Electron-Deficient Ternary and Quaternary Pnictides Rb4Zn7As7, Rb4Mn3.5Zn3.5Sb7, Rb7Mn12Sb12, and Rb7Mn4Cd8Sb12 with Corrugated Anionic Layers

The ternary pnictides Rb4Zn7As7 and Rb7Mn12Sb12 and their quaternary derivatives Rb4Mn3.5Zn3.5Sb7 and Rb7Mn4Cd8Sb12 have been prepared by reactions of the elements at 600 °C. They crystallize in two new structure types: orthorhombic Rb4Zn7As7-type (space group Cmcm, Z = 4; a = 4.1883(4) Å, b = 24.844(2) Å, c = 17.6056(17) Å for Rb4Zn7As7; a = 4.3911(8) Å, b = 26.546(5) Å, c = 18.743(4) Å for Rb4Mn3.5Zn3.5Sb7) and monoclinic Rb7Mn12Sb12-type (space group C2/m, Z = 2; a = 26.544(12) Å, b = 4.448(2) Å, c = 16.676(8) Å, β = 103.183(8)° for Rb7Mn12Sb12; a = 27.009(4) Å, b = 4.5752(7) Å, c = 16.727(3) Å, β = 103.221(2)° for Rb7Mn4Cd8Sb12). These related structures contain corrugated anionic layers built up by connecting ribbons of edge-sharing tetrahedra in a zigzag-like manner with chains of Mn-centered square pyramids located at the hinges. Homoatomic pnicogen–pnicogen bonding occurs in the form of Pn2 pairs. The compounds are formally deficient by one electron per formula unit, as confirmed by band structure calculations which reveal the location of the Fermi level just below a small gap in Rb4Zn7As7 or a pseudogap in Rb7Mn12Sb12.