Structural stability, optical, and dielectric properties of the (Ba_1/5Pb_1/5Sr_1/5RE_1/5K_1/5)TiO₃ high-entropy ceramic

A high-entropy matrix with highly polarizable elements sharing a rare-earth element at the same crystallographic site was designed using the chemical formula Ba_1/5Pb_1/5Sr_1/5RE_1/5K_1/5TiO₃ (BPSREKTO), where rare-earth (RE) = La, Nb, Sm, Gd, Dy, Ho, Y, and Lu. Single-phase stability was observed only in the BPSREKTO with RE = La, Nd, and Sm high-entropy compounds. The crystal structure, optical properties, and ferroelectric nature of the single-phase ceramic compounds were investigated. Elemental and structural analyses revealed that all the cations were homogeneously distributed in a global centrosymmetric cubic structure (S.G. Pm \bar 3 m). Optical absorption showed that the RE = Nd compound is more photoactive in the 200–1000 nm wavelength range, unlike the RE = La, Sm high-entropy compounds. The introduction of RE elements in high-entropy ceramic (HEC) systems affects the indirect bandgap of BPSREKTO with RE = La, Nd, and Sm. It was also found that cationic disorder increases the Urbach energy, leading to a decrease in the indirect energy bandgap in the HEC compound compared to the homologue BaTiO₃/SrTiO₃ single-phase. The dielectric spectra show a broad peak in the dielectric constant and dielectric loss, which are shifted in temperatures with increasing frequencies due to a relaxor ferroelectric transition typical of the diffuse phase transitions. This relaxor behavior was unexpected, because the global crystal structure was centrosymmetric, implying an increase in the number of polar nanoregions (PNRs). These PNRs coexisting with non-polar regions (NPRs) were observed using piezo-force microscopy. Furthermore, the slim polarization loop confirmed the relaxor behavior of BPSREKTO with RE = La, Nd, and Sm. These ferroelectric features make these RE-modified HEC materials good candidates for high-energy storage applications.