Ge Pairs and Sb Ribbons in Rare-Earth Germanium Antimonides RE12Ge7-xSb21 (RE = La-Pr)

The ternary rare-earth germanium antimonides RE12Ge7-xSb21 (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) angstrom, b=15.2050(12)-14.9777(7) angstrom, C=34.443(3)-33.9376(16) angstrom, in the progression from RE=La to Pr), integrating complex features found in RE6Ge5-xSb11+x and RE12Ga4Sb23. A three-dimensional polyanionic framework, consisting of Ge pairs and Sb ribbons, outlines large channels occupied by columns of face-sharing RE6 trigonal prisms. These trigonal prisms are centered by additional Ge and Sb atoms to form GeSb3 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 Ce12Ge6.5Sb21 indicate antiferromagnetic coupling but no long-range ordering down to 2 K.