Nuclear magnetic resonance (NMR) spectroscopy: 25 Mg magic-angle spinning (MAS) NMR spectroscopy was performed at a magnetic field of 11.7 T corresponding to a resonance frequency of 30.6 MHz. The measurements were carried out in 1.3 mm rotors at a spinning speed of 22 kHz with a rotor-synchronized Hahn-echo pulse sequence and a π/2 pulse duration of 3.7 µs. The recycle delay was 15 s for MgSc2Se4/MgY2Se4 and 1 s for MgEr2Se4/MgTm2Se4. The spectra were referenced to an aqueous solution of 5M MgCl2.