Crystalline samples of rare earth carbodiimides were synthesized by solid-state metathesis reactions of rare earth trichlorides with lithium cyanamide in sealed silica ampules. Two distinct structures were determined by single-crystal X-ray diffraction. The structure determined for Sm-2(CN2)(3) [C2/m, Z = 2, a = 14.534(2) angstrom, b = 3.8880(8) angstrom, c = 5.2691(9) angstrom, beta = 95.96(2)degrees R-1 = 0.0267, and wR(2) = 0.0667] was assigned for RE2(CN2)(3) compounds with RE = Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er, and the structure determined for Lu-2(CN2)(3) [R 32, Z = 3, a = 6.2732(8) angstrom, c = 14.681(2) angstrom, R-1 = 0.0208, and wR(2) = 0.0526] was assigned for the smallest rare earth ions with RE = Tm, Yb, and Lu by powder X-ray diffraction. Both types of crystal structures are characterized by layers of [NCN](2-) ions whose arrangements can be derived from the motif of a closest packed layer of sticks. These layers alternate with layers of rare earth ions in a one-by-one sequence. Different tilting arrangements of the N-C-N-axes relative to the stacking directions (c) and different arrangements of RE3+ ions within metal atom layers account for the two distinct structures in which Sm3+ and Lu3+ ions adopt the coordination numbers 7 and 6, respectively.