NEW LANTHANIDE SELENOPHOSPHATES - INFLUENCE OF FLUX COMPOSITION ON THE DISTRIBUTION OF [PSE4](3-) [P2SE6]4(-) UNITS AND THE STABILIZATION OF THE LOW-DIMENSIONAL COMPOUNDS A(3)REP(2)SE(8), AND A(2)(RE)P2SE7 (A = RB, CS RE = CE, GD)/

The reaction of Ce or Gd with a molten mixture of A(2)Se/P2Se5/Se (A = Rb, Cs) produced the quaternary compounds Rb3CeP2Se8 (I) and Cs3GdP2S e8 (II), as well as Rb2CeP2Se7 (III) and Rb2GdP2Se7 (IV). The orange c rystals of I, II and the red crystals of III, IV are air- and water-se nsitive. Compounds I, II crystallize in the monoclinic space group P2( 1)/c, and for I, a = 9.6013(2) Angstrom, b = 18.0603(1) Angstrom, c = 10.0931(1) Angstrom, 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) Angstrom, b = 7.212(1) Angstrom, c = 20.299(2) Angstrom, b eta = 98.23(1)degrees, and Z = 4. Compounds I, II have a 1-D structure with [(RE)P2Se8](n)(3n-) chains separated by A(+) cations. The RE3+ l anthanide cation is in a bicapped trigonal prismatic coordination with four tridentate [PSe4](3-) ligands. Compounds III, IV possess a 2-D s tructure. Each layer consists of [RE(PSe4)](x) ''chains'' interstitche d in two dimensions by hexadentate [P2Se6](4-) ligands. The trivalent RE cation is in a square antiprismatic coordination. The compounds wer e characterized with differential thermal analysis, far-IR, and solid- state UV/vis diffuse reflectance spectroscopy. Magnetic measurements i ndicate that the compounds obey the Curie law with mu(eff) values clos e to those of the free RE3+ ions.