Screening the spinel structures for different charge carriers revealed that the migration number served as a viable descriptor as illustrated in Fig. . However, for the lanthanide spinel compounds, the transition metal ionic radius was found to play a role in terms of stability and migration energy barriers . The effect of lattice distortions due to atomic size mismatches is another important consideration, as it can reduce the barriers associated with certain ion migration pathways . Furthermore, solids characterized by lower densities, such as high-temperature polymorphs and glasses, have demonstrated enhanced ionic conductivity due to 'paddlewheel' dynamics . Therefore, identifying a suitable descriptor is a daunting task due to the complex nature of migration reactions involving different diffusion mechanisms and pathways, highlighting the need for classification methods, such as categorization based on mobile ions or lattice structures.