Superspace approach applied to a neutron-diffraction study of the holographic data storage material Sr0.61Ba0.39Nb2O6

We investigated the incommensurately modulated structure of Sr0.61Ba0.39Nb2O6 by means of single-crystal neutron diffraction. With the measured 620 satellite reflections we could verify the two modulation vectors Q1,2=(0.3075,±0.3075,0.5)known from X-ray-diffraction experiments. As neutrons are much more sensitive to oxygen atoms than X-rays, we performed structural refinement of the oxygen atoms using the superspace approach. The largest amplitude for positional modulation was found for those oxygen atoms lying in the same plane (z=const.) as the Sr and Ba atoms. The final R values for main and satellite reflections are 0.063 and 0.153, respectively.