Defects analysis at the nanometric scale in Ca3Co4O9 thin films

Abstract

We report on the nature and origin of structural defects at a nanometric scale in incommensurate Ca3Co4O9 thin films deposited by pulsed laser ablation on Al2O3(001) substrates. X-ray diffraction suggests that the deposited films have a well defined texture and that are free of spurious phases. However, cross section scanning high resolution transmission electron microscopy observations show the presence of regions with different kinds of stacking. Such regions present different chemical compositions from that of Ca3Co4O9 and are not detectable in diffraction mode. The local chemical analysis and the interplane distance measurement suggest that these defects correspond to the formation of the CaCo2O4 spurious phase. This phase has a similar structure and close lattice parameters with those of Ca3Co4O9. The origin of the formation of CaCo2O4 is discussed in terms of (i) strains due to the substrate which tend to suppress the incommensurability of the system, and (ii) local chemical nonstoichiometry.