In the one-step synthesis, magnesium powder, scandium/thulium/erbium/yttrium powder and selenium powder were hand-milled in a molar ratio of 1:2:4 for 15 min. Then, 0.5 g of each powder mixture was pressed into a pellet (Ø = 10 mm) under a pressure of 300 MPa for 30 min. After pressing, each pellet was wrapped into a platinum foil and vacuum sealed in a separate quartz glass ampule, prior backed out at 800 °C under dynamic vacuum to remove moisture. To perform the solid-state reaction, the ampules were heated at 950 °C for 20 h (180 ° C h -1 heating rate) in a furnace (Nabertherm with controller P 300). When the samples reached room temperature again, the pellets were collected by breaking the ampules and afterwards ground to powders.