Neutron-Diffraction Studies of La2C3, Ce2C3, Pr2C3, and Tb2C3

The neutron diffraction data of the rare-earth sesquicarbides with the Pu2C3-type structure (D5c type), La2C3, Ce2C3, Pr2C3, and Tb2C3, were analyzed using the complete-matrix least-squares method. These sesquicarbides contain the C2 groups with the C–C distances, 1.236±0.009, 1.276±0.005, 1.239±0.003, and 1.240±0.005 A, respectively, for La2C3, Ce2C3, Pr2C3, and Tb2C3. These values are, except for Ce2C3, significantly longer than the C–C distance in CaC2, 1.191 A, but are shorter than the average C–C distance in the rare-earth dicarbides, 1.278 A. The paramagnetic scattering analyses show that all metal atoms in these sesquicarbides are in their trivalent Hund ground state, except Ce in Ce2C3, whose possible valency is about 3.4. The screening constants for the 4f hydrogenic radial wave functions of Pr and Tb as determined from the diffuse scattering analyses of their sesquicarbides are 40 and 43, respectively. The valency and metallic bonds in these sesquicarbides are briefly discussed.