beta-Bi-4(P2Se6)(3): A new ternary selenophosphate obtained in a P2Se5 flux

Bi metal dissolves in excess molten phosphorus selenide at 500 degrees C to yield black, rodlike, air- and water-stable beta-Bi-4(P2Se6)(3) crystals. The phase crystallizes in the triclinic space group P (1) over bar (No. 2) with the room-temperature cell parameters a = 12.2303(8) Angstrom, b = 6.76 40(4) Angstrom, c = 17.866 Angstrom, ct = 90.493(6)degrees beta = 94.133(6) degrees, and gamma = 91.163(6)4 The new structure is a complicated, three-d imensional framework built from [Bi4Se22] ribbons connected to each other b y sharing selenium atoms. The [P2Se6](4-) ethane-like units are found insid e and between the [Bi4Se22] ribbons. The compound is a semiconductor with a band gap of 1.32 eV. It melts with decomposition at 512 degrees C, convert ing partially to Bi2Se3 and a PxSey glassy material. Far-IR and Raman spect ra show characteristic stretches for the [P2Se6](4-) ligand and Bi-Se vibra tions. The structural and chemical relationships of this compound with the a-form are discussed.