Crystal Structures, Phase-Transition, and Photoluminescence of Rare Earth Carbodiimides

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 (R3̅c) modification for RE = Tm−Lu. The space group R3̅c is confirmed by single-crystal structure determination on Lu2(CN2)3 and indexed powder patterns of RE = Tm, Yb and Lu. The use of diverse chemical syntheses conditions for Tm2(CN2)3 revealed the dimorphic character of this compound. In addition, pressure experiments on Tm2(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 Gd2(CN2)3:Ln samples is presented with different activators (Ln = Ce, Tb) revealing a broad band emission of Gd2(CN2)3:Ce, quite similar to that of the well-known YAG:Ce.