Reducing the positional modulation of NbO6-octahedra in SrxBa1-xNb2O6 by increasing the Barium content: A single crystal neutron diffraction study at ambient temperature for x=0.61 and x=0.34

摘要

We report on the influence of the Barium content on the modulation amplitudein SrxBa1-xNb2O6 compounds by comparing Sr0.61Ba0.39Nb2O6 (SBN61) andSr0.34Ba0.66Nb2O6 (SBN34). Our single crystal neutron diffraction resultsdemonstrate that the amplitude of the positional modulation of the NbO6octahedra is reduced with increasing barium content, indicating that the originof the modulation is the partial occupation of the pentagonal channels by Srand Ba atoms. By increasing the Sr content the bigger Ba atoms are replaced bythe smaller Sr atoms, which leads to a larger deformation of the surroundinglattice and hence to a larger modulation amplitude. The more homogeneous thefilling of these channels with one atomic type (Ba) the lower the modulationamplitude. Our results also show that the structure can be described with atwo-dimensional incommensurate harmonic modulation. No second order modulationhas been observed, both by single crystal diffraction measurements and q-scans.The positional modulation of the Nb atoms is much smaller than that of theoxygen atoms, such that the modulation can be seen as a rotational modulationof almost rigid NbO6-octahedra.