Electron diffraction characterization of nanocrystalline materials using a Rietveld-based approach. Part I. Methodology

Abstract

Transmission electron microscopy is a powerful experimental tool, very effective for the complete characterization of nanocrystalline materials by employing a combination of imaging, spectroscopy and diffraction techniques. Electron powder diffraction (EPD) pattern fingerprinting in association with chemical information from spectroscopy can be used to deduce the identity of the crystalline phases. Furthermore, EPD has similar potential to X-ray powder diffraction (XRPD) for extracting additional information regarding material specimens, such as microstructural features and defect structures. The aim of this paper is to extend a full-pattern fitting procedure, broadly used for analysing XRPD patterns, to EPD. The interest of this approach is twofold: in the first place, the relatively short times involved with data acquisition allow one to speed up the characterization procedures. This is a particularly interesting aspect in the case of metastable structures or kinetics studies. Moreover, the reduced sampling volumes involved with electron diffraction analyses can better reveal surface alteration layers in the analysed specimen which might be completely overlooked by conventional bulk techniques. The first step forward to have an effective application of the proposed methodology concerns establishing a reliable calibration protocol to take into correct account the instrumental effects and thus separate them from those determined by the structure, microstructure and texture of the analysed samples. In this paper, the methodology for determining the instrumental broadening of the diffraction lines is demonstrated through a full quantitative analysis based on the Rietveld refinement of the EPD. In this regard, a CeO2 nanopowder reference specimen has been used. The results provide indications also on the specific features that a good calibration standard should have.

Electron diffraction characterization of nanocrystalline materials using a Rietveld-based approach. Part II. Application to microstructural analysis

“…We present here the results of Rietveld refinement for both XRPD and EPD data. The results from the two techniques are in agreement with each other, implying that the instrumental broadening function determined in Part I (Sinha et al, 2022) is also still validated by the results obtained with nc-Y 2 O 3 .…”

“…The option of manually removing the beam stop covering portions of the SAED signal is also available. These aspects are discussed in detail in our previous paper (Part I; Sinha et al, 2022). For thin films, it is also possible to perform quantitative texture analysis using EPD (Gemmi et al, 2011;Boullay et al, 2014) The background was fitted with a fourth-order polynomial curve, convoluted with an additional peak at zero Q to compensate for the strong signal from the direct beam.…”

“…nc-Y 2 O 3 , X-ray diffraction patterns were acquired to assess the electron microscopy results. The reader is referred to Part I (Sinha et al, 2022) for the relevant experimental details for this type of test. 2.…”

“…Similarly to X-ray powder diffraction, the diffracted electron intensity encodes structural information that can be modelled to yield the material microstructure and relevant parameters (Billinge & Levin, 2007). In Part I of this series of papers, electron powder diffraction (EPD)-based Rietveld refinement (Rietveld, 1967) was successfully applied to diffraction line profile analysis (Sinha et al, 2022). Such EPD analyses could support and complement the results from bulk characterization techniques.…”

“…In Part I (Sinha et al, 2022), the instrumental broadening function corresponding to different selected-area apertures and camera length combinations have been determined. As a next step, in this paper, we demonstrate the possibility of elucidating the microstructural features of different types of nanocrystalline materials through electron diffraction.…”