Local tetragonality and atomic structure in Nb3Sn superconductor studied by high resolution electron microscopy

The atomic structure of Nb3Sn in a commercial superconducting composite tape is studied by a 400 kV high resolution electron microscope. The images are interpreted by theoretical contrast calculations. The [001] structure image shows an image pattern similar to the [001] projection of the crystal, which consists of Nb atoms in the center corresponding to the projected Nb chain along [001] and a circular pattern corresponding to Nb and Sn atoms arranged in the perimeter. The [111] structure images show a hexagonal arrangement in the thin region and a trigonal arrangement in the thick region of the wedged crystal; the bright spots correspond to the small triangles of Nb atoms and to the central Sn atoms in the [111]-projection of the crystal. Different degrees of local distortion in the [111]-structure image and heavily strained domains in the [001]-structure image are observed. These phenomena are discussed in connection with the superconducting properties. (author)