Relationship between structure and intrinsic magnetic parameters of nanocrystalline Sm2(Fe,Ga)17C2

The nanocrystalline carbides Sm2Fe17−xGaxC2 (x ≤ 2) were prepared with a two-step method: high-energy milling with annealing at 1050 °C and carbonation at 420 °C by a solid–solid reaction with heavy hydrocarbon. The Rietveld analysis reveals an anisotropic volume expansion of the R 3 ¯ m unit cell with modifications of the atomic positions compared to those of the non-carbonated alloys. The average iron magnetic moment, for carbonated alloys, deduced from high magnetic field investigations at 4 K, decreases slightly with x. The Curie temperature TC enhancement after carbonation is reduced by the presence of Ga. TC follows the reduction of the Wigner–Seitz cell (WSC) volume of the carbon site affected also by the presence of Ga. A detailed analysis of the Mössbauer spectra has been carried out with a model based on the WSC volume correlation with isomer shift and the statistical distribution of Ga simulated by the multinomial law. The isomer shift sequence of each Fe non-equivalent site, δ{6c} > δ{18h} > δ{18f} > δ{9d} for x < 1 and δ{18h} > δ{6c} > δ{18f} > δ{9d} for x ≥ 1, reflects the perturbation brought about by C and Ga, in perfect agreement with the structural analysis. Upon carbonation the isomer shift increases for 18f and 18h results not only from the volume effect but also from the 4s(Fe)–2p(C) inter-atomic charge transfer, modulated by the 4s(Fe)–4p(Ga) transfer. The hyperfine field sequence H{6c} > H{9d} > H{18f} > H{18h}, deduced from the isomer shift assignment, is in agreement with the number of Fe neighbors.