Invited
The incommensurate modulation and competing polar/anti-polar modes in tetragonal tungsten bronzes

The nanoscale structure of Sr0.61Ba0.39Nb2O6, a classic uniaxial relaxor ferroelectric crystallizing with the tetragonal tungsten bronze (TTB) structure and exhibiting an incommensurate modulation, has been determined by atomistic refinements using combined data from variable-temperature neutron total scattering, extended X-ray absorption fine structure, and 3D single-crystal diffuse scattering. We found the modulation to arise from the intergrowth of structural slabs featuring distinct types of octahedral rotations directed to minimize octahedral deformations. This modulation involves displacements of the A-cations (Sr, Ba) but exerts no significant effect on the polar displacements of Nb. Our results demonstrated the coexistence of competing polar and anti-polar modes for off-centering of Nb above the nominal ferroelectric transition temperature of 350 K. The nanoscale-correlated anti-polar distortions are suppressed below the transition. We identified coupling between the polar off-centering of Nb on its two symmetrically distinct sites as a fundamental characteristic controlling the ferroelectric-to-relaxor crossover in TTBs.