SIMULTANEOUS NEUTRON AND X-RAY REFINEMENT OF ORTHO-II SUPERSTRUCTURE IN YBA2CU3O6.5

Clearly defined Lorentzian-like profiles of the Ortho-II superstructur e diffraction peaks have been observed with neutrons for the first tim e. The superstructure has been refined by simultaneously fitting neutr on and x-ray measurements on the same crystal. The combination of both x-ray and neutron information is important since the formation of the Ortho-II phase is driven by oxygen ordering which can be most clearly seen by the neutrons whilst the x-rays are more sensitive to the heav y ion displacements.