The True Nature of SmSb 2 O 4 Cl: Syntheses and Crystal Structures of Sm 2 [Sb 4 O 8 ]Cl 2 and Eu 2 [Sb 4 O 8 ]Cl 2

The two lanthanoid oxidoantimonate(III) chlorides SmSb 2 O 4 Cl and EuSb 2 O 4 Cl are accessible from solid‐state reactions of Sb 2 O 3 with Ln 2 O 3 and Ln Cl 3 ( Ln =Sm and Eu) at 750 °C for two days. They crystallize in the centrosymmetric tetragonal space group P 4/ ncc with the lattice parameters a =787.13(4) pm, c =1765.24(12) pm for SmSb 2 O 4 Cl and a =783.56(4) pm, c =1764.05(12) pm for EuSb 2 O 4 Cl with Z =8. Both can also be described with the crystal‐chemical formula Ln 2 [Sb 4 O 8 ]Cl 2 for Z =4, since they comprise isolated [Sb 4 O 8 ] 4− rings. This structural motif has some very close similarities to the known series of non‐centrosymmetric Ln Sb 2 O 4 Cl representatives ( Ln =Gd–Lu), crystallizing in the tetragonal space group P 42 1 2. All lanthanoid(III) cations have eight oxygen atoms as nearest neighbors arranged as square prisms [ Ln O 8 ] 13− , which are connected to layers by four parallel edges according to with fluorite‐like topology. The Sb 3+ cations together with three oxygen atoms each and their lone‐pair of electrons form ψ 1 ‐tetrahedra [SbO 3 ] 3− . Four of these [SbO 3 ] 3− entities join to rings with four bridging and four terminal oxygen atoms. Both centrosymmetric representatives, in contrast to the series of non‐centrosymmetric ones, have a doubled lattice parameter c and several more symmetry elements, which will be discussed in detail.