Incommensurate and commensurate magnetic structures of the ternary germanide CeNiGe₃

The structural properties of CeNiGe₃ have been investigated via electron diffraction and neutron powder diffraction (NPD). This ternary germanide crystallizes in the orthorhombic SmNiGe₃-type structure (Cmmm space group). Electrical resistivity, ac- and dc-magnetization measurements show that CeNiGe₃ orders antiferromagnetically below T_N = 5.5(2) K and exclude the occurrence at low temperatures of a spin-glass state for CeNiGe₃ as previously reported. Specific heat measurements and NPD both reveal two magnetic transitions, observed at T_N1 = 5.9(2) K and T_N2 = 5.0(2) K. Between T_N1 and T_N2, the Ce magnetic moments in CeNiGe₃ are ordered in a collinear antiferromagnetic structure associated with the k₁ = (100) wavevector and showing a relationship with the magnetic structure of the Ce₃Ni₂Ge₇ ternary germanide. Below T_N2, this k₁ = (100) commensurate magnetic structure coexists with an incommensurate helicoîdal magnetic structure associated with k₂ = (00.409(1)1/2). This last magnetic structure is highly preponderant below T_N2 (93(5)% in volume). At 1.5 K, the Ce atoms in CeNiGe₃ carry a reduced ordered magnetic moment (0.8(2) μ_B). This value, smaller than that obtained in Ce₃Ni₂Ge₇, results from an important hybridization of the 4f(Ce) orbitals with those of the Ni and Ge ligands.