Rare earth carbodiimides with the general formula RE2(CN2)(3) crystallize with two modifications. A monoclinic (C2/m) modification is obtained for RE = Y, Ce-Tm and a rhombohedral (R (3) over barc) modification for RE = Tm-Lu, The space group R (3) over barc is confirmed by single-crystal structure determination on Lu-2(CN2)(3) and indexed powder patterns of RE = Tm, Yb and Lu. The use of diverse chemical syntheses conditions for Tm-2(CN2)(3) revealed the dimorphic character of this compound. In addition, pressure experiments on Tm-2(CN2)(3) have induced a phase-transition from rhombohedral to monoclinic. This transformation comprises an increase of the coordination number of Tm from 6 to 7, and a unit-cell volume reduction in the order of 20 %. The photoluminescence behavior of lanthanide doped Gd-2(CN2)(3):Ln samples is presented with different activators (Ln = Ce, Tb) revealing a broad band emission of Gd-2(CN2)(3):Ce, quite similar to that of the well-known YAG:Ce.