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 (D3d5 − R3̄m), having the following crystallographic parameters in the hexagonal coordinates: the unit-cell dimensions, a = 3.595 ± 0.006 Å and c = 18.19 ± 0.03 Å at 300°K; three Tb2C formula units per unit cell; zTb = 0.2593 ± 0.0006; and zC = 0. Tb2C becomes ferromagnetic below Tc = 266 ± 2°K. The ordered moments are directed along the trigonal c axis, and the saturation moment is 8.6 ± 0.2 μB per Tb. The temperature variation of the ordered moment in the range 260°K < Tc < 5°K is represented by 8.6[1 − (T / Tc)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 9 μB, the value expected for the free Tb3+ ion.