Unusual variability of the lattice constant in polycrystalline epitaxial growth of superconducting A15 Nb-Si.

Films of A15 Nb-Si have been grown by electron beam coevaporation onto polycrystalline, A15 Nb-Ir. The films have been studied by X-ray diffraction and Auger sputter profiling. The Nb- Si films are found to grow lattice matched to the Nb-Ir with no tetragonal distortion even when the composition of the Nb-Si is such that there is an expected lattice mismatch of up to 0.15angstroms. Auger analysis failed to locate chemical impurities which could be identified as the cause of the variability of the lattice parameter. In addition, the variability does not appear to be due to the formation of non A15 phases which change the composition of the A15 material.