Study of superconducting Ba-Ge-Co compounds

We prepared samples with starting composition Ba-8-Ge46-x-COx (x = 0, 4 and 6) by direct melting. These Ge-based compounds were characterized by X-ray diffraction and WDS, and we found two superconducting transitions at T-C = 10 and 4 K in the Co-free sample. Co-doping results in the suppression of T-C to 7 K. The superconducting volume fraction also decreases with increasing Co-doping. For Co-doped samples, there is no 4 K superconducting transition. X-ray refinement shows that the compounds are mixtures of several phases. The dominant phase is diamond Ge, and we found no Ge-clathrate phases. Besides diamond Ge, there are also several weak diffractions from an unknown Ba-Ge phase, and most of them were indexed on the basis of a monoclinic unit cell. Diffraction peaks for Ba2Ge, BaGe, BaGe2, BaGe2O5, BaGeO3, Ba3GeO5, alpha-BaGeO3, BaGe2O5, Ba2Ge5O12, beta-BaGeO3, Ba2GeO4 and BaGe4O9 were carefully searched for but not seen in the samples. For the Co-doped sample, besides the main diamond Ge phase, there is also a semiconducting phase CoGe2. With increasing Co content, the CoGe2 content increases. The WDS results agree with this result. The main phase composition for the Co-free sample is Ba0.01Ge99.9. We also discuss the origin of two superconducting transitions in Ge-based compounds. (C) 2004 Elsevier B.V. All rights reserved.