Charge distribution in nano-scale grains of magnesium aluminate spinel

Charge distribution in magnesium aluminate spinel (MAS) results in the formation of a space-charge region that plays a critical role in assigning functional properties. Significant theoretical advances explaining this phenomenon have been accomplished, even though quantitative experimental support from nano-scale granular MAS is only indirect. In this work, the electrostatic potential distribution in nano-scale grains of nonstoichiometric MAS (MgO·0.95Al₂O₃ and MgO·1.07Al₂O₃) was measured by off-axis electron holography (OAEH) and compared to the distribution of cations and defects in this material as measured by electron energy-loss spectroscopy (EELS). In this manner, we studied the roles of composition, grain size, and applied electric field (EF) on the formation of a space-charge region. We quantitatively demonstrated that regardless of grain size, the vicinity of MgO·0.95Al₂O₃ grain boundaries presented an excess of Mg⁺² cations, whereas the vicinity of MgO·1.07Al₂O₃ grain boundaries included an excess of Al⁺³ cations. The degree of structural disorder (ie, the inversion parameter, i) indicated that as-synthesized MAS were significantly disordered (i between 0.37 and 0.41), with values decreasing toward equilibrium ordering values following annealing (i between 0.27 and 0.31). The application of an external ~150 V/cm EF during annealing further enhanced lattice ordering (i between 0.16 and 0.19). Such variations in the distribution of cations and defects should determine the space-charged potential (SCP). However, using these measurements to calculate the SCP was not possible due to the wide range of values reported for formation energies of defects (0.82-8.78 eV). Consequently, we correlated local ionic ordering with electrostatic potential in nonstoichiometric MAS. The magnitudes of the SCP in both MgO·0.95Al₂O₃ and MgO·1.07Al₂O₃ decreased following annealing from −3.4 ± 0.3 V and 2.0 ± 0.2 V to −2.0 ± 0.2 V and 1.6 ± 0.1 V, respectively.