Abstract
Undoped LaFeAsO, the parent compound of the newly foundhigh-Tc superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at∼160 K. The anomaly can be suppressed by F doping with simultaneous appearance ofsuperconductivity appears correspondingly, suggesting a close association of the anomalywith the superconductivity. We examined the crystal structures, magnetic propertiesand conductivity of undoped (normal conductor) and 14 at.% F-doped LaFeAsO(Tc = 20 K) by synchrotron x-ray diffraction (XRD), DC magnetic measurements, and ab initiocalculations demonstrated that the anomaly is associated with a phase transition from tetragonal(P4/nmm) toorthorhombic (Cmma) phases at ∼160 K as well as an antiferromagnetic spin ordering transition at∼140 K. These transitions can be explained by spin configuration-dependent potential energysurfaces derived from the ab initio calculations. The suppression of the transitions is ascribed tointerrelated effects of geometric and electronic structural changes due to doping byF− ions.
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