Study of superconducting Ba–Ge–Co compounds Yang Li a, * , J.H. Ross Jr. a, * , J.A. Larrea b , Elisa Baggio-Saitovitch b a Department of Physics, Texas A&M University, College Station, TX 77843-4242, USA b Centro Brasileiro de Pesquisas Fisicas, Rua Dr. Xavier Siguad, 150, 22290-180 Rio de Janeiro, RJ, Brazil Abstract We prepared samples with starting composition Ba 8 –Ge 46x –Co x (x ¼ 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 ¼ 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 2 Ge, BaGe, BaGe 2 , BaGe 2 O 5 , BaGeO 3 , Ba 3 GeO 5 , a-BaGeO 3 , BaGe 2 O 5 , Ba 2 Ge 5 O 12 , b-BaGeO 3 , Ba 2 GeO 4 and BaGe 4 O 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 0:01 Ge 99:9 . We also discuss the origin of two su- perconducting transitions in Ge-based compounds. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Ge–Ba compounds; Superconductivity; Co-doping; Clathrate Ge- and Si-based compounds present variegated polymorphs of the same stoichiometric composition that differ in the germanium and silicon metal coordination. The different bonding possibilities of Ge and Si give rise to slightly different structural phases that can differ very much in their physical behaviour. For semiconducting Si- and Ge-based compounds, such as materials with clathrate structure, superconductivity could realize by doping and modify their bonding coordinations and electron structures [1–4]. It is intriguing in theoretical and practical interest although there are some contro- versies for Ge-based clathrates [2,3]. Superconductivity in these materials has not been so exhaustively explored. In order to help clarify the controversy on these prob- lems, we research transport and magnetic properties on (Ge, Ba)-based compounds. Here, we report two Type II superconducting transitions with T C  10 K and T C  4 K, respectively, in (Ge, Ba)-based compounds. We also report Co-doping effect on superconductivity for (Ge, Ba)-based compounds. Ba–Ge–Co compounds with starting compositions Ba 8 Ge 46x Co x (x ¼ 0, 4 and 6) were synthesized by di- rectly high temperature reaction of elements, and also investigated by X-ray diffractions. Samples are com- prised of mixtures of Ge (diamond) and several un- known phases. A high percentage of Ge with diamond structure can be produced for all the samples. Diffrac- tion peaks for Ba 2 Ge, BaGe, BaGe 2 , BaGe 2 O 5 , BaGeO 3 , Ba 3 GeO 5 , a-BaGeO 3 , BaGe 2 O 5 , Ba 2 Ge 5 O 12 , b-BaGeO 3 , Ba 2 GeO 4 and BaGe 4 O 9 were carefully searched for but not seen in the samples. For Co-free Ba–Ge sample, several weak diffractions from an unknown Ba–Ge phase were indexed on the basis of a monoclinic unit cell. For Co-doped samples, there is no such unknown phase, but there is CoGe 2 phase with the PdSn 2 struc- ture. With increasing Co content, the CoGe 2 content * Corresponding authors. Tel.: +1-979-845-7823; fax: +1- 979-845-2590. E-mail addresses: yli@physics.tamu.edu (Y. Li), jhross@ tamu.edu (J.H. Ross Jr.). 0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.225 Physica C 408–410 (2004) 869–871 www.elsevier.com/locate/physc