BISMUTH(II) CHALCOGENOMETALLATES(III) BI2 M4X8, COMPOUNDS WITH BI-2(4+) DUMBBELLS (M = AL, GA AND X = S, SE)

The ternary bismuth(II) chalcogenometallates(III) Bi2M4X8 (With M = Al , Ga and X = S, Se) were synthesized from the binary chalcogenides M2X 3 and Bi2X3 and elementary bismuth. All compounds are diamagnetic semi conductors with E-g (opt.) = 1.8-2.7eV. The phases (except Bi2Al4Se8) are thermodynamically stable and decompose peritectically above 965-10 20 K. Bi2Al4Se8 is metastable below 825 K and is obtained only by rapi d quenching from T > 825 It. The isotypic compounds crystallize in a n ew tetragonal tP28 structure type (P4/nnc). The characteristic unit is the hitherto unknown clustercation Bi-2(4+), with the mean bond lengt h (d) over bar(Bi-Bi) = 314.2 pm, the Raman frequency 102 cm(-1) less than or equal to v(s) less than or equal to 108 cm(-1) and the mean fo rce constant of (f) over bar = 0.68 N cm(-1). The Electron Localizatio n Function, ELF, shows the covalent Ei-Bi bond, the lone electron pair s of the psi-octahedrally coordinated Bi(II) cations, and the polar ch aracter of the Bi-X bonds.