Indirect EELS imaging reaching atomic scale – CaO planar faults in CaTiO3

Indirect EELS analysis is based on an enhanced “spatial difference” method which separates ELNES of a defect from ELNES of the matrix [H. Gu, Ultramicroscopy 76 (1999) 159, 173.]. Combined with the “spectrum imaging” technique [C. Colliex et al., Microchem. Acta 114/115 (1993) 71], the spatial difference method is employed in the STEM to investigate CaO planar faults in nonstoichiometric CaTiO3 which can locally form Ca4Ti3O10. Although inelastic imaging used as 1D profiling resolves individual faults separated by 1.36 nm from each other, the effective EELS probe size is worse than the elastic HADF imaging by a factor of 3 for the same microscope conditions. However, quantitative analysis of the EELS profiling yields additional information about equivalent probe size, and more importantly, provides an alternative way to separate the EELS spectra. O-K ELNES in the separated spectra reveal convincingly the differences between the perovskite and the rock-salt structures, and their corresponding ELNES profile maps distinguish between these two structures. In this indirect way, EELS analysis reaches effectively the state-of-art by obtaining a high quality spectrum reflecting the electronic structures in a 0.22 nm layer of defect structure.