Electron-deficient ternary and quaternary pnictides Rb_4Zn _7As_7, Rb_4Mn_(3.5)Zn _(3.5)Sb_7, Rb_7Mn_(12)Sb_(12), and Rb_7Mn_4Cd_8Sb_(12) with corrugated anionic layers

摘要

The ternary pnictides Rb_4Zn_7As_7 and Rb_7Mn_(12)Sb_(12) and their quaternary derivatives Rb_4Mn_(3.5)Zn_(3.5)Sb_7 and Rb _7Mn_4Cd_8Sb_(12) have been prepared by reactions of the elements at 600 C. They crystallize in two new structure types: orthorhombic Rb_4Zn_7As_7-type (space group Cmcm, Z = 4; a = 4.1883(4) ?, b = 24.844(2) ?, c = 17.6056(17) ? for Rb_4Zn_7As_7; a = 4.3911(8) ?, b = 26.546(5) ?, c = 18.743(4) ? for Rb_4Mn _(3.5)Zn_(3.5)Sb_7) and monoclinic Rb _7Mn_(12)Sb_(12)-type (space group C2/m, Z = 2; a = 26.544(12) ?, b = 4.448(2) ?, c = 16.676(8) ?, β = 103.183(8) for Rb_7Mn_(12)Sb_(12); a = 27.009(4) ?, b = 4.5752(7) ?, c = 16.727(3) ?, β = 103.221(2) for Rb_7Mn_4Cd_8Sb_(12)). 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 Pn_2 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 Rb _4Zn_7As_7 or a pseudogap in Rb_7Mn _(12)Sb_(12).