Long-Range and Local Structure of Sr_xBa_1–xNb₂O₆ (x = 0.33 and 0.67) across the Ferroelectric–Relaxor Transition

Abstract

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 x Ba1–x Nb2O6 (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 x Ba1–x Nb2O6 (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.

Incommensurate Modulation and Competing Ferroelectric/Antiferroelectric Modes in Tetragonal Tungsten Bronzes

“…As in previous reports, , the average magnitudes of vectors describing the local Nb off-centering, Δ Nb , are larger for Nb1 sites. A PDD for the angle between Δ Nb1 and the c -axis (Figure c), calculated per steradian, peaks at zero, whereas a similar PDD for Δ Nb2 reveals significant (15–30°) deviations of these vectors from the c -direction, with larger inclinations observed at higher temperatures.…”

“…An experimental study of the local structure in SBN by Li et al 14 employed a combination of Rietveld and both small-and large-box refinements from neutron total scattering data. These analyses also highlighted significant differences for the displacements of Nb1 and Nb2, with the former decreasing gradually upon heating and the latter remaining constant, which appears to contradict the trends obtained in the MD simulations.…”

“…Recently, several studies considered not only the average structure of SBN , but also its local atomic order. , Pasćiak et al employed molecular dynamics (MD) simulations validated by qualitative comparison with the experimental atomic pair distribution function (PDF) obtained from neutron total scattering to investigate local structural distortions. Static and dynamic effects were separated by averaging multiple MD snapshots.…”

The Local Structure and Metal-Insulator Transition in a Ba3Nb5−xTixO15 System

“…However, low electrical conductivity limits their thermoelectric performance determined by the thermoelectric figure of merit ZT = S 2 T/κρ, where S is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity and κ is the thermal conductivity [12]. They have also been studied for their dielectric response showing a crossover from ferroelectric to relaxor behavior with Sr substitution [13,14]. The "filled" TTB niobates, with a random occupation of the A1 and A2 structural sites, have also been studied for their dielectric response [15].…”

Crystallization and Texturing of SrxBa1–xNb2O6 Thin Films Prepared by Aqueous Solution Deposition─An In Situ X-ray Diffraction Study

“…They have a spontaneous and switchable polarization and are studied for their use in ferroelectric random-access memories, piezoelectric sensors and transducers, pyroelectric detectors, electro-optic modulators, and more. − Currently, the dominating materials used are lead zirconium titanate (PZT)-based, which contains about 60 wt % of toxic lead. − A big effort has therefore been placed on finding alternative lead-free ferroelectrics to replace PZT, and one option is the tetragonal tungsten bronze (TTB) Sr x Ba 1– x Nb 2 O 6 (SBN) system . SBN has shown good dielectric and electro-optic properties and a ferroelectric relaxor behavior for high Sr content ( x > 0.5). ,− SBN has an unfilled TTB structure, giving an inherent disorder, great flexibility and tunable properties. − For example, changes in the Sr content ( x ) are known to change the ferroelectric properties, including the Curie temperature ( T C ), which varies approximately linearly from 60 to 250 °C with increasing x in the range 0.25–0.75 . Furthermore, the ferroelectric polarization in SBN is uniaxial along the c -axis, making textured ceramics desirable to enhance the ferroelectric properties. , …”