  We report on the influence of the Barium content on the modulation amplitude
in SrxBa1-xNb2O6 compounds by comparing Sr0.61Ba0.39Nb2O6 (SBN61) and
Sr0.34Ba0.66Nb2O6 (SBN34). Our single crystal neutron diffraction results
demonstrate that the amplitude of the positional modulation of the NbO6
octahedra is reduced with increasing barium content, indicating that the origin
of the modulation is the partial occupation of the pentagonal channels by Sr
and Ba atoms. By increasing the Sr content the bigger Ba atoms are replaced by
the smaller Sr atoms, which leads to a larger deformation of the surrounding
lattice and hence to a larger modulation amplitude. The more homogeneous the
filling of these channels with one atomic type (Ba) the lower the modulation
amplitude. Our results also show that the structure can be described with a
two-dimensional incommensurate harmonic modulation. No second order modulation
has been observed, both by single crystal diffraction measurements and q-scans.
The positional modulation of the Nb atoms is much smaller than that of the
oxygen atoms, such that the modulation can be seen as a rotational modulation
of almost rigid NbO6-octahedra.
