Grain‐Boundary Characterization in a Nonstoichiometric Fine‐Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space‐Charge Layers

The grain‐boundary chemistry of fine‐grained spinel MgO· n Al 2 O 3 (mean grain size below micron) has been investigated by STEM microanalysis. We have quantified the concentration of each element across the grain boundaries. Stoichiometry variations are observed from the grain‐boundary region to the bulk. The Al/Mg ratio increases from 2.1 in the bulk to 2.35 at the grain‐boundary regions. X‐ray quantification allows us to reveal and to characterize the space‐charge layer in the subgrain boundary. The grain‐boundary cores are negatively charged due to vacancies in excess, and in the subgrain‐boundary region, an opposite, positive space‐charge layer is obtained. The point defect composition and the characteristic (sign, space‐charge potential Φ ∞ ) of the space‐charge layer are discussed.