New transition-metal doped germanium clathrates

We have investigated transition-metal substitutions into germanium clathrates, and describe the properties of two different Fe-substituted Ge clathrates, one with the chiral Ba6Ge25-type clathrate structure, and one based on the Ba6Ga16Ge30 clathrate with the type-I structure. In both cases Fe exhibits a high-spin local moment, with 5.5 mu(B) and 5.6 mu(B) per Fe. We observe ferromagnetic ordering, with T-c up to 278 K in the most heavily substituted Ba6Ga16Ge30 clathrate. X-ray powder diffraction and electron microprobe measurements confirmed the formation of these substituted phases. Ni-doped type I clathrates were found to be diamagnetic. Strong competition from Mn11Ge8 makes it difficult to produce pure Mn-doped clathrates, however a composite material with partially substituted chiral structure is described. Similar efforts to produce Co, Cr, and V-substituted clathrates have not been successful.