  We report the synthesis and characterization of antimony doped oxypnictide
superconductor, LaO0.8F0.2FeAs1-xSbx (x = 0.05 and 0.10). The parent compound
LaOFeAs with fluorine doping exhibits superconductivity at maximum transition
temperature ~ 28.5 K [11]. Here we partially substitute As by Sb
(LaO0.8F0.2FeAs1-xSbx) and observe enhancement of the transition temperature to
30.1 K. This is the only instance so far where Tc increases with doping in the
conducting layer (FeAs) and this leads to the highest transition temperature in
any La-based oxypnictide. XRD and EDAX measurements confirm phase purity of the
samples and the presence of Sb. The magneto-resistance measurements show that
the value of upper critical field Hc2(0) to be about 73 T corresponding to a
coherence length of 22 angstrom. The Seebeck coefficient measurements indicate
electron transport with strong contribution from electron-electron correlation.
These results provide interesting insight to the origin of superconductivity in
these novel series of compounds.
