Nd4N2Se3 und Tb4N2Se3: Zwei nicht‐isotype Lanthanoid(III)‐Nitridselenide

Nd₄N₂Se₃ and Tb₄N₂Se₃: Two non‐isotypical Lanthanide(III) Nitride Selenides The non‐isotypical nitride selenides M₄N₂Se₃ 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 °C 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, β = 90.039(2)°) 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, β = 130.377(2)°), respectively. The crystal structures differ fundamentally in the linkage of the structure dominating N^3‐ centred (M³⁺)₄ tetrahedra. In Nd₄N₂Se₃, the [NNd₄] units are edge‐linked to bitetrahedra which are cross‐connected to [N(Nd1)(Nd2)]³⁺ layers via their remaining four corners, whereas the [NTb₄] tetrahedra in Tb₄N₂Se₃ share cis‐oriented edges to form strands [N(Tb1)(Tb2)]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 (N^3‐ und Se^2‐). Each of the two independent kinds of Se^2‐ 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.