Quaternary strontium copper(I) lanthanoid(III) selenides with cerium and praseodymium:: SrCuCeSe3 and SrCuPrSe3, unequal brother and sister

Quaternary strontium copper(I) lanthanoid(III) selenides are formed by the oxidation of elemental strontium, copper and the corresponding lanthanoid with selenium. Orange to red needle-shaped single crystals of SrCuPrSe3 and SrCuCeSe3 have been synthesized by heating mixtures of Sr, Cu, Pr / Ce and Se with CsI as a flux in evacuated silica tubes to 800 degreesC for 7 d. Both compounds crystallize orthorhombically in space group Pnma with four formula units per unit cell, but with unlike lattice constants (a = 1097.32(6), b = 416.51(2), c = 1349.64(8) pm for SrCuPrSe3 and a = 846.13(5), b = 421.69(2), c = 1663.42(9) pm for SrCuCeSe3) and therefore different structure types. The Pr3+ cations in SrCuPrSe3 are surrounded octahedrally by six Se2- anions forming chains of edge-sharing [PrSe6](9-) octahedra that are joined by common vertices. Together with [CuSe4](7-) tetrahedra they form [CuPrSe3](2-) layers piled up parallel (001). Between those layers the Sr2+ cations are coordinated by seven Se2- anions in the shape of capped trigonal prisms linking the structure in the third dimension. On the other hand in SrCuCeSe3 the Ce3+ cations as well as the Sr2+ cations adopt a coordination number of seven. Since the bonding distances between cerium and selenium match with those of strontium and selenium the two crystallographically independent sites of these cations are occupied statistically by Ce3+ and Sr2+ with equal ratios. Nevertheless, there is a close structural relationship between SrCuPrSe3 and SrCuCeSe3. Similar to SrCuPrSe3 where Cu+ and Pr3+ cations together with Se2- anions form [CuPrSe3](2-) layers parallel (001), the Cu+ cations and [(Cel/Srl)Se-7](11.5-) polyhedra in SrCuCeSe3 build strongly puckered layers which are connected by (Ce2)(3+)/(Sr2)(2+) cations. The copper selenium part in both compounds correlates as well, with [CuSe4](7-) tetrahedra linked by common vertices to form [CuSe3](5-) chains running along [010].