<p>Polycrystalline magnesium aluminate spinel, or simply spinel, with near theoretical density is a transparent ceramic material with multiple applications in extreme environmental conditions, which require an enhanced fracture toughness. For this reason, in this study, the effect of 500&#xa0;ppm doping level of calcium (Ca) and yttrium (Y) on the segregation behavior and mechanical properties of spinel was quantitatively investigated. Calcium and yttrium doping reduced the grain-boundary plane anisotropy for grain boundaries with rotations about the [111] axis. More tilt boundaries and fewer twist boundaries were found in the doped samples compared to the undoped condition. Direct observations by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging revealed that yttrium atoms preferentially occupy aluminum sites at grain boundaries in spinel. Quantitative electron energy-loss spectrometry (EELS) analysis in the vicinity of grain boundaries indicated that calcium atoms preferentially occupy magnesium sites in the Ca-doped spinel samples and confirmed the substitution of aluminum by yttrium atoms in the Y-doped sample. Quantitative X-ray energy-dispersive spectrometry (XEDS) analysis employing the ζ-factor method indicated that the maximum segregation levels at grain boundaries were 0.8 ± 0.1 calcium atoms/nm² (0.23 ± 0.03 monolayers) and 2.4 ± 0.06 yttrium atoms/nm² (0.45 ± 0.11 monolayers). Enhanced indentation fracture toughness was found in samples with calcium and yttrium doping compared to undoped spinel. The enhanced fracture toughness in Ca- and Y-doped spinel samples, in comparison with undoped spinel, was primarily attributed to more pronounced crack deflections and a more tortuous crack path.</p> Graphical abstract <p></p>