A Structural Comparison of Two Thallium Copper Erbium Sulfides: TlCu3Er2S5 and Tl2Cu5Er3S8

Abstract

Orange TlCu3Er2S5 crystallizes in the orthorhombic space group Cmcm (a = 391:27(2), b = 1382:91(7), c = 1630:54(8) pm, Z = 4). Its crystal structure contains one Tl+, one Er3+, two Cu+, and three S2- ions as crystallographically unique components. All Cu+ cations are coordinated by four S2- anions to form [CuS4]7- tetrahedra, which have vertex- and edge-connectivity to build up 2 ∞{[Cu3S5]7-} layers parallel to the (010) plane. The Er3+ cations are octahedrally surrounded by six S2- anions. These [ErS6]9- units share trans-oriented edges forming linear chains, which are further linked by common vertices yielding 1 ∞{[Er2S10]14-} double strands. Further fusion via common vertices results in 2 ∞{[Er2S5]4-} layers parallel to the (010) plane. Alternatingly stacked along [010] and condensed via S2- anions, these two types of layers form a three-dimensional framework, leaving tunnels along [100], where the Tl+ cations fit in. These are each coordinated by eight S2- anions in the shape of bicapped trigonal prisms. Tl2Cu5Er3S8 was obtained as black single crystals with metallic luster (monoclinic, Cm, a = 1381:69(7), b = 390:73(2), c = 1435:98(7) pm, β = 111:132(3)°, Z =2). Four of the five crystallographically different Cu+ cations are tetrahedrally surrounded by S2- anions, whereas (Cu3)+ is situated in sulfur triangles. Connected via common vertices and edges, the copper-sulfur polyhedra form strongly corrugated 2 ∞{[Cu5S8]11-} layers parallel to the (201) plane. All Er3+ cations are again coordinated by six S2- anions in the shape of [ErS6]9- octahedra. Their condensation via edges and vertices leads to 2 ∞{[Er3S8]7-} layers spreading out parallel to the (010) plane. Both types of anionic layers result in a three-dimensional framework through sulfide fusion, which leaves tunnels running along [010] occupied by Tl+ cations. The two crystallographically different Tl+ cations both have eight coordinating S2- anions forming bicapped trigonal prisms. Remarkably enough, the ion closest to (Tl1)+ is not a S2- anion (shortest Tl+-S2- bond: 305 pm), but a (Cu3)+ cation at a distance of just 291 pm.