Extended energy-loss fine structure spectroscopy of structural modifications in Nd2CuO4−δ at the oxygen K edge

The discovery of the superconducting electron-doped compound Nd₁₈₅Ce₀₁₅CuO_4?δ has stimulated great interest in its micro- and crystal structure, since the superconducting properties depend on parameters such as nonstoichiometry, phase composition, heat treatment and microstructure. The work presented herein is focused on the determination of the oxygen environment in the undoped parent compound Nd₂CuO₄ and in the structural modification Nd₂CuO₃₅ The analysis of the oxygen K (O 1s) edge extended electron energy-loss fine structure (EXELFS) of the tetragonal parent compound Nd₂CuO₄ and of the orthorhombic modification Nd₂CuO₃₅ is reported by using electron energy-loss spectroscopy in combination with transmission electron microscopy. Nd₂CuO₃₅ is produced by in situ heating and reduction of Nd₂CuO₄ in the transmission electron microscope. The EXELFS of the O 1s electron energy-loss edges is analysed with the classical extended X-ray absorption fine structure (EXAFS) treatment and compared with ab initio multiple scattering EXAFS calculations for both structural modifications. Highly accurate information on the local atomic environment of the oxygen atoms in Nd₂CuO₃₅ is obtained from EXELFS analysis using Nd₂CuO₄ as a standard. The results are in accordance with the structural data gained from X-ray diffraction analysis. This applies especially to the more complicated structure of Nd₂CuO₃₅ determined recently.     

Electron microscopy of crack/particle interactions in Al2O3/SiC nanocomposites

Crack/particle interactions in alumina/silicon carbide nanocomposites have been investigated by scanning electron microscopy and transmission electron microscopy, with cracks induced by Vickers microindentation. Intergranular cracks are frequently deflected into grains by SiC particles on grain boundaries inclined to the average direction of crack propagation. This mechanism is proposed to explain the change in the fracture mode from intergranular fracture for monolithic alumina to predominantly transgranular fracture for the nanocomposites. Neither stress-induced microcracking around SiC particles nor significant crack deflection by intragranular particles was found to occur in the nanocomposites. It is argued that an addition of nanoparticles may not be a promising approach for increasing the toughness of alumina.