Four oxtselenides of praseodymium: Pr10OSe14, Pr2OSe2, Pr2O2Se, and Pr4O4Se3

By reacting elemental praseodymium with selenium and selenium dioxide (SeO2 ) as oxygen source in suitable stoichiometric ratios, it is possible to pre pare the single-phase praseodymium(III) oxyselenides Pr10OSe14, Pr2OSe2, Pr 2O2Se, and Pr4O4Se3 each within seven days at 750 degreesC in torch-sealed evacuated silica tubes. The addition of equimolar amounts of CsCl as flux g uarantees quick and complete reactions to single-crystalline, water- and ai r-resistant products. Pr10OSe14 (tetragonal, I4(1)/acd; a = 1568.74(8), c = 2073.4(1) pm; Z = 8) crystallizes as dark-red polyhedra. Pr2OSe2 (monoclin ic, P2(1)/c; a = 882.05(6), b = 732.89(5), c = 732.94(5) pm, beta = 100.288 (7)degrees; Z = 4) and Pr2O2Se (trigonal, P (3) over bar m1; a = 401.12(3), c = 705.51(5) pm; Z = 1) accumulate as yellowish green platelets with rect angular and hexagonal cross-sections, respectively. Pr4O4Se3 (orthorhombic, Amm2; a = 849.92(6), b = 402.78(3), c = 1292.57(9) pm; Z = 2) forms lath-s haped, pleochroitic crystals with a strong tendency for twinning, which app ear green along [001], but red along [100] and [010]. All the crystal structures of these oxyselenides are dominated by [OPr4] te trahedra, whose condensation rate strongly increases with growing oxygen co ntent. Se2- anions, in the case of Pr4O4Se, (equivalent to{(Pr3+)(4)(O2-)(4 )(Se2-)[Se-2](2-)}) as well as [Se2]2- dumb-bells, take care of charge bala nce and three-dimensional cross-linkage. In the oxygen-poor Pr10OSe14 the [ OPr4](10+) tetrahedra occur isolated and are embedded in the complex anioni c matrix framework (3)(infinity){(Pr6Se14)(10-)}. The oxygen-rich links in this row show according to (2)(infinity){[OPr3/3Pr1/1]}Se-2 (equivalent to Pr2OSe2), (2)(infinity){([OPr4/4](2))}Se (equivalent to Pr2O2Se), and (2)(i nfinity){([OPr4/4](4))}-[Se-2]Se (equivalent to Pr4O4Se3) [OPr4] tetrahedra which are connected to more or less dense layers via corners and respectiv ely one, three and four common edges.