Magnetic and Crystal Structures of the Trigonal Tb2C

Neutron powder‐diffraction measurements have shown that Tb₂C is isostructural with Y₂C, i.e., crystallizes with an anti‐CdCl₂‐type trigonal structure ${\mathrm{(D}}_{\text{3d}}^5\text{\hspace{0.17em}−\hspace{0.17em}R3̄m)}$ , having the following crystallographic parameters in the hexagonal coordinates: the unit‐cell dimensions, $\text{a\hspace{0.17em}=\hspace{0.17em}3.595\hspace{0.17em}±\hspace{0.17em}0.006 Å}$ and $\text{c\hspace{0.17em}=\hspace{0.17em}18.19\hspace{0.17em}±\hspace{0.17em}0.03 Å}$ at 300°K; three Tb₂C formula units per unit cell; $z_{\mathrm{Tb}}\text{\hspace{0.17em}=\hspace{0.17em}0.2593\hspace{0.17em}±\hspace{0.17em}0.0006}$ ; and $z_C\text{\hspace{0.17em}=\hspace{0.17em}0}$ . Tb₂C becomes ferromagnetic below $T_c\text{\hspace{0.17em}=\hspace{0.17em}266\hspace{0.17em}±\hspace{0.17em}2°}\mathrm{K}$ . The ordered moments are directed along the trigonal $c$ axis, and the saturation moment is ${\text{8.6\hspace{0.17em}±\hspace{0.17em}0.2 μ}}_B$ per Tb. The temperature variation of the ordered moment in the range $\text{260°}\mathrm{K}{\mathrm{\hspace{0.17em}<\hspace{0.17em}T}}_c\text{\hspace{0.17em}<\hspace{0.17em}5°}\mathrm{K}$ is represented by ${\text{8.6[1\hspace{0.17em}−\hspace{0.17em}(T\hspace{0.17em}/\hspace{0.17em}T}}_c{)}^2]$ in Bohr magnetons. The ferromagnetic ordering accompanies a small‐angle magnetic scattering inferring an additional ordered moment with the modulation repetition of roughly 100 Å. The maximum composite ordered moment is estimated to be about ${\text{9 μ}}_B$ , the value expected for the free Tb³⁺ ion.