Detecting Structural and Bonding Changes in EELS Near-Edge Structures with a Monochromated TEM

Abstract

The enhanced energy resolution in electron energy loss spectroscopy measurements achieved in transmission electron microscopes equipped with monochromators provides new opportunities for the study of the properties of materials and their defects with an unprecedented spatial resolution. The current systems offering an energy resolution comparable to the one of soft x-ray absorption spectroscopy (about 0.1eV) make it possible to increase the sensitivity to changes in the near-edge structure while maintaining the capability to study small objects with a probe size better than 2nm [1]. With this improved performance, it is therefore possible to study changes in the electron energy loss spectra due to modifications of the structure or the bonding of the sample with minimal contributions due to the poor energy resolution of the instrument. This capability is a necessity for the study of transition metal oxides, and in particular compounds with perovskite-related structures, where subtle changes in the atomic arrangement or distortions in the bond angles can lead to dramatic changes in the physical properties. These structural and bonding effects are studied here by probing the variations of the O-K and Ti-L 23 edges in a series of transition metal oxides.Experiments were carried out on a FEI Tecnai-200FEG equipped with a monochromator[1] and a high-resolution electron energy loss spectrometer [2]. The first applications on a broad range of materials (including oxides, nitrides and fullerenes) and details of the performance of the system have been described elsewhere [3][4][5] and show that an energy resolution of about 0.1eV can be achieved yielding additional information on the near-edge structure spectra.The O-K edge in TiO 2 -brookite (figure 1a) shows two well-resolved peaks attributed to the O2p hybridization with the Ti3d states that are split into t 2g -e g components due to the octahedral crystal field. Experiments with and without the monochromator[3,4] supported by calculations make it possible to deduce that the width of these peaks is intrinsic to the electronic structure of this material and is due to dispersion of the hybridized states and the strong distortions of the octahedra. In the perovskite structures such as CaTiO 3 and SrTiO 3 ( figure 1b and ref. [6]), however, the t 2g peak (at 531eV) is significantly narrower and the e g peak intensity (at 533-534 eV) is significantly reduced due to the lower distortion of the octahedral environment around the cations. This sensitivity to bonding changes can be exploited when studying the variations of the local electronic structure due to substitutions of cations in perovskites. These effects can be probed in the Ba(Ti,Ru)O 3 compound where some of the Ti atoms are substituted by Ru. The main features of the O-K edge of the perovskite spectra are maintained in Ba(Ti,Ru)O 3 (figure 2a) since there are general similarities with the spectra of CaTiO 3 (figure 1b) and BaTiO 3 . Differences exist, however, in the intensity of the t 2g and e g peaks suggesting th...