Crystal and magnetic structure of the oxypnictide superconductor LaFeAsO1-xFx: A neutron-diffraction study

High-resolution and high-flux neutron as well as x-ray powder-diffraction experiments were performed on the oxypnictide series LaO1-xFxFeAs with 0 <= x <= 0.15 in order to study the crystal and magnetic structure. The magnetic symmetry of the undoped compound corresponds to those reported for REOFeAs (with RE a rare earth) and for AFe(2)As(2)(A=Ba, Sr) materials. We find an ordered magnetic moment of 0.63(1) mu B at 2 K in LaOFeAs, which is significantly larger than the values previously reported for this compound. A sizable ordered magnetic moment is observed up to a F doping of 4.5% whereas there is no magnetic order for a sample with a F concentration of x=0.06. In the undoped sample, several interatomic distances and FeAs4 tetrahedra angles exhibit pronounced anomalies connected with the broad structural transition and with the onset of magnetism supporting the idea of strong magnetoelastic coupling in this material.