Magnetic Ordering in Nitrides with the η-Carbide Structure: (Ni,Co,Fe)2(Ga,Ge)Mo3N

Compositions in the series Ni2-xCoxGeMo3N (0 ex e2),Co2Ge1-xGaxMo3N (0<x e0.7),Co2-xFexGeMo3N (0 e x e 2), and Co2-xFexGe0.5Ga0.5Mo3N (0 e x e 0.8) have been synthesized by the reductive nitridation of binary oxides and studied by appropriate combinations of magnetometry, transport measurements, neutron diffraction, and M€ossbauer spectroscopy. All of these compositions adopt the cubic η-carbide structure (a ∼ 11.11 A ̊ ) and show a resistivity of ∼10 -3 Ω cm. No long-range magnetic order was observed in Ni 2-xCoxGeMo3N, although evidence of spin freezing was observed in Co2GeMo3N. The introduction of gallium into this composition leads to the onset of antiferromagnetic ordering at 90 K in Co2Ge0.3Ga0.7Mo3N. The magnetic structure consists of an antiferromagnetic arrangement of ferromagnetic Co4 groups, with an ordered magnetic moment of 0.48(9) μB per cobalt atom. The same magnetic structure is found in Co0.5Fe1.5GeMo3N and Co1.2Fe0.8Ge0.5Ga0.5Mo3N. The former orders above room temperature with an average moment of 1.08(3) μB per transition-metal site, and the latter at 228 K with an average moment of 1.17(4) μB per site. The magnetic behavior of these compounds is discussed in terms of the electron count within each series.