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

In this work we report the results of the neutron diffraction investigation performed on the germanides R15Ge9C, for R = Ce, Pr and Nd (La15Ge9Fe-type, hP50, P63mc, Z = 2), to refine the crystal superstructure of these compounds and determine their magnetic structures. The interstitial carbon atoms occupy mainly the 2b Wyckoff site in the position (1/3 2/3 ∼1/2) and also, with a smaller occupancy rate, the Wyckoff site 2a 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 TC = 10, 30 and 80 K as Curie temperature of Ce15Ge9C, Pr15Ge9C and Nd15Ge9C, respectively. Ce15Ge9C and Nd15Ge9C present a ferromagnetic alignment of the R moments along the c-axis and an antiferromagnetic spin arrangement within the (a–b) plane. For Pr15Ge9C the ferromagnetic contribution is found within the (a–b) plane, as previously observed for the isotypic compound Tb15Si9C. The carbides crystal structure possesses four inequivalent rare earth sites carrying different magnetic moments, leading to mean values of 0.9 μB/Ce, 1.1 μB/Pr and 2.2 μB/Nd for Ce15Ge9C, Pr15Ge9C and Nd15Ge9C, respectively. The magnetic structures of these R15Ge9C compounds differ strongly from those of their parent R5Ge3 germanides, but present strong similarities with the structures of the R15Si9C compounds. The overall results indicate and confirm the drastic influence of carbon insertion in the rare earth environment.