Nd4N2Se3 and Tb4N2Se3:: Two non-isotypical lanthanide(III) nitride selenides

The non-isotypical nitride selenides M(4)N(2)S(e)3 of neodymium (Nd4N2Se3) and terbium (Tb4N2Se3) are formed by the reaction of the respective rare-earth metal with sodium azide (NaN3), selenium and the corresponding rare-earth tribromide (MBr3) at 900 degreesC in evacuated silica ampoules after seven days. Each of them crystallizes monoclinically in the space group C2/c with Z = 4 for Nd4N2Se3 (a = 1300.47(4), b = 1009.90(3), c = 643.33(2) pm, beta = 90.039(2)degrees) and in the space group C2/m with Z = 2 for Tb4N2Se3 (a = 1333.56(5), b = 394.30(2), c = 1034.37(4) pm, beta = 130.377(2)degrees), respectively. The crystal structures differ fundamentally in the linkage of the structure dominating N3- centred (M3+)(4) tetrahedra. In Nd4N2Se3, the [NNd4] units are edge-linked to bitetrahedra which are cross-connected to (2)(infinity)[N(Nd1)(2/2)(k)(Nd2)(2/2)(e)](3+) layers via their remaining four corners, whereas the [NTb4] tetrahedra in Tb4N2Se3 share cis-oriented edges to form strands (1)(infinity)[N(Tb1)(3/3)(k)(Tb2)(1/1)(t)](3+). Both structures contain two crystallographically different M3+ cations, that show coordination numbers of six and seven (Nd4N2Se3) or twice six (Tb4N2Se3), respectively, relative to the anions (N3- and Se2-). Each of the two independent kinds of Se2- anions provide the three-dimensional linkage as well as the charge balance. The particular axial ratio a/c and the monoclinic reflex angle offer two choices for fixing the unit cell of Tb4N2Se3.