Über Sesquiselenide der Lanthanoide: Einkristalle von Ce 2Se3 im C-, Gd2Se3 im U- Und Lu 2Se3 im Z-Typ

Abstract

Single crystals of lanthanoid sesquiselenides (M2Se 3; here: M = Ce, Gd, Lu) are accessible through conversion of the elements (lanthanoid and selenium) in molar ratios of 2:3 within seven days at 850°C from evacuated silica ampoules if equimolar amounts of NaCl serve as a flux. In the case of Ce2Se3 (a = 897.74(6) pm) und Gd2Se3 (a = 872.56(5) pm) the cubic C-type (I4̄3d, Z = 5.333) forms as dark red beads, whereas the orthorhombic Z-type (Fddd, Z = 16) emerges for Lu2Se3 (a = 1125.1(1), b = 798.06(8), c = 2387.7(2) pm) as orange-yellow bricks. Upon oxidation of monochloride hydrides (MClHx or AyMClHx; M = Ce, Gd, Lu; x = 1; A = Li, Na; y = 0.5) with selenium in arc-welded tantalum ampoules the same main products appear with C-Ce2Se3 and Z-Lu2Se 3, even with a surplus of NaCl or LiCl as fluxing agent. In the case of Gd2Se3, however, black-red needles of the orthorhombic U-type (Pnma, Z = 4; a = 1118.2(1), b = 403.48(4); c = 1097.1(1) pm) are yielded instead of C-Gd2Se3. C-Ce2Se 3 crystallizes in a cation-deficient Th3P 4-type structure (Ce2S3 type) according to Ce2.667□0.333Se4 (Z = 4) or with Z = 5.333 for the empirical formula Ce2Se3. Here, Ce3+ is coordinated by eight Se2- anions trigon-dodecahedrally. In U-Gd2Se3 (U2S3 type) two crystallographically independent Gd3+ cations with coordination numbers of 7 (Gd1) and 7+1 (Gd2), respectively, are present, exhibiting mono- or bicapped trigonal prisms as coordination polyhedra. The crystal structure of Z-Lu2Se3 (Sc2S3 type) shows two different Lu3+ cations as well, which now both reside in octahedral coordination of six Se2- anions each.