Microsc. Microanal. 23, 314–320, 2017 doi:10.1017/S1431927616012691
© MICROSCOPY SOCIETY OF AMERICA 2017
In Situ Atom Probe Deintercalation of Lithium-Manganese-Oxide
Björn Pfeiffer,* Johannes Maier, Jonas Arlt, and Carsten Nowak Institute of Materials Physics, Georg-August-University Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
Abstract: Atom probe tomography is routinely used for the characterization of materials microstructures, usually assuming that the microstructure is unaltered by the analysis. When analyzing ionic conductors, however, gradients in the chemical potential and the electric field penetrating dielectric atom probe specimens can cause significant ionic mobility. Although ionic mobility is undesirable when aiming for materials characterization, it offers a strategy to manipulate materials directly in situ in the atom probe. Here, we present experimental results on the analysis of the ionic conductor lithium-manganese-oxide with different atom probe techniques. We demonstrate that, at a temperature of 30 K, characterization of the materials microstructure is possible without measurable Li mobility. Also, we show that at 298K the material can be deintercalated, in situ in the atom probe, without changing the manganese-oxide host structure. Combining in situ atom probe deintercalation and subsequent conventional characterization, we demonstrate a new methodological approach to study ionic conductors even in early stages of deintercalation.
Key words: atom probe, ionic conductor, in situ deintercalation, lithium-manganese-oxide, microstructure
INTRODUCTION Atom probe tomography is routinely used for three- dimensional (3D) chemical mapping of materials micro- structures. The material is analyzed by subsequent field evaporation of individual atoms or molecules from the surface. This way, the subjacent material is exposed to the surface, so that it is accessible for field evaporation, too. Thereby, the 3D information on the materials micro- structure is reconstructed from a series of surface states, and although surface reconstruction effects are expected, the spatial resolution often is high enough to reveal lattice planes in crystalline materials. Thus, it is generally assumed that the materials microstructure is not significantly changed during the analysis and can be accessed using appropriate algorithms for the 3D reconstruction of the analyzed volume (Bas et al., 1995). However, if not all species in a multicomponent system
are field evaporated and there is significant mobility of some species in the material, the chemical composition can change during analysis. This is particularly relevant for materials containing highly mobile species like hydrogen (Kesten et al., 2002) and for ionic conductors (Escher et al., 2006). As ionic conductors are usually oxides or semiconductors, mobile species can not only be driven by gradients in the chemical potential or mechanical stress fields, but also by electric fields penetrating dielectric atom probe specimens (Silaeva et al., 2013, 2014; Greiwe et al., 2014). Therefore, the influence of the electric field between specimen and counter electrode in an atom probe has to be considered. Although mobility of species during atom probe analysis is undesirable when
*Corresponding author.
bpfeiffer@ump.gwdg.de Received September 5, 2016; accepted December 12, 2016
aiming for microstructural characterization (cf. Schmitz et al., 2010; Santhanagopalan et al., 2015; Devaraj et al., 2015; Maier et al., 2016), it offers a strategy for controlled specimen manipulation in an atom probe. Here, we present enhanced atom probe analyses of
lithium-manganese-oxide (LiMn2O4; or LMO) as model system for an ionic conducting material relevant to electro- chemical energy conversion (Zhang et al., 2015). In the initial state, the LMO has a cubic spinel structure resulting in an isotropic diffusion network for the mobile Li-ions. This Li mobility is central for the atom probe analyses presented here. Adjusting the specimen base temperature and the operation of electric field for field evaporation enables the operation of the atom probe in different modes. We show that at 30K and high electric fields, conventional microstructural analysis of specimens is possible without measurable Li mobility. This is particularly important as this aspect is not addressed in the literature on atom probe analysis of lithium-(nickel)-manganese-oxide so far (Devaraj et al., 2015; Maier et al., 2016). In contrast, for a temperature of 298K and lower electric fields, we demonstrate that in situ deintercalation experiments can be performed in the atom probe without changing the Mn–O host structure. By combining both modes we show that during deintercalation of the LMO complex, microstructural features develop and we discuss their impact on Li transport in LMO.
MATERIALS ANDMETHODS
LMO At room temperature, LMO with the stoichiometry LiMn2O4, has a cubic spinel structure (space group Fd3m) with lattice parameter a=8.24 Å. The O atoms are located at
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