-ion conductors Li6CaLa2Ta2O12 and Li6BaLa2Ta2O12

High-purity specimens of Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂ have been successfully synthesized by solid-state reactions. The analytical chemical compositions of these samples were in good agreement with the nominal compositions of Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂. The Rietveld refinements verified that these compounds have the garnet-type framework structure with the lattice constants of a = 12.725(2) Å for Li₆CaLa₂Ta₂O₁₂ and a = 13.001(4) Å for Li₆BaLa₂Ta₂O₁₂. All of the diffraction peaks of X-ray powder diffraction patterns were well indexed on the basis of cubic symmetry with space group Ia-3d. To make a search for Li sites, the electron density distributions were precisely examined by using the maximum entropy method. Li⁺ ions occupy partially two types of crystallographic site in these compounds: (i) tetrahedral 24d sites, and (ii) distorted octahedral 96h sites, the latter of which are the vacant sites of the ideal garnet-type structure. The present Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂ samples exhibit the conductivity σ = 2.2 × 10^? 6 S cm^? 1 at 27 °C (E_a = 0.50 eV) and σ = 1.3 × 10^? 5 S cm^? 1 at 25 °C (E_a = 0.44 eV), respectively.     

Chemical diffusion of water in the double perovskites Ba4Ca2Nb2O11 and Sr6Ta2O11

The mechanism and kinetics of water incorporation in the double perovskites Ва₄Ca₂Nb₂O₁₁ and Sr₆Ta₂O₁₁ has been investigated (T = 300÷500 °C and aH₂O = 1 · 10^− 3÷2.2 · 10^− 2). The formation of hydration products Ba₄Ca₂Nb₂O₁₁·xH₂O and Sr₆Ta₂O₁₁·xH₂O (0.2 < x < 0.50) was limited by the diffusion of H₂O. It has been found that the concentration dependences of D˜_H2O are the same for both samples: small increasing of D˜_H2O with increasing x. The temperature dependences of the chemical diffusion coefficients of water for compositions of Ba₄Ca₂Nb₂O₁₁·0.35H₂O and Sr₆Ta₂O₁₁·0.35H₂O could be described with close activation energies of E_a = 0.38 ± 0.03 eV and E_a = 0.49 ± 0.03 eV, respectively. The chemical diffusion coefficients of water are nearly one order of magnitude smaller for tantalate Sr₆Ta₂O₁₁. This result correlates with lower oxygen and proton conductivities in Sr₆Ta₂O₁₁ as the consequence of lower mobilities.     

Soft chemistry synthesis and characterizations of fully oxidized and reduced NdBaCo2O5+δ phases δ = 0, 1

The double ordered perovskites NdBaCo₂O₅ and NdBaCo₂O₆ were prepared by soft chemistry. The samples were characterized from a structural and chemical point of view, concomitantly with their physical properties. Upon reduction, NdBaCo₂O₅ is formed with a tetragonal unit cell (a = b = 3.94 Å, c = 7.57 Å) and presents an antiferromagnetic behavior. Upon oxidation, a complete stoichiometric ordered phase NdBaCo₂O₆ with a tetragonal unit cell (a = b = 3.88 Å, c = 7.63 Å) could be obtained with a ferromagnetic and a metallic behavior. Finally it is shown that these phases are able to reversibly catch and release oxygen, suggesting a high anionic conductivity.     

Modeling and preparation of practical optical filters

Multilayer bandpass and bandstop filters have been produced using electron beam evaporation. Initially bandstop filter is modeled with non absorbing zinc sulphide (ZnS) and zinc selenide (ZnSe). When the absorption data was incorporated for the said materials significant absorption was observed at shorter wavelengths of the spectral band restricting the practical usage of the filter. ZnS and ZnSe were then replaced by dispersive silicon dioxide (SiO₂), tantalum penta oxide (Ta₂O₅) and titanium dioxide (TiO₂) along with their absorption and the filters are optimized to get desired bandpass and bandstop data. Bandpass and bandstop filters with desired performance were experimentally characterized with two combinations SiO₂/Ta₂O₅/glass and SiO₂/TiO₂/glass. The measured average transmission for both combination (bandpass) in the pass band was about 92% with T < 1% in the stop band. Slightly narrow bandwidth was observed for SiO₂/TiO₂/glass as compared to SiO₂/Ta₂O₅/glass which is attributed to layers densification. Similarly T_avg ⩾ 80% was achieved for two combinations of bandstop filters with T < 0.1% in the stop band. The structure and surface morphology of the prepared filters were characterized by X-ray diffraction and scanning electron microscopy. XRD analysis reveals amorphous structure. SEM analysis also reveals that the layers are compact and have good surface quality.     

Microstructure and varistor properties of ZnO–V2O5–MnO2 ceramics with Ta2O5 addition

The effect of Ta₂O₅ addition on microstructure, electrical properties, and dielectric characteristics of the quaternary ZnO–V₂O₅–MnO₂ vaistor ceramics was investigated. Analysis of the microstructure indicated that the quaternary ZnO–V₂O₅–MnO₂–Ta₂O₅ ceramics consisted of mainly ZnO grain and minor secondary phases such as Zn₃(VO₄)₂, ZnV₂O₄, TaVO₅, and Ta₂O₅. As the amount of Ta₂O₅ increased, the sintered density increased from 94.8 to 97.2% of the theoretical density (5.78 g/cm³ for ZnO), whereas the average grain size decreased from 7.7 to 6.0 μm. The ceramics added with 0.05 mol% Ta₂O₅ exhibited the highest breakdown field (2715 V/cm) and the highest nonlinear coefficient (20). However, further increase caused α to abruptly decrease. The Ta₂O₅ acted as a donor due to the increase of electron concentration in accordance with the amount of Ta₂O₅. The donor concentration increased from 1.97×10¹⁸ to 3.04×10¹⁸cm^?3 with increasing the amount of Ta₂O₅ and the barrier height exhibited the maximum value (0.95 eV) at 0.05 mol% Ta₂O₅.     

Thermal stabilities of metal bottom electrodes for Ta2O5 metal-oxide-metal capacitor structure

Thermal stabilities of various metal bottom electrodes were examined by using a Ta₂O₅ metal-oxide-metal (MOM) capacitor structure. After depositing 10-nm thick Ta₂O₅ on metal-electrode/poly-Si, we performed rapid thermal oxidation (RTO) at 850 °C for 60 s in an O₂ ambient. A chemical-vapor-deposition (CVD) WSi₂ electrode showed satisfactory thermal stability after the RTO, while other examined electrode materials exhibited thermal degradation caused by oxidation failure or interfacial reaction between the substrate poly-Si and the Ta₂O₅. After post-annealing at 650 °C for 30 min (in N₂ condition) with CVD TiN top electrode, an effective oxide thickness (T_ox) of ∼32 Å and a leakage current density of ∼10⁷ A/cm² at 1.25 V were obtained from the MOM capacitor with the WSi₂ bottom electrode. Other electrode materials, such as TiN, TiSi_x, WN_x, W, and Ta, were severely oxidized during the RTO in the MOM structures, and very poor capacitor properties were obtained in terms of T_ox and leakage current.     

Interfacial reactions of Ba2YCu3O6+z with coated conductor buffer layer,LaMnO3

Chemical interactions between the Ba₂YCu₃O_6+x superconductor and the LaMnO₃ buffer layers employed in coated conductors have been investigated experimentally by determining the phases formed in the Ba₂YCu₃O_6+x–LaMnO₃ system. The Ba₂YCu₃O_6+x–LaMnO₃ join within the BaO–(Y₂O₃–La₂O₃)–MnO₂–CuO_x multi-component system is non-binary. At 810 °C (p_O2 = 100 Pa) and at 950 °C in purified air, four phases are consistently present along the join, namely, Ba_2?x(La_1+x?yY_y)Cu₃O_6+z, Ba(Y_2?xLa_x)CuO₅, (La_1?xY_x)MnO₃, (La,Y)Mn₂O₅. The crystal chemistry and crystallography of Ba(Y_2?xLa_x)CuO₅ and (La_1?xY_x)Mn₂O₅ were studied using the X-ray Rietveld refinement technique. The Y-rich and La-rich solid solution limits for Ba(Y_2?xLa_x)CuO₅ are Ba(Y_1.8La_0.2)CuO₅ and Ba(Y_0.1La_1.9)CuO₅, respectively. The structure of Ba(Y_1.8La_0.2)CuO₅ is Pnma (No. 62), a = 12.2161(5) Å, b = 5.6690(2) Å, c = 7.1468(3) Å, V = 494.94(4) Å³, and D_x = 6.29 g cm^?3. YMn₂O₅ and LaMn₂O₅ do not form solid solution at 810 °C (p_O2 = 100 Pa) or at 950 °C (in air). The structure of YMn₂O₅ was confirmed to be Pbam (No. 55), a = 7.27832(14) Å, b = 8.46707(14) Å, c = 5.66495(10) Å, and V = 349.108(14) Å³. A reference X-ray pattern was prepared for YMn₂O₅.     

Lithium-ionic diffusion and electrical conduction in the Li7La3Ta2O13 compounds

The electrical properties and the mechanism of lithium ionic diffusion in the Li₇La₃Ta₂O₁₃ compounds were investigated. The bulk and total conductivity at 300 K of the Li₇La₃Ta₂O₁₃ compound are about 3.3 × 10^? 6 S/cm and 2.6 × 10^? 6 S/cm, respectively. The activation energy of bulk and total conductivity is in the range of 0.38–0.4 eV. A prominent internal friction peak in Li₇La₃Ta₂O₁₃ compounds was observed around 280 K at 0.5 Hz, which is actually composed of two subpeaks (P₁ peak at lower temperature and P₂ peak at higher temperature). From the shift of peak position with frequency, the activation energy of 1.0 eV and the pre-exponential factor of relaxation time in the order of 10^? 18–10^? 21 s were obtained if one assumes Debye relaxation processes. These values of relaxation parameters strongly suggest the existence of interaction between the relaxation species (here lithium ions or vacancies). Based on the coupling model, the relaxation activation energies are deduced as 0.45 eV and the pre-exponential factor of relaxation time as 10^? 15 s. Judging from these relaxation parameters and the similarity of structure between Li₇La₃Ta₂O₁₃ and Li₅La₃Ta₂O₁₂ compounds, the P₁ and P₂ peaks are suggested to be related with the lithium ionic diffusion between 48g?48g and 24d?48g.     

A single-crystal study of the electrochemically Li-ion intercalated spinel-type Li4Ti5O12

Single crystals of Li_4 + xTi₅O₁₂ were prepared by means of electrochemical Li-ion intercalation technique using parent Li₄Ti₅O₁₂ single crystals. The obtained Li_4 + xTi₅O₁₂ (x = 1.35) crystallizes in the cubic spinel-related type structure, space group Fd3?m, and lattice parameters of a = 8.346(2) Å and V = 581.3(5) Å³ and Z = 8. The Li-ion intercalated sites were successfully determined to be both the 8a and 16c sites by using the difference Fourier synthesis map. The structure was determined by single-crystal X-ray structure analysis and refined to the conventional value of R = 3.7% for 132 independent observed reflections. The chemical composition has been determined to be Li_5.35Ti₅O₁₂ from the result of site-population refinements. In addition, theoretical electron density distributions and total energy were calculated for three postulated compounds of “Li_4.5Ti_4.5O₁₂” and “Li_4.5 + xTi_4.5O₁₂” with x = 1.5 and 3.0.     

Ionic conductivities of Nasicon-type glass-ceramic superionic conductors in the system Na2O–Y2O3–XO2–SiO2 (X = Ti,Ge, Te)

Glass-ceramics of the titanium-, germanium- or tellurium-containing Na₅RSi₄O₁₂-type (R = rare earth; Y) Na⁺-superionic conductors (N5YXS) were prepared by crystallization of glasses with the composition Na_3+3xY_1 ? xX_ySi_3 ? yO₉ (X = Ti; NYTiS, Ge; NYGeS, X = Te; NYTeS), and the effects of X elements on the separation of the phase and the microstructural effects on the conduction properties of glass-ceramics were discussed. The combination of x and y was most varied in N5YGeS and more limited in the order of N5YTeS > N5YTiS. Their conductivities and activation energies are of the order of 10^? 2 S/cm at 300 °C and of 15 to 24 kJ/mol, respectively. The conductivity of the glass-ceramic N5YXS decreases giving the order N5YGeS > N5YTeS > N5YTiS. It is considered that this order corresponds to the N5 single phase region. Large enhancement of electrical conductivity was observed in the glass-ceramics as the grain growth was promoted with increase of heating temperature and heating time for crystallization.     

Influence of the orthorhombic distortion on the van Hove singularities in the DOS and ARPES spectra of layered cuprates

Four atom states Cu3d_{x² −  y²}, Cu4s, O_a2p_xare involved in a tight-binding model for the superconducting CuO₂plane. The orthorhombic distortion is taken into account by the differences of Cu–O hopping amplitudes and single-site oxygen energies ε_aand ε_bof two oxygen positions in the elementary cell as well. An effective ‘oxygen’ Hamiltonian including only the electron amplitudes at the oxygen ions is derived. Simple expressions for the constant energy contours and the Fermi surface are obtained and they qualitatively describe the photoemission spectra. Extended saddle points nearp = (π,0) andp = (0,π) are observed in qualitative agreement with the ARPES data. The van Hove singularities of the density of states (DOS) related to the extended saddle points are calculated by a Monte Carlo method. It is found that the splitting of the singularity of the DOS at the bottom of the conduction band is created by the energy difference ε_a −  ε_b = 2δ.     

Submillimeter-wave measurements of N2 and O2 pressure broadening for HO2 radical generated by Hg-photosensitized reaction

The N₂ and O₂ pressure broadening coefficients of the pure rotational transitions at 625.66 GHz (N_KaKc=10_1?9–10_0?10, J=10.5–10.5) and 649.70 GHz (N_KaKc=10_2?9–9_2?8, J=9.5–8.5) in the vibronic ground state X²A′ of the perhydroxyl (HO₂) radical have been determined by precise laboratory measurements. For the production of HO₂, the mercury-photosensitized reaction of the H₂ and O₂ precursors was used to provide an optimum condition for measurement of the pressure broadening coefficient. The Superconducting Submillimeter-wave Limb Emission Sounder (SMILES) was designed to monitor the volume mixing ratio of trace gases including HO₂ in the Earth's upper atmosphere using these transitions. The precise measurement of pressure broadening coefficient γ in terms of the half width at half maximum is required in order to retrieve the atmospheric volume mixing ratio. In this work, γ coefficients of the 625.66 GHz transition were determined for N₂ and O₂ at room temperature as γ(N₂)=4.085±0.049 MHz/Torr and γ(O₂)=2.578±0.047 MHz/Torr with 3σ uncertainty. Similarly, the coefficients of the 649.70 GHz transition were determined as γ(N₂)=3.489±0.094 MHz/Torr and γ(O₂)=2.615±0.099 MHz/Torr. The air broadening coefficients for the 625.66 GHz and 649.70 GHz lines were estimated at γ(air)=3.769±0.067 MHz and 3.298±0.099 MHz respectively, where the uncertainty includes possible systematic errors. The newly determined coefficients are compared with previous results and we discuss the advantage of the mercury-photosensitized reaction for HO₂ generation. In comparison with those of other singlet molecules, the pressure broadening coefficients of the HO₂ radical are not much affected by the existence of an unpaired electron.     

Photoionization studies of the atmospherically important species N and OH at the Elettra synchrotron radiation source

In this paper angularly resolved photoelectron spectra (PES) and constant-ionic-state (CIS) spectra are presented for the atmospherically important species N and OH.The natural width Γ, line shape parameters q and ρ² and discrete oscillator strengths f have been measured for the members of the N*[2s2p³(⁵S), np] (⁴P) ← N(⁴S) autoionizing resonances for n = 5–10. The n = 5 parameters calculated in this work are in good agreement with the values obtained previously whereas for the resonances with n = 6–10 the values of these parameters are reported for the first time. The asymmetry parameter (β) for the first band of N atoms, the N⁺(³P) ← N(⁴S) ionization, has also been measured in the photon energy range of the above autoionizing resonances.For OH, CIS spectra have been recorded for the first photoelectron band corresponding to the ionization OH⁺(X³Σ⁻, v⁺ = 0) ← OH(X²Π, v″ = 0). In these spectra, rotationally partially resolved bands associated with OH*(a¹Δ3d, v′ = 0) ← OH(X²Π, v″ = 0) resonances have been observed. Suggestions for their assignment are made on the basis of their positions and band simulations which use rotational line strength calculations.     

On the rate constants of OH + HO2 and HO2 + HO2: A comprehensive study of H2O2 thermal decomposition using multi-species laser absorption

Hydrogen peroxide (H₂O₂) and hydroperoxy (HO₂) reactions present in the H₂O₂ thermal decomposition system are important in combustion kinetics. H₂O₂ thermal decomposition has been studied behind reflected shock waves using H₂O and OH diagnostics in previous studies (Hong et al. (2009) [9] and Hong et al. (2010) [6,8]) to determine the rate constants of two major reactions: H₂O₂ + M → 2OH + M (k₁) and OH + H₂O₂ → H₂O + HO₂ (k₂). With the addition of a third diagnostic for HO₂ at 227 nm, the H₂O₂ thermal decomposition system can be comprehensively characterized for the first time. Specifically, the rate constants of two remaining major reactions in the system, OH + HO₂ → H₂O + O₂ (k₃) and HO₂ + HO₂ → H₂O₂ + O₂ (k₄) can be determined with high-fidelity.No strong temperature dependency was found between 1072 and 1283 K for the rate constant of OH + HO₂ → H₂O + O₂, which can be expressed by the combination of two Arrhenius forms: k₃ = 7.0 × 10¹² exp(550/T) + 4.5 × 10¹⁴ exp(?5500/T) [cm³ mol^?1 s^?1]. The rate constants of reaction HO₂ + HO₂ → H₂O₂ + O₂ determined agree very well with those reported by Kappel et al. (2002) [5]; the recommendation therefore remains unchanged: k₄ = 1.0 × 10¹⁴ exp(?5556/T) + 1.9 × 10¹¹⁺exp(709/T) [cm³ mol^?1 s^?1]. All the tests were performed near 1.7 atm.     

Synthesis and electrical transport properties of Lu2+xTi2−xO7−x/2 oxide-ion conductors

The Lu_2+xTi_2−xO_7−x/2 (x = 0; 0.052; 0.096; 0.286; 0.44; 0.63; 33.3–49 mol% Lu₂O₃) nanoceramics with partly disordered pyrochlore-type structure are prepared by sintering freeze-dried powders obtained by a co-precipitation technique with 1600 °C annealing. Similar to pyrochlore-like compositions in the zirconate system, some of the new titanates are good oxide-ion conductors in air. The new solid-state electrolytes have oxide-ion conductivity in the interval of 1.0 × 10^− 3 – 2.5 × 10^− 3 S/cm at 740 °C in air. This value of conductivity is comparable with that of ZrO₂/Y₂O₃ ceramics. The conductivity of Lu_2+xTi_2−xO_7−x/2 depends on the chemical composition. The highest ionic conductivity is exhibited by nearly stoichiometric Lu_2+xTi_2−xO_7−x/2 (x = 0.096; 35.5 mol% Lu₂O₃) material containing ∼ 4.8 at.% Lu_Ti anti-site defects.     

Enhanced performance of bi-layer Nb2O5 coated TiO2 nanoparticles/nanowires composite photoanode in dye-sensitized solar cells

Dye sensitized solar cells (DSSCs) were fabricated based on coumarin NKX-2700 dye sensitized bi-layer photoanode and quasi-solid state electrolyte sandwiched together with cobalt sulfide coated counter electrode. A novel bi-layer photoanode has been prepared using composite mixtures of 90 wt.% TiO₂ nanoparticles + 10 wt.% TiO₂ nanowires (TNPWs) as active layer and Nb₂O₅ is coated on the active layer, which acts as scattering layer. Hafnium oxide (HfO₂) was applied over the TNPWs/Nb₂O₅ photoanode film, as a blocking layer. TiO₂ nanoparticles (TNPs), TiO₂ nanowires (TNWs) and TNPWs/Nb₂O₅ were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The sensitizing organic dye coumarin NKX-2700 displayed maximum absorption wavelength (λ_max) at 525 nm, which could be observed from the UV–vis spectrum. DSSC-1 fabricated with composite bi-layer photoanode revealed enhanced photo-current efficiency (PCE) as compared to other DSSCs and illustrated photovoltaic parameters; short-circuit current J_SC = 18 mA/cm², open circuit voltage (V_OC) = 700 mV, fill factor (FF) = 64% and PCE (η) = 8.06%. The electron transport and charge recombination behaviors of DSSCs were investigated by electrochemical impedance spectra (EIS) and the results illustrated that the DSSC-1 showed the lowest charge transport resistance (R_tr) and the longest electron lifetime (τ_eff). Therefore, in the present investigation, it could be concluded that the novel bi-layer photoanode with blocking layer increased the short circuit current, electron transport and suppressed the recombination of charge carriers at the photoanode/dye/electrolyte interface in DSSC-1.     

Crystal growth and structural properties of the spinel-type Li1 + xMn2 − xO4 (x = 0.10, 0.14)

Single crystals of the lithium-rich lithium manganese oxide spinels Li_1 + xMn_2 − xO₄ with x = 0.10 and 0.14 have been successfully synthesized in high-temperature molten chlorides at 1023 K. The single-crystal X-ray diffraction study confirmed the cubic Fd3¯m space group and the lattice parameters of a = 8.2401(9) Å for x = 0.10 and a = 8.2273(10) Å for x = 0.14 at 300 K, respectively. The crystal structures have been refined to the conventional values R = 3.7% for x = 0.10 and R = 3.1% for x = 0.14, respectively. Low-temperature single-crystal X-ray diffraction experiments revealed that these single crystal samples showed no phase transition between 100 and 300 K. The electron-density distribution images in these compounds by the single-crystal MEM analysis clearly showed strong covalent bonding features between the Mn and O atoms due to the Mn–3d and O–2p interaction.     

Effect of composition change of metals in transition metal sites on electrochemical behavior of layered Li[Co1 −2x(Li1 / 3Mn2 / 3)x(Ni1 / 2Mn1 / 2)x]O2 solid solutions

Five compositions of Li[Co_1 −2x(Li_1 / 3Mn_2 / 3)_x(Ni_1 / 2Mn_1 / 2)_x]O₂ solid solutions ( x = 0.1, 0.2, 0.3, 0.4, and 0.5) were synthesized using a sol–gel method with three end members of LiCoO₂, Li₂MnO₃(Li[Li_1 / 3Mn_2 / 3]O₂), and Li[Ni_0.5Mn_0.5]O₂. The compositions of metals in transition metal sites were changed to see the effect of them on electrochemical behavior of the solid solutions. All the samples were nano-sized semi-spherical shaped particles with a layered structure. The reduction of cobalt content (the increase of other metals) in the sites increases the lattice parameters, a and c, resulting in the shift of Raman and XRD peak positions. The discharge capacity fading turned serious at higher Co contents, but it was significantly diminished with the decrease of Co content. At lower Co contents, the capacity increased with cycle numbers. The most stable voltage profile was obtained from the composition of Li[Li_1 / 15Co_3 / 5Ni_1 / 10Mn_7 / 30]O₂ (x = 0.2).     

A comparative study of the Ruddlesden-Popper series,Lan+1NinO3n+1 (n = 1, 2 and 3), for solid-oxide fuel-cell cathode applications

A comparative investigation of the much-studied La₂NiO_4+δ (n = 1) phase and the higher-order Ruddlesden-Popper phases, La_n+1Ni_nO_3n+1 (n = 2 and 3), has been undertaken to determine their suitability as cathodes for intermediate-temperature solid-oxide fuel cells. As n is increased, a structural phase transition is observed from tetragonal I4/mmm in the hyperstoichiometric La₂NiO_4.15 (n = 1) to orthorhombic Fmmm in the oxygen-deficient phases, La₃Ni₂O_6.95 (n = 2) and La₄Ni₃O_9.78 (n = 3). High temperature d.c. electrical conductivity measurements reveal a dramatic increase in overall values from n = 1, 2 to 3 with metallic behavior observed for La₄Ni₃O_9.78. Impedance spectroscopy measurements on symmetrical cells with La_0.9Sr_0.10Ga_0.80Mg_0.20O_3−δ (LSGM-9182) as the electrolyte show a systematic improvement in the electrode performance from La₂NiO_4.15 to La₄Ni₃O_9.78 with ∼ 1 Ω cm² observed at 1073 K for the latter. Long-term thermal stability tests show no impurity formation when La₃Ni₂O_6.95 and La₄Ni₃O_9.78 are heated at 1123 K for 2 weeks in air, in contrast to previously reported data for La₂NiO_4.15. The relative thermal expansion coefficients of La₃Ni₂O_6.95 and La₄Ni₃O_9.78 were found to be similar at ∼ 13.2 × 10^− 6 K^− 1 from 348 K to 1173 K in air compared to 13.8 × 10^− 6 K^− 1 for La₂NiO_4.15. Taken together, these observations suggest favourable use for the n = 2 and 3 phases as cathodes in intermediate-temperature solid-oxide fuel cells when compared to the much-studied La₂NiO_4+δ (n = 1) phase.     

Electrical conductivity and thermal expansion of the oxy-cuspidine Gd4Al2O9 substituted with Ca and Sr

The series of Gd_4 ? xM_xAl₂O_9 ? x/2 (M = Ca, Sr) with x = 0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd_4 ? xCa_xAl₂O_9 ? x/2 and Gd_4 ? xSr_xAl₂O_9 ? x/2 show the monoclinic cuspidine structure with space group of P2₁/c up to 0.05–0.1 and 0.01–0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd₄Al₂O₉, GdAlO₃ and SrGd₂Al₂O₇ appear as additional phases. The highest electrical conductivity obtained at 900 °C yielded σ = 1.49 × 10^? 4 S/cm for Gd_3.95Ca_0.05Al₂O_8.98. In comparison, the conductivity of pure Gd₄Al₂O₉ was σ = 1.73 × 10^? 5 S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd₄Al₂O₉ at 1000 °C was 7.4 × 10^? 6 K^? 1. The phase transition between 1100 and 1200 °C reported earlier changes with increasing substitution of Ca and Sr.