La4N2S3: A new nitride sulfide of lanthanum with unprecedented crystal structure

The oxidation of lanthanum powder with sulfur and cesium azide (CsN3) in the presence of lanthanum tribromide (LaBr3) yields lanthanum nitride sulfide with the composition La4N2S3 when appropriate molar ratios of the reactants are used. Additional cesium bromide (CsBr) as a flux secures fast reactions (7 d) at 900 degrees C in evacuated silica tubes as well as the formation of almost black single crystals. The orthorhombic crystal structure (Pnnm, Z = 2) was determined from single crystal X-ray diffraction data (a = 641.98(4), b = 1581.42(9), c = 409.87(3) pm). Two crystallographically different La3+ cations are present, La1 resides in sixfold coordination of two N3- and four S2- anions forming a trigonal prism and La2 is coordinated by two N3- and five S2- in the shape of a monocapped trigonal prism. However, the main feature of the crystal structure comprises N3--centred (La3+)(4) tetrahedra which arrange as pairs [N2La6](12+) of edge-shared [NLa4](9+) units and which are further connected via four vertices to form double chains (1)(infinity){[N2La2La'(4/2)](6+)}. They get bundled along [001] like a hexagonal rod packing and are held together by two crystallographically different S2- anions. Further motifs for the connectivity of [NM4](9+) tetrahedra in crystal structures of nitride chalcogenides and halides of the rare-earth elements (M = Sc, Y, La; Cc - Lu) with ratios of N : M = I : 2 are presented and discussed for comparison.