  Low-temperature synthesis is a promising and potentially effective method for
improving superconducting properties. We report on the fabrication of
polycrystalline samples of SmFeAsO1-xFx with nominal x content varying in a
wide range of x = 0-0.35 synthesized at 900 deg C. This synthesis temperature
is around 300 deg C lower than the conventional synthesis temperature. The
variation in the lattice parameters and transition temperature (Tc) of various
F-doped samples indicates that reduction of the unit cell volume (V) seems to
be the main reason for the rise of Tc up to 57.8 K. Magnetoresistance
measurements showed that the upper critical field slope (dHc2/dT) increased
with increasing F concentration up to x = 0.2, where it reached a maximum value
of -8 T/K corresponding to a coherence length of 10 angstrom. At still higher F
doping levels, dHc2/dT and the low field Jc decreased; above 0.5 T, however, Jc
had almost the same value. Compared with previous reports, the present
synthesis route with low synthesis temperatures and commonly available FeF2 as
the source of F is more effective at introducing F into the SmFeAsO system and
thereby resulting in improved superconducting properties for the system. In
addition, this new sample preparation method also reduces unnecessary problems
such as the evaporation of F and reaction between the crucible and
superconductor during the solid-state reaction.
