| dc.description.abstract | Ferroelectric ceramics are promising materials for a wide range of piezoelectric applications, including high-permittivity dielectrics, sensors, transducers, actuators, and electro-optic devices. Among commercially viable ceramics, lead zirconate titanate (Pb(Zr Ti )O , PZT) has dominated due to its superior piezoelectric and dielectric properties, high electromechanical coupling, ease of processing, and low cost.
However, the toxicity and volatility of lead during processing pose serious health and environmental concerns. Global legislation against lead-based products has accelerated research into lead-free alternatives, with emphasis on perovskite-based ceramics. These materials combine excellent properties with relatively simple structures, facilitating structure-property relationship studies.
One of the most promising candidates is sodium bismuth titanate (Na½Bi½TiO , NBT) and its solid solutions, which exhibit strong ferroelectric polarization (~40 C/cm²), promising piezoelectric strain (~0.08%), and a longitudinal piezoelectric coefficient (d ~ 80 pC/N).
Structural Complexity of NBT
NBT was discovered six decades ago, but its structure remained unclear due to contradictory reports.
X-ray and neutron diffraction suggested a rhombohedral (R3c) structure at room temperature, transforming to tetragonal (P4bm) at ~230 °C.
High-resolution XRD revealed monoclinic (Cc) features at room temperature.
Electron diffraction showed planar disorders linked to in-phase octahedral tilts, not explained by R3c or Cc.
Local structural studies (EXAFS, diffuse scattering) revealed differences from bulk diffraction results.
A breakthrough came when electric-field sensitivity of morphotropic phase boundary (MPB) compositions was discovered. Even pure NBT showed electric-field-induced structural transitions, highlighting the intrinsic tendency of electric fields to influence NBT’s structure.
Key Findings
Room Temperature Characterization
XRD and neutron diffraction: electric-field-induced transition from monoclinic (Cc) to rhombohedral (R3c).
First-principles calculations: monoclinic phase is unstable, suggesting microstructural effects (nano-domains).
Electron diffraction and HRTEM: suppression of in-phase tilted regions in poled samples, correlating monoclinic features with strain heterogeneity.
High-Temperature Phase Transitions
Poling introduced sharp anomalies at the depolarization temperature (T ), linked to in-phase octahedral tilts.
Transition from normal to relaxor ferroelectric behavior observed.
Intermediate cubic-like phase (~300 °C) showed persistence of tilted superlattice reflections, indicating non-truly cubic structure.
BaTiO -Substituted NBT
Subtle compositional phase boundary at x = 0.03, disappearing upon poling.
Excess Bi improved piezoelectric properties but lowered T .
At x = 0.2: crossover from modulated to non-modulated tetragonal phase.
At x ~ 0.7: transition from normal to relaxor ferroelectric state, driven by Bi³ lone pair effects.
K½Bi½TiO -Substituted NBT
Pre-MPB compositions (x < 0.2): electric field induced rhombohedral structure.
Post-MPB compositions (x > 0.2): negligible electric field effect, persistent relaxor behavior.
FWHM analysis of {200} peaks confirmed tetragonal emergence beyond x = 0.15. | |
