Epitaxial growth of Nb3Ge on Nb3Ir and Nb3Rh

The possibility of using epitaxy to prepare the high Tsub(c) superconductor Nb3Ge in an A15 crystalline structure at the 3:1 stoichiometry was investigated. Nb3Ir polycrystalline films with the A15 structure deposited on sapphire were used as substrates for the epitaxial growth of Nb3Ge because of the favorable lattice parameter match. The experimental results clearly show that epitaxial growth indeed occurs and helps to extend the range of homogeneity of the A15 phase up to 26.3 at.% Ge as compared with the thermodynamic equilibrium boundary at 19 at.% Ge. Nb3Rh films were also used as substrates and found to be inferior to Nb3Ir because of the multiphase nature of the films. In addition to extending the A15 phase boundary epitaxy results in a considerable rise in the superconducting transition temperature for Ge-rich samples together with a reduction in the transition width. The work suggests that polycrystalline epitaxy can be an important tool in the synthesis of thin-film intermetallic compounds. (author)