The MgB2Se4 spinels (B = Er, Tm, Y) were initially prepared according to our reported onestep synthesis of MgSc2Se4 by reacting stoichiometric amounts of the elemental powders at 950 °C for 20 h. However, although high-purity MgSc2Se4 was obtained with this procedure, Rietveld refinement results based on the X-ray diffraction (XRD) patterns of the other three spinels (Figure -S2, Supporting Information) reveal a significant fraction of impurities.