- or B - Type Structure

The B-type lanthanoid(III) chloride oxoselenates(IV) MCl[SeO3] (M = Sm–Lu) crystallize in the orthorhombic space group Pnma (no. 62) with Z = 4 in the structure type of HoCl[TeO3]. Their lattice constants are decreasing following the lanthanoid contraction from a = 730.01(7), b = 707.90(7), c = 895.64(9) pm for SmCl[SeO3] to a = 714.63(7), b = 681.76(7), c = 864.05(9) pm for LuCl[SeO3]. In contrast to NdCl[SeO3], the only representative of the A-type structure, where the coordination numbers of the Nd3+ cations are 7+2 and 8, the B-type structure is dominated by pentagonal bipyramids [MO5Cl2]9− (CN(M3+) = 7), which are connected via trans-oriented O· · ·O edges to 1 ∞{[MO4/2O1/1Cl2/1]} chains (e = edge-sharing, t = terminal) running parallel to the [010] direction. Their inclination relative to each other allows for an alternating interconnection of these chains via Cl− and ψ1-tetrahedral [SeO3]2− anions to form a three-dimensional structure. The distances within the [SeO3]2− groups are in the normal range (d(Se–O) = 165 – 172 pm), while those of the O2− and Cl− anions to the central M3+ cation diminish in dependence of the increasing atomic number (d(M–O) = 226 – 244 pm / 216 – 232 pm, d(M–Cl) = 277 – 278 pm / 266 – 270 pm, M = Sm / Lu). For the synthesis of the chloride oxoselenates(IV) MCl[SeO3] the respective lanthanoid sesquioxide (M2O3) and selenium dioxide (SeO2) were reacted with either an eutectic mixture of RbCl and LiCl or with the corresponding lanthanoid trichloride (MCl3) in evacuated silica ampoules for either five weeks at 500 ◦C or one week at 850 ◦C.