A neutron diffraction study of the R15Ge9C compounds (R=Ce, Pr, Nd)

Abstract In this work we report the results of the neutron diffraction investigation performed on the germanides R 15 Ge 9 C, for R = Ce, Pr and Nd (La 15 Ge 9 Fe-type, hP 50, P 6 3 mc , Z  = 2), to refine the crystal superstructure of these compounds and determine their magnetic structures. The interstitial carbon atoms occupy mainly the 2 b Wyckoff site in the position (1/3 2/3 ∼1/2) and also, with a smaller occupancy rate, the Wyckoff site 2 a at (0 0 ∼1/2). In the magnetic state, the three compounds display predominantly a ferromagnetic behavior with the propagation vector k  = [0 0 0]. These results are in agreement with the magnetization measurements, with T C  = 10, 30 and 80 K as Curie temperature of Ce 15 Ge 9 C, Pr 15 Ge 9 C and Nd 15 Ge 9 C, respectively. Ce 15 Ge 9 C and Nd 15 Ge 9 C present a ferromagnetic alignment of the R moments along the c -axis and an antiferromagnetic spin arrangement within the ( a – b ) plane. For Pr 15 Ge 9 C the ferromagnetic contribution is found within the ( a – b ) plane, as previously observed for the isotypic compound Tb 15 Si 9 C. The carbides crystal structure possesses four inequivalent rare earth sites carrying different magnetic moments, leading to mean values of 0.9 μ B /Ce, 1.1 μ B /Pr and 2.2 μ B /Nd for Ce 15 Ge 9 C, Pr 15 Ge 9 C and Nd 15 Ge 9 C, respectively. The magnetic structures of these R 15 Ge 9 C compounds differ strongly from those of their parent R 5 Ge 3 germanides, but present strong similarities with the structures of the R 15 Si 9 C compounds. The overall results indicate and confirm the drastic influence of carbon insertion in the rare earth environment.