The ability to resolve spatially and identify chemically atoms in defects would greatly advance our understanding of the correlation between structure and property in materials1. This is particularly important in polycrystalline materials, in which the grain boundaries have profound implications for the properties and applications of the final material2. However, such atomic resolution is still extremely difficult to achieve, partly because grain boundaries are effective sinks for atomic defects and impurities3, 4, 5, which may drive structural transformation of grain boundaries and consequently modify material properties6, 7. Regardless of the origin of these sinks, the interplay between defects and grain boundaries complicates our efforts to pinpoint the exact sites and chemistries of the entities present in the defective regions, thereby limiting our understanding of how specific defects mediate property changes. Here we show that the combination of advanced electron microscopy, spectroscopy and first-principles calculations can provide three-dimensional images of complex, multicomponent grain boundaries with both atomic resolution and chemical sensitivity. The high resolution of these techniques allows us to demonstrate that even for magnesium oxide, which has a simple rock-salt structure, grain boundaries can accommodate complex ordered defect superstructures that induce significant electron trapping in the bandgap of the oxide. These results offer insights into interactions between defects and grain boundaries in ceramics and demonstrate that atomic-scale analysis of complex multicomponent structures in materials is now becoming possible.
Figures at a glance
Figure 1: Chemical and structural analysis of a Σ = 5, (310)[001] grain boundary. 
a, EELS spectrum from an energy-loss range containing the calcium L2,3, titanium L2,3 and oxygen K edges, showing the presence of calcium and titanium in the grain boundary (GB) region. The spectra with the background subtracted are shown at bottom. Inset, sketch of the MgO bicrystal. a.u., arbitrary units. b, High-resolution medium-voltage (400-kV) TEM image of the bicrystal viewed along the [001] direction. The grain boundary is atomically flat over extended regions up to several tens of nanometres in size. Comparing experimental images with simulated ones (inset) identifies the bright spots as sites of atomic columns.
Figure 2: Atomic-column imaging of the Σ = 5 grain boundary. 
a, b, Atomic-resolution HAADF images viewed from the [001] (a) and [1
0] (b) directions. The spots with low image contrast are indicated by arrows in a. We note that the distance between the (620) planes is as small as 0.66 Å, making them invisible in b. c, Line profile showing image intensity along the line I–II (a). The presence of entities giving rise to the spots with low image contrast is confirmed by clear peaks in the profile. d, h, Magnified HAADF images of the analysed region overlaid with the determined structural unit of the grain boundary observed from the [001] (d) and [1 
0] (h) directions. In the structural unit, orange lines indicate planes containing titanium. e–g, i–k, Core-loss images of the calcium L2,3, titanium L2,3 and oxygen K edges viewed from the [001] (e–g) and [1 
0] (i–k) directions. The core-loss images were made at the same places as the HAADF images in d and h.
Figure 3: Formation of an ordered defect superstructure at the grain boundary. 
a, Periodic supercell used to model the pure, Σ = 5 grain boundary. The boundary structural unit is indicated by the dashed polygon. b, The optimal model of the grain boundary with segregated calcium. The substitution and vacancy sites considered for titanium and, respectively, magnesium are numbered 1 to 11. c, d, The determined grain boundary viewed from the [001] (c) and [1
0] (d) directions. The dots with low image contrast in Fig. 2a are recognized as calcium interstitials and are marked by arrows (yellow arrows show Cai locations and blue arrows show from where the Ca atoms were displaced). e–h, Comparison showing a good match between simulated HAADF images obtained using the determined grain boundary and the corresponding low-pass-filtered images: [001] projection (e, f); [1 
0] projection (g, h).
Figure 4: Calculated free energy of grain boundary as a function of the chemical potential of oxygen (μO). 
We consider a pure MgO grain boundary, a calcium-doped grain boundary, and calcium-doped grain boundaries with, respectively, Ti2+ segregated to column 1 (
), Ti3+ charge-compensated by 
at column 1 (2 
+ 
(Rel)) and Ti4+ charge compensated by 
at column 1 ( 
+ 
). The free energy of the 2 
+ 
grain boundary before relaxation (2 
+ 
(Unr)) is given for comparison. The magnesium-rich and oxygen-rich ends of the chemical potential scale correspond to the cases in which MgO is in equilibrium with metallic magnesium and, respectively, O2. Inset, structure transformation through optimization in the case of 2 
+ 
.
