Anisotropic Strain in Rare-Earth Substituted Ceria Thin Films Probed by Polarized Raman Spectroscopy and First-Principles Calculations.

Lattice strain in oxygen ion conductors can be used to tune their functional properties for applications in fuel cells, sensors, or catalysis. However, experimental measurements of thin film strain in both in- and out-of-plane directions can be experimentally challenging. We propose a method for measuring strain in rare-earth doped ceria thin films by polarized Raman spectroscopy. We study epitaxial CeO2 films substituted by La, Gd, and Yb grown on MgO substrates with BaZrO3 and SrTiO3 interlayers, where different levels of strain are generated by annealing at distinct temperatures. The films show in-plane compression and out-of-plane expansion, resulting in a lowering from the bulk cubic to tetragonal lattice symmetry. This leads to the splitting of the F2g Raman mode in the cubic phase to B2g and Eg modes in the tetragonal lattice. The symmetry and frequency of these modes are determined by polarized Raman in the backscattering and right-angle scattering geometries as well as by first-principal calculations. The frequency splitting of the two modes is proportional to the strain measured by X-ray diffraction and its magnitude agrees with first-principles calculations. The results offer a fast, nondestructive, and precise method for measuring both in- and out-of-plane strain in ceria and can be readily applied to other ionic conductors.

High- and low-temperature phases in isostructural 4-chloro-3-nitroaniline and 4-iodo-3-nitroaniline.

The structures of 4-chloro-3-nitroaniline, C6H5ClN2O2, (I), and 4-iodo-3-nitroaniline, C6H5IN2O2, (II), are isomorphs and both undergo continuous (second order) phase transitions at 237 and 200 K, respectively. The structures, as well as their phase transitions, have been studied by single-crystal X-ray diffraction, Raman spectroscopy and difference scanning calorimetry experiments. Both high-temperature phases (293 K) show disorder of the nitro substituents, which are inclined towards the benzene-ring planes at two different orientations. In the low-temperature phases (120 K), both inclination angles are well maintained, while the disorder is removed. Concomitantly, the b axis doubles with respect to the room-temperature cell. Each of the low-temperature phases of (I) and (II) contains two pairs of independent molecules, where the molecules in each pair are related by noncrystallographic inversion centres. The molecules within each pair have the same absolute value of the inclination angle. The Flack parameter of the low-temperature phases is very close to 0.5, indicating inversion twinning. This can be envisaged as stacking faults in the low-temperature phases. It seems that competition between the primary amine-nitro N-H···O hydrogen bonds which form three-centred hydrogen bonds is the reason for the disorder of the nitro groups, as well as for the phase transition in both (I) and (II). The backbones of the structures are formed by N-H···N hydrogen bonding of moderate strength which results in the graph-set motif C(3). This graph-set motif forms a zigzag chain parallel to the monoclinic b axis and is maintained in both the high- and the low-temperature structures. The primary amine groups are pyramidal, with similar geometric values in all four determinations. The high-temperature phase of (II) has been described previously [Garden et al. (2004). Acta Cryst. C60, o328-o330].