Ge pairs and Sb ribbons in rare-earth germanium antimonides RE12Ge(7-x)Sb21 (RE = La-Pr).

The ternary rare-earth germanium antimonides RE(12)Ge(7-x)Sb(21) (RE = La-Pr; x = 0.4-0.5) are synthesized by direct reactions of the elements. Single-crystal X-ray diffraction studies indicate that they adopt a new structure type (space group Immm, Z = 2, a = 4.3165(4)-4.2578(2) A, b = 15.2050(12)-14.9777(7) A, c = 34.443(3)-33.9376(16) A in the progression from RE = La to Pr), integrating complex features found in RE(6)Ge(5-x)Sb(11+x) and RE(12)Ga(4)Sb(23). A three-dimensional polyanionic framework, consisting of Ge pairs and Sb ribbons, outlines large channels occupied by columns of face-sharing RE(6) trigonal prisms. These trigonal prisms are centered by additional Ge and Sb atoms to form GeSb(3) trigonal-planar units. A bonding analysis attempted through a Zintl-Klemm approach suggests that full electron transfer from the RE atoms to the anionic substructure cannot be assumed. This is confirmed by band-structure calculations, which also reveal the importance of Ge-Sb and Sb-Sb bonding. Magnetic measurements on Ce(12)Ge(6.5)Sb(21) indicate antiferromagnetic coupling but no long-range ordering down to 2 K.