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The potential of the perovskite system Nd1–xSrxCoO3−δ(x= 1/3 and 2/3) as cathode material for solid oxide fuel cells (SOFCs) has been investigated via detailed structural, electrical, and electrochemical characterization. The average structure of x= 1/3 is orthorhombic with a complex microstructure consisting of intergrown, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the temperature range 373–1073 K, as observed by neutron powder diffraction. A higher Sr content of x= 2/3 leads to stabilization of the cubic perovskite with a homogeneous microstructure and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation temperature. Both materials are p-type electronic conductors with high total conductivities of 690 and 1675 S·cm–1at 473 K in air for x= 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x= 2/3 loses more oxygen on heating than x= 1/3, associated with a greater loss of p-type charger carriers. However, composite cathodes prepared with Nd1/3Sr2/3CoO3−δand Ce0.8Gd0.2O2−δpresent lower ASR values (0.10 Ω·cm2at 973 K in air) than composites prepared with Nd2/3Sr1/3CoO3−δand Ce0.8Gd0.2O2−δ(0.34 Ω·cm2). The high activity for the oxygen electrochemical reaction at intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very promising SOFC cathode for real devices.

The potential of interactive layers of mixed-conducting oxides for improving the performance of air electrodes of solid oxide cells in the intermediate-temperature range is demonstrated. Active layers of Ce0.9Gd0.1O2−δ(CGO), Ce0.8Pr0.2O2−δ(CPO), and SrFe0.9Mo0.1O3−δ(SFM) with thickness in the range 200–400 nm are deposited on CGO-based electrolyte by spray pyrolysis, followed by deposition of a SFM/CGO composite air electrode by painting. The morphologies and phase composition of the active layers are examined by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy microanalysis. The electrochemical performance of the electrolyte–electrode assemblies is determined by impedance spectroscopy in the range 600–800 °C. Significant improvements in the performance of the electrode process and the geometrically normalized ohmic conductance are observed for the assembly with a CPO active layer with mixed-oxide-ionic–electronic conductivity, especially in the low-temperature range, attributable to extension of the surface path of the electrochemical reactions. The CGO intermediate layer also improves performance but to a lesser degree, most likely due to better ionic-current collection in comparison to the assemblies with either SFM as the active layer or no active layer.

The BaZr0.7Ce0.2Y0.1O3−δ–BaPrO3−δperovskite system, of interest for high-temperature electrochemical applications involving mixed protonic–electronic conductivity, forms a solid-solution with a wide interval of Ba substoichiometry in the range Ba(Ce0.2Zr0.7)1–xPrxY0.1O3−δ, 0 ≤ x≤ 1. Structural phase transitions mapped as a function of temperature and composition by high-resolution neutron powder diffraction and synchrotron X-ray diffraction reveal higher symmetry for lower Pr content and higher temperatures, with the largest stability field observed for rhombohedral symmetry (space group, R3̅c). Rietveld refinement, supported by magnetic-susceptibility measurements, indicates that partitioning of the B-site cations over the A and B perovskite sites compensates Ba substoichiometry in preference to A-site vacancy formation and that multiple cations are distributed over both sites. Electron–hole transport dominates electrical conductivity in both wet and dry oxidizing conditions, with total conductivity reaching a value of ∼0.5 S cm–1for the x= 1 end-member in dry air at 1173 K. Higher electrical conductivity and the displacement of oxygen loss to higher temperatures with increasing Pr content both reflect the role of Pr in promoting hole formation at the expense of oxygen vacancies. In more reducing conditions (N2) and at low Pr contents, conductivity is higher in humidified atmospheres (∼0.023 atm pH2O) indicating a protonic contribution to transport, whereas the greater electron–hole conductivity with increasing Pr content results in lower conductivity in humidified N2due to the creation of protonic defects and the consumption of holes.

Members of the perovskite solid solution BaZr1–xPrxO3−δ(0.2 ≤ x≤ 0.8) with potential high-temperature electrochemical applications were synthesized via mechanical activation and high-temperature annealing at 1250 °C. Structural properties were examined by Rietveld analysis of neutron powder diffraction and Raman spectroscopy at room temperature, indicating rhombohedral symmetry (space group R3̅c) for members x= 0.2 and 0.4 and orthorhombic symmetry (Imma) for x= 0.6 and 0.8. The sequence of phase transitions for the complete solid solution from BaZrO3to BaPrO3is Pm3̅m→ R3̅c→ Imma→ Pnma. The structural data indicate that Pr principally exists as Pr4+on the B site and that oxygen content increases with higher Pr content. Electrical-conductivity measurements in the temperature range of 250–900 °C in dry and humidified (pH2O ≈ 0.03 atm) N2and O2atmospheres revealed an increase of total conductivity by over 2 orders of magnitude in dry conditions from x= 0.2 to x= 0.8 (σ ≈ 0.08 S cm–1at 920 °C in dry O2for x= 0.8). The conductivity for Pr contents x> 0.2 is attributable to positively charged electronic carriers, whereas for x= 0.2 transport in dry conditions is n-type. The change in conduction mechanism with composition is proposed to arise from the compensation regime for minor amounts of BaO loss changing from predominantly partitioning of Pr on the A site to vacancy formation with increasing Pr content. Conductivity is lower in wet conditions for x> 0.2 indicating that the positive defects are, to a large extent, charge compensated by less mobile protonic species. In contrast, the transport mechanism of the Zr-rich composition (x= 0.2), with much lower electronic conductivity, is essentially independent of moisture content.

A full understanding of the structural glass variations and of the nucleation and growth of crystalline phases in glass-ceramics (GCs) induced by compositional changes is highly relevant for applications such as sealants for solid oxide fuel cells, bio- and dental materials, and photonics. Systematic substitution of Ca with Sr in diopside (Di-CaMgSi2O6) has a significant impact on the crystalline-phase assemblage after heat treating at 850 °C for 500 h. A high percentage of Di crystallized from Sr-free glass; however, with increasing Sr content the tendency toward crystallization decreased, and Sr-akermanite (Sr-Ak) was preferentially crystallized. In this study, the structural transformations in the Di-based-GC system upon the substitution of calcium with strontium have been examined by X-ray powder diffraction (XRD), 29Si and 27Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and Raman spectroscopies. XRD Rietveld refinement results indicate that up to a maximum of ∼25 at. % of Sr occupies the Ca site of the Di structure. 27Al MAS NMR spectra reveal that most of the Al exists in amorphous glassy phase after crystallization. Measured mean quadrupolar coupling constant values augment with increasing Sr content, indicating an increase in order in the glassy phase, which is manifested by a strong preference of Mg2+for nonbridging oxygens (NBO), resulting in the formation MgO4tetrahedra. At higher Sr substitution levels (>50%), the decrease in Mg2+coordination from six to four is evident from the decrease in average Mg–O bond lengths and the systematic variation in intensity of characteristic Mg–O Raman vibrational bands.

The types of structures formed when two or more cations occupy the sodium sites in the NaCl unit cell in a non-random manner are reviewed with reference to the cation arrangement in layers of close-packed octahedra and the cation arrangement in the NaCl subcell. Factors influencing the stability of particular structure types, including bond valence and radius ratios, are discussed.

Li3Ni2NbO6has a partially ordered rock salt superstructure in which Nb occupies fully one set of octahedral sites and Li/Ni are distributed nonrandomly over three other sets of octahedral sites (Matheret al.,J. Mater. Chem.5, 1177 (1995)). Over the temperature range 25–1100°C the Li/Ni site occupancy distribution is largely unchanged. Above 1100°C, Nb increasingly occupies the Li/Ni sites until, above 1300°C, the cations are completely disordered. The transition appears to be fully continuous, with no evidence of a transition enthalpy by DTA. Cr3+ions are able to substitute partially into both ordered and disordered structures by a range of mechanisms. The temperature range of the order–disorder transition decreases with Cr content, as shown in the phase diagram of the vacancy solid solutions, Li3−xNi2−xCrxNbO6(0

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