CsTmCl2[SeO3]: a quinary phase in the system CsCl/TmOCl/SeO2

Abstract

CsTmCl2[SeO3] was prepared by reaction of a stoichiometric mixture of Tm2O3, TmCl3, SeO2 and CsCl in an evacuated torch-sealed silica tube at 830°C for 7 days as colourless single crystals (monoclinic, P21/n; a=658.92(5), b=689.26(6), c=1752.5(1)pm, β=99.093(7)°, Z=4; κ-CCD, Rall=0.035, GooF=1.090). The structure is characterized by layers parallel to (001) consisting of parallel zigzag strands ∞1{[TmOe4/2Ot1/1Clt2/1]5−}(≡∞1{[TmO3Cl2]5−}) with both chloride anions occupying apical positions in the pentagonal bipyramid [TmO5Cl2] (d(Tm–O)=225–240pm, d(Tm–Cl)=261pm, 2×). Four of the five equatorial oxygen atoms are utilized to share trans-edges, whereas one remains terminal and is syndiotactically ordered along the chain direction [010]. Within the layers these different chains are held together by Se4+ cations via both an edge and a vertex of a trigonal [SeO3]2− pyramid. The Cs+ coordination sphere can be described as a (2+1)-capped trigonal prism (d(Cs–Cl)=336–385pm, 6×) with all three oxygen atoms (d(Cs–O)=320, 383 and 407pm) being the triangular face of a single [SeO3]2− anion. The crystal structure is strongly influenced by the stereochemical lone-pair effect at the Se4+ cations reflected by the formation discrete of ψ1-tetrahedral [SeO3E]2− groups (E=non-binding electron pair) with Se–O distances of 166, 170 and 171pm.