Superstructure and magnetic properties of R15X9C compounds (R = rare earth; X = Si and Ge)

Abstract We report the synthesis of the new compounds R 15 Si 9 C with R = Sm, Gd–Er, Y and R 15 Ge 9 C with R = Ce, Pr and Nd. These compounds crystallize in the hexagonal La 15 Ge 9 Fe structure type (hP50, P6 3 mc, Z  = 2) which is an ordered superstructure of La 5 Ge 3 (Mn 5 Si 3 -type, hP16, P6 3 /mcm, Z  = 2). The superstructure is absent in a single-phase R 5 Ge 3 C 0.33 alloy with R = Sm and Gd. The addition of interstitial carbon modifies the magnetic properties significantly as regards the magnetic transition temperature and the nature of the magnetic ordering. Carbon doping induces mostly a ferromagnetic or ferrimagnetic configuration. The preponderance of ferromagnetic interactions in the superstructure compounds, with respect to the parent phases, can tentatively be attributed to a change in the sign of the RKKY interactions. The dense Kondo lattice state of Ce in ferrimagnetic Ce 5 Ge 3 is preserved in Ce 15 Ge 9 C. Large coercive fields (∼4–5 kOe) at low temperatures are observed in most compounds, which exceed ∼50 kOe in the two Sm based compound.