The complete‐matrix least‐squares analysis on the neutron diffraction intensities of CaC2, YC2, LaC2, CeC2, TbC2, YbC2, LuC2, and UC2 yielded the C☒C distances in the C2 groups, 1.191±0.009, 1.275±0.002, 1.303±0.012, 1.283±0.004, 1.293±0.009, 1.287±0.010, 1.276±0.012, and 1.340±0.007 A, respectively. The statistical averages of the neutron scattering amplitudes obtained from the dicarbide data and those reported previously are 0.488±0.007, 0.796±0.009, 0.832±0.014, 0.482±0.006, 0.756±0.020, 1.262±0.012, 0.73±0.02, and 0.851±0.022, all in 10—12 cm with positive sign, for Ca, Y, La, Ce, Tb, Yb, Lu, and U, respectively.
The paramagnetic scattering analyses revealed that all metal atoms in these dicarbides are in their trivalent Hund ground state, except Ca (divalent), Yb (possible valency of 2.8), and U (possible tetravalent). The scattering data also deduced the screening constants for the 4f electrons in Ce, Tb, and Yb as 39, 43, and 43, respectively, which are compared with those obtained by other methods. The bond numbers for the rare‐earth dicarbides are briefly discussed.