Structural and Electrical Characterization of LaSrAl1-xMgxO4-δ Layered Perovskites Obtained by Mechanical Synthesis.

This work presents the structural and electrical characterization of K2NiF4-type layered perovskites of LaSrAl1-xMgxO4-δ composition to be used as oxide-ion electrolytes for a solid-oxide fuel cell (SOFC). These perovskites were prepared by mechano-chemical synthesis (ball milling), an alternative to traditional synthesis methods such as citrate-nitrates and solid-state reaction. With these methods, two things are avoided: first, the use of nitrate salts, which are more environmentally harmful than oxide precursors, and second, it saves the series of long thermal treatments associated with solid-state reactions. After grinding the precursors, nanometric particles were obtained with a combination of crystalline regions and amorphous regions; this effect was determined by XRD and TEM, showing that Mg has a positive effect on the phase formation by only mechanical synthesis. R2C4: After sintering, it was found by XRD that the sample x = 0.1 only presents the diffraction peaks corresponding to the desired phase, which shows a phase purity greater than 97%, even higher than that of the standard undoped sample. For x = 0.2 and 0.3, there was a segregation of impurities, possibly by the local migration of La and Sr heavy cations; this was determined by SEM and EDS. The electrical characterization of the sintered pellets was carried out by electrochemical impedance spectroscopy, which determined that the incorporation of Mg in the structure improves the ionic conductivity by three orders of magnitude, obtaining conductivities of 1.6 mS/cm at 900 °C for x = 0.2. Although the improvement in conductivity is considerable, many challenges such as densification, the segregation of impurities, and the study of mechanical and thermal properties must be carried out on these materials to endorse them as solid electrolytes in SOFC.

Protonic Conduction in the BaNdInO4 Structure Achieved by Acceptor Doping.

The potential of calcium-doped layered perovskite compounds, BaNd1-x Ca x InO4-x/2 (where x is the excess Ca content), as protonic conductors was experimentally investigated. The acceptor-doped ceramics exhibit improved total conductivities that were 1-2 orders of magnitude higher than those of the pristine material, BaNdInO4. The highest total conductivity of 2.6 × 10-3 S cm-1 was obtained in the BaNd0.8Ca0.2InO3.90 sample at a temperature of 750 °C in air. Electrochemical impedance spectroscopy measurements of the x = 0.1 and x = 0.2 substituted samples showed higher total conductivity under humid environments than those measured in a dry environment over a large temperature range (250-750 °C). At 500 °C, the total conductivity of the 20% substituted sample in humid air (∼3% H2O) was 1.3 × 10-4 S cm-1. The incorporation of water vapor decreased the activation energies of the bulk conductivity of the BaNd0.8Ca0.2InO3.90 sample from 0.755(2) to 0.678(2) eV in air. The saturated BaNd0.8Ca0.2InO3.90 sample contained 2.2 mol % protonic defects, which caused an expansion in the lattice according to the high-temperature X-ray diffraction data. Combining the studies of the impedance behavior with four-probe DC conductivity measurements obtained in humid air, which showed a decrease in the resistance of the x = 0.2 sample, we conclude that experimental evidence indicates that BaNd1-x Ca x InO4-x/2 is a fast proton conductor.

Anomalous X-ray diffraction study of Pr-substituted BaCeO3†-†δ.

The effect of Pr doping on the crystal structure and site occupancy was studied for the nominally synthesized BaCe1 - xPrxO3 - δ (x = 0, 0.2, 0.4, 0.6 and 0.8) perovskites using anomalous X-ray powder diffraction (AXRD) data and Rietveld analysis. Crystal structure parameters were accurately determined using 10,000 eV photons, and the Pr occupancy was refined using data collected with 5962 eV photons, close to the Pr LIII absorption edge. BaCe1 - xPrxO3 - δ crystallizes in the Pnma (No. 62) space group for all x values. Pr cations are mainly located at the Ce sites (perovskites B site), but a small fraction of them increasingly substitute some of the Ba ions at the A site as Pr content increases. The Pr doping introduces electronic defects (Pr(+3)/Pr(+4)) and oxygen vacancies needed for H2O incorporation and H-ionic conductivity. A decrease in the orthorhombic distortion would produce the opposite effects on the electronic and ionic mobility. The electronic mobility should increase due to an improvement in the overlap of the (Ce/Pr)4f-O2p orbital, while the proton mobility should decrease as a consequence of a larger hopping distance.