Effect of Substitutional Anions on the Magnetic Properties of the Mononitrides of Uranium and Gadolinium

The mononitrides of uranium and gadolinium have the NaCl-type structure and are good electrical conductors. UN is antiferromagnetic of type I (TN∼50 K); GdN is ferromagnetic (TC∼70 K). In UN limited oxygen substitution raises TN, while carbon substitution lowers TN, and UN0.9C0.1 does not order. Oxygen substitution in GdN lowers the paramagnetic Curie temperature θ and GdN0.95O0.05 and GdN0.90O0.10 do not order or at most have short-range antiferromagnetic order; carbon substitution in GdN raises TC or θ. The different orderings in UN and GdN and the variations in TN or TC due to anion substitution are accounted for by a simple model, originally proposed for UX compounds, in terms of variations in the number Z of conduction electrons per cation. Assuming U4+ and Gd3+ cations and N∼3− anions, and RKKY-type interactions between the localized f electrons, the observed ordering in UN is most stable for Z∼1 and in GdN for Z∼0.1. Substituting O−2 anions for nitrogen increases Z in both UN and GdN; carbon substitution decreases Z. The variations of magnetic ordering and θ with Z are discussed for anion-substituted GdN and UN as well as other monopnictides of Gd and U.