Study of superconducting Ba–Ge–Co compounds

Abstract

We prepared samples with starting composition Ba 8–Ge 46− x –Co x ( 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 Ba 2Ge, BaGe, BaGe 2, BaGe 2O 5, BaGeO 3, Ba 3GeO 5, α-BaGeO 3, BaGe 2O 5, Ba 2Ge 5O 12, β-BaGeO 3, Ba 2GeO 4 and BaGe 4O 9 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 CoGe 2. With increasing Co content, the CoGe 2 content increases. The WDS results agree with this result. The main phase composition for the Co-free sample is Ba 0.01Ge 99.9. We also discuss the origin of two superconducting transitions in Ge-based compounds.