As shown in Figure , clearly fewer intensive peaks of impurities could be found in the XRD patterns of the final products using this two-step procedure, although the formation energy is much lower for the spinels built from the binaries. The performed Rietveld refinements (Figure ) confirm the reduced fraction of B2O2Se and unreacted binaries in MgTm2Se4 (4.3 wt% and 0 wt%), MgEr2Se4 (3.0 wt% and 0.6 wt%) and MgY2Se4 (2.0 wt% and 8.9 wt%), as well as the spinel-type structure with Fd-3m space group (additional crystallographic data in Tables ). The spinel structure consists of a cubic dense packing of Se-anions, where the B-ions reside in 16d sites (octahedra) and the Mg-ions occupying the 8a sites (tetrahedra). To further investigate the position of the Mg-ions, 25 Mg magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was carried out. The 25 Mg nucleus has a nuclear spin I = 5/2 and a large nuclear quadrupolar moment Q = 201 mb and thus is very sensitive to asymmetric local environments. The 25 MAS NMR spectra (Figure ) show only a single isotropic peak for all spinels, indicating that Mg occupies exclusively the tetrahedral site. In case of the Sc-and Y-based spinels, representing diamagnetic compounds, the isotropic peak is very narrow (width of 1.5 ppm and 2.5 ppm, respectively) and the spinning sideband patterns are very weak. These characteristics reveal that the local environment, i.e. the surrounding Se tetrahedron, is highly symmetric. Instead, for the lanthanide-based spinels, consistent with their reported paramagnetism, much broader isotropic peaks with large negative shifts and broader spinning sideband patterns were observed. Furthermore, energy disperse X-ray spectroscopy (EDS) images superimposed to scanning electron micrographs (SEM) display the uniform distribution of the elements Mg, Sc/Tm/Er/Y and Se in the samples. The spinel stoichiometry, which is ideally 1:2:4, is confirmed by the observed atomic ratios (Figure ), including small deficits especially in the Se fraction (0-17.5%), probably due to Se evaporation from the sample during the hightemperature synthesis. The SEM images show that the particles mostly agglomerate. For this reason, to verify the size of the single particles, transmission electron microscopy (TEM) images were collected (Figures ), which demonstrate a typical particle size in the low micrometer range for all spinel powders (MgSc2Se4: 1-3 µm, MgTm2Se4: 0.5-1.5 µm, MgEr2Se4: 0.5-3 µm, MgY2Se4: 1-10 µm). Additionally, the selected area electron diffraction (SAED) patterns recorded during the TEM measurements confirm the spinel structure as well.