New Lanthanide Selenophosphates. Influence of Flux Composition on the Distribution of [PSe(4)](3)(-)/[P(2)Se(6)](4)(-) Units and the Stabilization of the Low-Dimensional Compounds A(3)REP(2)Se(8), and A(2)(RE)P(2)Se(7) (A = Rb, Cs; RE = Ce, Gd).

The reaction of Ce or Gd with a molten mixture of A(2)Se/P(2)Se(5)/Se (A = Rb, Cs) produced the quaternary compounds Rb(3)CeP(2)Se(8) (I) and Cs(3)GdP(2)Se(8) (II), as well as Rb(2)CeP(2)Se(7) (III) and Rb(2)GdP(2)Se(7) (IV). The orange crystals of I, II and the red crystals of III, IV are air- and water-sensitive. Compounds I, II crystallize in the monoclinic space group P2(1)/c, and for I, a = 9.6013(2) Å, b = 18.0604(1) Å, c = 10.0931(1) Å, beta = 90.619(1) degrees, and Z = 4. Compounds III, IV crystallize in the monoclinic space group P2(1)/n, and for IV, a = 10.137(2) Å, b = 7.212(1) Å, c = 20.299(2) Å, beta = 98.23(1) degrees, and Z = 4. Compounds I, II have a 1-D structure with (RE)P(2)Se(8)()(3)(n)()(-) chains separated by A(+) cations. The RE(3+) lanthanide cation is in a bicapped trigonal prismatic coordination with four tridentate PSe(4)(-) ligands. Compounds III, IV possess a 2-D structure. Each layer consists of RE(PSe(4))() "chains" interstitched in two dimensions by hexadentate P(2)Se(6)(-) ligands. The trivalent RE cation is in a square antiprismatic coordination. The compounds were characterized with differential thermal analysis, far-IR, and solid-state UV/vis diffuse reflectance spectroscopy. Magnetic measurements indicate that the compounds obey the Curie law with &mgr;(eff) values close to those of the free RE(3+) ions.