Migration of Mg-ions within the spinel structure (Figure ) typically occurs via an interstitial mechanism, transitioning from initial tetrahedral sites through an empty octahedron to final tetrahedral sites (tet-oct-tet), as illustrated in Figure . The energy difference between tetrahedral and octahedral sites, known as site preference energy, influences the migration barriers, with lower barriers expected when site preference energy is minimized. However, Mg lanthanide spinels defy this trend, exhibiting lower barriers when site preference energy is maximized. This phenomenon is attributed to the stabilization of the triangular transition state with increasing ionic radius rB of the metal B. Despite its importance, the transition state energy is not usually explicitly considered in the design rule for identifying good Mg-ion conductors.