Synthesis and Characterization of the “Metallic Salts” A5Pn4 (A = K, Rb, Cs and Pn = As, Sb, Bi) with Isolated Zigzag Tetramers of Pn44- and an Extra Delocalized Electron

Abstract

The isostructural title compounds were prepared by direct reactions of the corresponding elements, and their structures were determined from single-crystal X-ray diffraction data in the monoclinic space group C2/m, Z = 2 (K₅As₄, a = 11.592(2) Å, b = 5.2114(5) Å, c = 10.383(3) Å, β = 113.42(1)°; K₅Sb₄, a = 12.319(1) Å, b = 5.4866(4) Å, c = 11.258(1) Å, β = 112.27(7)°; Rb₅Sb₄, a = 12.7803(9) Å, b = 5.7518(4) Å, c = 11.6310(8) Å, β = 113.701(1)°; K₅Bi₄, a = 12.517(2) Å, b = 5.541(1) Å, c = 11.625(2) Å, β = 111.46(1)°; Rb₅Bi₄, a = 12.945(4) Å, b = 5.7851(9) Å, c = 12.018(5) Å, β = 112.78(3)°; Cs₅Bi₄, a = 12.887(3) Å, b = 6.323(1) Å, c = 12.636(1) Å, β = 122.94(2)°). The compounds contain isolated and flat zigzag tetramers of Pn₄^4- (Pnictide (Pn) = As, Sb, Bi) with a conjugated π-electron system of delocalized electrons. All six compounds are metallic (“metallic salts”) and show temperature-independent (Pauli-like) paramagnetism due to a delocalized electron from the extra alkali-metal cation in the formula. At low temperatures (around 9.5 K) and low magnetic fields the bismuthides become superconducting.