Magnetic order in CeI2
CeI2 is a halide with metallic conductivity according to the formula (Ce3+)(I-)2(e-). Magnetic interactions are mediated by the conduction electrons and are significantly stronger than for insulating rare-earth halides. CeI2 crystallizes in the MoSi2 layer-type structure with space group I4/mmm and lattice parameters a=3.87212(5) Å and c=13.8646(2) Å at 15 K. At TN=10.0(2) K, a phase transition to a 3D antiferromagnetically ordered state occurs with an incommensurate k vector of (0, 0.2761(1), 1) at 1.5 K. The k vector is temperature-dependent and, as the sublattice magnetization, reaches saturation below 4 K. CeI2 has one magnetic sublattice with moments parallel to the a axis. The amplitude is sine-modulated along the b axis and amounts to 1.87(1) μB per Ce3+ ion. The crystal-field splitting reduces the ordered moment slightly below the 2F5/2 free-ion value of g·J=15/7 for Ce3+.