Electron microscopic investigation of a V₅Si₃ needle-shaped precipitate in V₃Si (A15 structure)

Abstract

A lattice defect had been found by TEM in the superconducting compound V₃Si (A15 structure). As a first approximation, it has been analysed as an a/2〈100〉 faulted dipole, by matching experimental micrographs with computer simulations. This faulted dipole accounts for the elastic deformation of the V₃Si matrix caused by a needle-shaped precipitate. The purpose of this article is to assess the chemical and crystallographic natures of the precipitate. First, the structure factor contrast is studied in CTEM, together with high-magnification bright-field micrographs which enable us to observe the geometrical features of the cross-section of the precipitates. Then, the chemical nature of the precipitates is assessed by electron energy loss spectroscopy and its crystallographic nature is determined by convergent beam electron microdiffraction in a Philips EM 400 microscope equipped with a field-emission gun: the precipitates belong to the tetragonal V₅Si₃ phase. The orientation relationships between the V₅Si₃ precipitates and the V₃Si matrix are also assessed by convergent beam electron microdiffraction.