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