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

Abstract

The neutron diffraction data of the rare‐earth sesquicarbides with the Pu₂C₃‐type structure (D5_c 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.