High-precision quantitative atomic-site-analysis of functional dopants in crystalline materials by electron-channelling-enhanced microanalysis

Abstract

Knowledge of the location and concentration of impurity atoms doped into a synthesized material is of great interest to investigate the effect of doping. This would usually be investigated using X-ray or neutron diffraction methods in combination with Rietveld analysis. However, this technique requires a large-scale facility such as a synchrotron radiation source and nuclear reactor, and can sometimes fail to produce the desired results, depending on the constituent elements and the crystallographic conditions that are being analysed. Thus, it would be preferable to use an element-selective spectroscopy technique that is applicable to any combination of elements. We have established a quantitative method to deduce the occupation sites and their occupancies, as well as the site-dependent chemical states of the doped elements, using a combination of transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, and electron energy-loss spectroscopy (EELS). The method is based on electron channelling phenomena where the symmetries of the Bloch waves excited in a crystal are dependent on the diffraction condition or incident beam direction with respect to the crystal axes. By rocking the incident electron beam with a fixed pivot point on the sample surface, a set of EDX/EELS spectra are obtained as a function of the beam direction. This is followed by a statistical treatment to extract the atom-site-dependent spectra, thereby quantitatively enabling the estimation of the site occupancies and chemical states of the dopants. This is an extension of the ‘ALCHEMI’ (Atom Location by Channelling Enhanced Microanalysis) method or ‘HARECXS/HARECES’ (High Angular Resolution Channelled X-ray/Electron Spectroscopy), and we further extended the method to be applicable to cases where the crystal of interest contains multiple inequivalent atomic sites for a particular element, applying the precise spectral predictions based on electron elastic/inelastic dynamical scattering theory. After introduction of conceptual aspects of the method, we describe the extension of the method together with the development of the theoretical calculation method. We then demonstrate several useful applications of the method, including luminescent, ferrite, and battery materials. We discuss the advantages and drawbacks of the present method, compared with those of the recently developed atomic column-by-column analysis using aberration-corrected scanning TEM and high-efficiency X-ray detectors.