Unraveling structural evolution in high-performance energy storage La-doped BaTiO₃ superparaelectric relaxor ferroelectric ceramics

Unraveling the doping- and temperature-dependent structural dynamics is pivotal for developing high-performance energy storage ceramics, yet the fundamental physical mechanisms governing these phenomena remain elusive. Here, we employ in situ polarized Raman spectra and spectroscopic ellipsometry to probe the structural evolution in La³⁺-doped BaTiO₃ ceramics. A-site substitution by La³⁺ induces significant lattice distortions and generates oxygen vacancies, which disrupt long-range ferroelectric order and promote relaxor behavior, with the system ultimately evolving into a superparaelectric (SPE) state. These perturbations manifest spectroscopically as broadening and low-frequency shifts of Raman-active phonon, accompanied a lattice structural transition from tetragonal to pseudocubic symmetry. Spectroscopic ellipsometry demonstrates a systematic increase in the optical bandgap with La doping, attributed to the charge compensation mechanisms and the ionic size mismatch. The structural evolution from relaxor ferroelectric to SPE and finally to paraelectric phases with temperature is revealed by distinct anomalies in phonon frequencies, depolarization ratio, and optical bandgap, while the constructed phase diagram shows that La-doping suppresses critical transformation temperatures. Notably, the A_1g phonon mode serves as a critical spectroscopic fingerprint for differentiating distinct structural evolution pathways. These studies present the systematical results on structural properties and optical/dielectric properties for BaTiO₃-based ceramic, establishing a design paradigm for advanced lead-free energy storage ceramic.