Crystallographic phase transition and high-T_c superconductivity in LaFeAsO:F

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