Co-Segregation of Aliovalent Dopants at α-Al₂O₃ Σ13 Grain Boundary

Impurity doping at grain boundary (GB) is a major strategy to control the properties of polycrystalline materials. The isovalent impurity with a low solubility limit is usually segregated to the specific sites at the GB core and forms the periodic GB structure [1]. However, the segregation behavior of aliovalent impurities is still unclear. Aliovalent impurities should form charged defects that will be compensated by the other charged defects, and therefore segregation behavior would be complex [2]. In this study, we investigated the segregation mechanism of counterbalancing charged defects of Ca_Al^1- and Si_Al¹⁺, which are typical sintering additives for Al₂O₃, at <i>α</i>-Al₂O₃ Σ13 &lt;&gt;/{} GB by scanning transmission electron microscopy (STEM) combined with systematic density functional theory (DFT) calculations.<br/><br/>Atomic-resolution annular dark/bright-field (ADF/ABF) STEM observations revealed that the atomic structure of Ca/Si-doped GB is significantly transformed from that in the pristine Σ13 GB. The energy dispersive x-ray spectroscopy (EDS) in STEM revealed that both Ca and Si are periodically segregated to the specific sites at the GB core. To understand the origin of this GB structural transformation, we first reconstructed the framework GB structure by referring to the ABF-STEM image. The GB energy of the framework structure is 4.17 J m^-2, which is much higher than that of the stable Σ13 GB structure (2.40 J m^-2). This result suggests that the framework structure may be stabilized by the co-segregation of Ca and Si. To determine the stable segregation sites of Ca_Al^1- and Si_Al¹⁺, the segregation energies of Ca_Al^1- and Si_Al¹⁺ are evaluated for all Al sites within 10 Å from the GB core. These atomic sites with minimum segregation energies agree with the Ca/Si segregation sites in STEM-EDS mapping. The minimum segregation energy of Ca_Al^1- is -4.63 eV, which is much lower than those of other Al sites. On the other hand, the minimum segregation energy of Si_Al¹⁺ is -0.46 eV, and the site dependence of segregation energy is small. These results suggest that the segregations of Ca_Al^1- and Si_Al¹⁺are induced by the structural stability and the local charge compensation with Si_Al¹⁺, respectively. Lastly, we reconstructed the Ca/Si co-segregated Σ13 GB and evaluated the GB energy, and the most stable structure is matched well with the STEM image. The GB energy of the Ca/Si co-doped GB decreases with the increase in the chemical potentials of Ca and Si, and it becomes lower than that of the pristine GB. This result suggests that, as increasing the doping amount of Ca and Si, they segregate to the GB core and induce the GB structural transformation.<br/><br/>[1] J. P. Buban et al., Science, 311, 212 (2006).<br/>[2] T. Futazuka et al., Phys. Rev. Mater. 4 073602 (2020).