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

We report the synthesis of the new compounds R15Si9C with R = Sm, Gd-Er, Y and R15Ge9C with R = Ce, Pr and Nd. These compounds crystallize in the hexagonal La15Ge9Fe structure type (hP50, P6(3)mc, Z = 2) which is an ordered superstructure of La5Ge3 (Mn5Si3-type, hP16, P6(3)/mcm, Z = 2). The superstructure is absent in a single-phase R5Ge3C0.33 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 Ce5Ge3 is preserved in Ce15Ge9C. Large coercive fields (similar to 4-5 kOe) at low temperatures are observed in most compounds, which exceed similar to 50 kOe in the two Sm based compound. (C) 2011 Elsevier B.V. All rights reserved.