  We report the antimony (Sb) doping effect in a prototype system of iron-based
supercon-ductors LaFeAsO1-yFy (y=0, 0.1, 0.15). X-ray powder diffraction
indicates that the lattice pa-rameters increase with Sb content within the
doping limit. Rietveld structural refinements show that, with the partial
substitution of Sb for As, while the thickness of the Fe2As2 layers increases
significantly, that of the La2O2 layers shrinks simultaneously. So a negative
chemical pressure is indeed "applied" to the superconducting-active Fe2As2
layers, in con-trast to the effect of positive chemical pressure by the
phosphorus doping. Electrical resis-tance and magnetic susceptibility
measurements indicate that, while the Sb doping hardly influences the SDW
anomaly in LaFeAsO, it recovers SDW order for the optimally-doped sample of
y=0.1. In the meantime, the superconducting transition temperature can be
raised up to 30 K in LaFeAs1-xSbxO1-yFy with x=0.1 and y=0.15. The Sb doping
effects are discussed in term of both J1-J2 model and Fermi Surface (FS)
nesting scenario.
