LuF[SeO3] and LuCl[SeO3]: Two non-isotypic halide oxoselenates(IV) of lutetium

Despite the formal similarity of LuF[SeO3] and LuCl[SeO3] both compounds show significant structural differences due to the different positions of the halide anions (X-) within the pentagonal bipyramids [LuO5X2](9-). However, both oxoselenates(IV) have these pentagonal bipyramids as basic modules in common that are connected via O2- edges to (1)(infinity){[(LuO4/2O1/1X2/2c)-O-e-X-t](4-)} chains. LuCl[SeO3] crystallizes orthorhombically in space group Pnma (no. 62; a = 714.63(7), b = 681.76(7) and c = 864.05(9) pm; Z = 4), The structure is isotypic to that one recently presented for ErCl[SeO3]. With a single Cl- anion in each an apical and an equatorial position, the (1)(infinity){[(LuO4/2O1/1Cl2/2c)-O-e-Cl-t](4-)} chains have to be inclined with an angle of about 54 degrees relative to each other to get connected alternately by common Cl- corners and bridging [SeO3](2-) pyramids. In contrast to that, LuF[SeO3] which crystallizes triclinically in space group P (1) over bar (no. 2; a = 644.85(6), b = 684.41(7), c = 427.98(4) pm, alpha = 94.063(5), beta = 96.484(5) and gamma 91.895(5)degrees; Z = 2) takes a structural motif already known from CsTmCl2[SeO3]. Owing to the apical position of both halide anions it is now possible to connect the (1)(infinity){[(MO4/2O1/1Cl2/2c)-O-e-Cl-t](5-)} chains directly via discrete Psi-tetrahedral [SeO3](2-) groups to (2)(infinity){(MCl2[SeO3])(-)} layers. The same layers are present in LuF[SeO3] and without the formal alkali-metal halide unit (CsCl) of the CsTmCl2[SeO3]-type compounds, the (2)(infinity){(LuF2[SeO3])(-)} layers can also be connected directly by common F- corners to a three-dimensional array. Torch-sealed evacuated silica ampoules were used for the synthesis of both lutetium(III) halide oxoselenates(IV). For LuF[SeO3] these vessels have been graphitized before to prevent them from oxosilicate-producing side-reactions with the applied fluoride. The synthesis of LuCl[SeO3] required Lu2O3 and SeO2 in a molar ratio of 1 : 6 with a surplus of an eutectic RbCl/LiCl mixture as fluxing agent and an annealing period of five weeks at a temperature of 500 degrees C, whereas Lu2O3, LuF3 and SeO2 (in a molar ratio of I : I : 3) with CsBr as flux were converted to LuF[SeO3] at 750 degrees C within six days.