Measuring Charge Distribution in Nanoscale Magnesium Aluminate Spinel by Electron Energy‐Loss Spectroscopy and Electron Holography

Abstract

Charge distribution resulting in the formation of a space charge zone (SCZ) in ionic materials has a critical role on functional properties [1]. Even though significant advances in theoretical models have been accomplished, experimental evidence in nanoscale granular materials is indirect. Here, we investigated the distribution of cations and defects on the formation of a SCZ in a nanoscale granular model system of non‐stoichiometric MgO•nAl 2 O 3 (MAS, n= 0.95 and 1.07). The SCZ was investigated experimentally by electron energy‐loss spectroscopy (EELS) and off‐axis electron holography (OAEH). EEL spectra were collected along directions perpendicular to grain boundaries (GB's), from which the magnesium‐to‐aluminum relative cation concentrations were calculated, as presented in Fig.1. We found that regardless of annealing processes, the vicinity of GB's of the Mg rich spinel has excess Mg +2 cations while the vicinity of GB's of the Al rich spinel has excess of Al +3 cations. Additionally, the cation distribution shows strong dependency on the grain size. For non‐stoichiometric MAS, cation concentration is proportional to the defect concentration, because deviation from stoichiometry results in adjacent defects that compensate for the electric charge [2, 3, 4]. In both materials, the cation distribution is inhomogeneous for grains smaller than 40 nm. For larger grains, the defect concentration approaches the bulk value at the center of the grain. Furthermore, excess of Mg (Al) cations at the vicinity of the GB decreased with increase of grain size. Maier et al. [1] calculated that for grain size at the scale of the Debye length (estimated at 9nm for non‐stoichiometric MAS studied here [7]), the GC is no longer electrically neutral, instead influenced by accumulation or depletion of charge at the boundaries. Due to the lack of accurate values for defect formation energy [5, 6], we applied OAEH to measure directly the electrostatic charge distribution in nano‐sized MAS. We show that charge distribution and the buildup of electrostatic potential between GB and core are linked to the spatial distribution of defects rather than the overall composition of MAS (Fig. 2). At the vicinity of GB's, excess Mg +2 or Al +3 cations accumulate depending on the composition, the magnitude of which increases with decreasing grain size. Indeed, the potential distributions show the relation between the excess cation species, grain size and the Debye length, in agreement with theoretical models [1].