Note that impurities with fractions >5 wt%, in particular found in MgY2Se4, or the interfacial resistances that are probably material-dependent (not considered in our fittings) could have reduced the calculated conductivities. Moreover, it is worth to mention that in case of MgEr2Se4, the fitted ionic resistance of the spinel R2ion and the fitted total ionic resistance (R1ion+R2ion) are comparably higher than their corresponding estimates Rion(MgEr2Se4) and Rion(SEs), directly calculated from the raw data (see Table ). In this simplified calculation, both, the total ionic resistance of the reference cell Rion(UiO66-MgIL) and the sandwich-type cell Rion(SEs) were directly calculated from the lowest data point at the intercept of the Re(Z) axis. From the difference of Rion(SEs) and Rion(UiO66-MgIL) the ionic resistance of the spinel Rion(MgEr2Se4) is given. The discrepancy between the fitted and directly calculated values results from the flattend shape of the Nyquist plots at the Re(Z) axes intercept, possibly induced due to an additional contribution from an interface or impurity. As this contribution is more integrated in the fitted values, the resistance determined directly from the raw data is lower, resulting in a slightly higher ionic conductivtiy (for example: 4.8x10 -5 S cm -5 at 25 °C) for MgEr2Se4. Figure shows the Arrhenius plots of the ionic conductivities for the temperature range between 0 °C and 60 °C. Applying the Arrhenius equation (Equation , experimental section), the Mg-ion migration barriers can be calculated to be Ea(MgSc2Se4) = 386±24 meV, Ea(MgTm2Se4) = 381±4 meV, Ea(MgEr2Se4) = 382±6 meV (raw data: 373±5 meV) and Ea(MgY2Se4) = 406±9 meV. These values are lower than published ones for other classes of Mg-ion SEs and close to our computationally derived migration barriers (Ea(th)) obtained from periodic density functional theory (DFT) calculations based on the SCAN exchange-correlation functional (Figure ). In addition, our computational results exhibit a decreasing trend of the Mg 2+ migration barrier with increasing B-ion radius rB, in accordance to previous predictions. This trend is not observed in the electrochemical results for the compounds synthesized here, since the experimental error exceeds the minor changes (<0.03 eV) of the migration barriers for the narrow range of the ionic radii rB.