Cu1.45Er0.85S2: A copper(I) erbium(III) sulfide with cation-deficient CaAl2Si2-type structure

Attempts to synthesize single-phase CuYS2-type copper(I) erbium(III) disulf ide (CuErS2) from 1:1:2-molar mixtures of the elements (Cu, Er and S) alter seven days at 900 degreesC in sealed evacuated silica tubes failed with eq uimolar amounts of CsCl working as flux and reagent. In these cases, quater nary CsCu3Er2S5 (orthorhombic, Cmcm; a=394.82(4), b=1410.9(1), c=1667.2(2) pm, Z=4) and ternary Cu1.45Er0.85S2 (trigonal, P (3) over bar m1; a = 389.5 1(4), c = 627.14(6) pm, Z = 1) become the unexpected by-products. Both emer ge even as yellow single crystals (lath-shaped fibres and platelets, respec tively, with triangular cross-section) and both crystal structures contain condensed [CuS4] and [ErS6] units as dominating building blocks. The ternar y sulfide Cu1.45Er0.85S2 exhibits CdI2-analogous layers (2)(infinity) [(Er3 +)(S2-)(6/3)](-)} of edge-shared [ErS6] octahedra (d(Er-S) = 272 pm, 6x) wh ich are piled up parallel (001) and interconnected by interstitial Cu+ cati ons in tetrahedral S2- coordination (d(Cu-S)= 236 pm, 1 x; 240 pm, 3x). The latter thereby form anionic layers (2)(infinity)(([(Cu+)(S2-)(4/4)](-))(2) } as well, consisting of [CuS4] tetrahedra which share three cis-oriented e dges. When the S2- anions arrange hexagonally closest-packed and the corres ponding layers are symbolized with capital Roman letters, the Er3+ cations (small Roman) and the Cu+ cations (small Creek letters) reside layerwise al ternatingly within half of the octahedral (Er3+) and tetrahedral (Cu+) void s according to...AcB alpha beta AcB alpha betaA.... Since both kinds of cat ions occupy only a certain percentage (Cu+: 72.6%, Er3+: 85.1%) of their re gular positions, the crystal structure of Cu1.45Er0.85S2 can be addressed a s a double cation-deficient CaAl2Si2-type arrangement according to (Er(0.85 )square (0.15))(Cu(1.45)square (0.55))S-2. The partial occupation could be established by both released site occupation factors in the course of the c rystal structure refinement and electron beam X-ray microanalysis (EDX).