Bismuth(II) chalcogenometallates(III) Bi2M4X8, compounds with Bi-2(4+) dumbbells (M = Al, Ga and X = S, Se)

Abstract

The ternary bismuth(II) chalcogenometallates(III) Bi2M4X8 (With M = Al, Ga and X = S, Se) were synthesized from the binary chalcogenides M2X3 and Bi2X3 and elementary bismuth. All compounds are diamagnetic semiconductors with E-g (opt.) = 1.8-2.7eV. The phases (except Bi2Al4Se8) are thermodynamically stable and decompose peritectically above 965-1020 K. Bi2Al4Se8 is metastable below 825 K and is obtained only by rapid quenching from T > 825 It. The isotypic compounds crystallize in a new tetragonal tP28 structure type (P4/nnc). The characteristic unit is the hitherto unknown clustercation Bi-2(4+), with the mean bond length (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 force constant of (f) over bar = 0.68 N cm(-1). The Electron Localization Function, ELF, shows the covalent Ei-Bi bond, the lone electron pairs of the psi-octahedrally coordinated Bi(II) cations, and the polar character of the Bi-X bonds.