Long-Range and Local Structure of Sr_xBa_{1- x}Nb₂O₆ (x = 0.33 and 0.67) across the Ferroelectric-Relaxor Transition

Tetragonal tungsten bronze (TTB), with a flexible chemical framework, is an important class of ferroelectric oxide that hosts both the frequency-independent ferroelectric behavior as well as the broad, frequency-dependent relaxor behavior. One particularly interesting composition is the archetype Sr_xBa_1-xNb₂O₆ (denoted as SBN100x), which display a crossover from ferroelectric (FE) to relaxor (RE) with x > 0.6. While the dielectric responses of TTBs have been extensively reported, the atomic origin that leads to the switch between RE and FE behavior remains unclear. In this contribution, the structural evolution of Sr_xBa_1-xNb₂O₆ (x = 0.33 and 0.67, a FE and RE compound, respectively), from the long-range average structure to the local atomic ordering, from 300 to 720 K, is probed with Rietveld analysis, small-box least-squares refinement, and Reverse Monto Carlo (RMC) modeling using neutron total scattering data. For both compounds, a static, distorted local atomic configuration, different from the long-range crystal structure (P4/mbm), is revealed well above the ferroelectric transition temperature, indicating local polarization. In particular, a net polarization at the Nb2 site is found only in the RE composition. Overall, the FE to RE crossover is explained in the context of the local cation configuration: the Sr/Ba size mismatch at A1 sites displaces neighboring oxygen, leading to net polarization in SBN67 and the observed RE behavior. These results demonstrate new findings regarding the microscopic origin of the FE to RE crossover in TTB.