  We investigated the influence of different Ag additions (up to 10 wt %) on
the superconducting properties of FeSe0.94. The structural investigations (XRD
and SEM) indicated that Ag is present in three different forms. Ag at grain
boundaries supports the excellent intergrain connections and reduces
superconducting transition width to values smaller than 1K at B=0 and smaller
than 2.74 K at B=14 T. Ag insertion in the crystal lattice unit cell provides
additional carriers and changes the electron hole imbalance in FeSe0.94. This
results in an increase in the magnetoresistive effect (MR) and critical
temperature (Tc). Reacted Ag forms a small amount (~1%) of Ag2Se impurity
phase, which may increase the pinning energy in comparison with that of the
undoped sample. The enhanced upper critical field (Bc2) is also a result of the
increased impurity scattering. Thus, unlike cuprates Ag addition enhances the
Tc, Bc2, pinning energy and MR making the properties of polycrystalline
FeSe0.94 similar to those of single crystals.
