Synthesis, structure, and properties of Cs4Th4P4Se26: A quaternary thoriumselenophosphate containing the (P2Se9)(6-) anion

Orange crystals of Cs4Th4P4Se26 were grown from the reaction of Th-232 and P in a Cs2Se3/Se Molten salt flux at 750 degreesC. Cs4Th4P4Se26 crystallize s in the orthorhombic space group Pbca with the unit cell parameters: a 12. 0130(6), b = 14.5747(7), c = 27.134(1) Angstrom; Z = 8. The compound exhibi ts a three-dimensional structure, consisting of dimeric [Th2Se13] polyhedra l units. The two crystallographically independent, nine-coordinate, bicappe d trigonal prismatic thorium atoms share a triangular face to form the dime r, and each dimer edge-shares two selenium atoms with two other dimers to f orm kinked chains along the [010] direction. While this structure shares fe atures of the previously reported Rb4U4P4Se26, including phosphorus in the 5+ oxidation state, careful inspection of the structure reveals that the se lenophosphate anion that knits the structure together in three directions i n both compounds is a unique (P2Se9)(6-) anion. The formula may be describe d best as [CS2Th2(P2Se9)(Se-2)(2)](2). The (P2Sc9)(6-) anion features a nea rly linear Se-Se-Se backbone with an angle of 171 degrees and Se-Se distanc es that are approximately 0.2-0.3 Angstrom longer than the typical single S e-Se bond. Magnetic studies confirm that this phase contains Th(IV). Raman data for this compound is reported, and structural comparisons will be draw n to its uranium analogue, Rb4U4P4Se26.