Structure and electrical transport properties of the ordered skutterudites MGe1.5S1.5 (M=Co, Rh, Ir)

High-resolution powder neutron diffraction data collected for the skutterudites MGe_1.5S_1.5 (M=Co, Rh, Ir) reveal that these materials adopt an ordered skutterudite structure (space group R3¯), in which the anions are ordered in layers perpendicular to the 111] direction. In this ordered structure, the anions form two-crystallographically distinct four-membered rings, with stoichiometry Ge₂S₂, in which the Ge and S atoms are trans to each other. The transport properties of these materials, which are p-type semiconductors, are discussed in the light of the structural results. The effect of iron substitution in CoGe_1.5S_1.5 has been investigated. While doping of CoGe_1.5S_1.5 has a marked effect on both the electrical resistivity and the Seebeck coefficient, these ternary skutterudites exhibit significantly higher electrical resistivities than their binary counterparts.