  We predict atomic, electronic, and magnetic structures of a hypothetical
compound LaOFeSb by first-principles density-functional calculations. It is
shown that LaOFeSb prefers a stripe-type antiferromagnetic phase (i.e., spin
density wave (SDW) phase) to the non-magnetic (NM) phase, with a larger Fe spin
moment and greater SDW-NM energy difference than those of LaOFeAs. The SDW
phase is found to favor the orthorhombic structure while the tetragonal
structure is more stable in the NM phase. In the NM-phase LaOFeSb, the
electronic bandwidth near the Fermi energy is reduced compared with LaOFeAs,
indicating smaller orbital overlap between Fe $d$ states and subsequently
enhanced intra-atomic exchange coupling. The calculated Fermi surface in the NM
phase consists of three hole and two electron sheets, and shows increased
nesting between two hole and two electron sheets compared with LaOFeAs.
Monotonous changes found in our calculated material properties of LaOFePn
(Pn=P, As, and Sb), along with reported superconducting properties of doped
LaOFeP and LaOFeAs, suggest that doped LaOFeSb may have a higher
superconducting transition temperature.
