Structural stability, optical, and dielectric properties of the (Ba1/5Pb1/5Sr1/5RE1/5K1/5)TiO3 (RE = Rare Earth, Y) high-entropy ceramic.

High entropy matrix with highly polarizable elements sharing a rare earth element at the same crystallographic site has been designed with chemist formula: Ba1/5Pb1/5Sr1/5RE1/5K1/5TiO3 (BPSREKTO) with RE=La3+, Nb3+, Sm3+, Gd3+, Dy3+, Ho3+, Y3+, and Lu3+. The single-phase stability was only found in the BPSREKTO with RE= La3+, Nd3+, and Sm3+ high-entropy compounds. Crystal structure, optical properties, and the ferroelectric nature were investigated over the single-phase ceramic compounds. Elemental and structure analysis reveals that whole cations are homogeneously distributed in a global centrosymmetric cubic structure (S.G. Pm-3m). The optical absorption showed that the RE=Nd3+ compound is more photoactive in the 200 – 1000 nm wavelength range unlike RE=La3+, Sm3+ HECs compounds. The rare earth introduction in the entropy ceramic systems affect more the indirect band gap for BPSREKTO with RE= La3+, Nd3+, and Sm3+, respectively. It also found that the cationic disorder increases the Urbach energy leading to decrease of the indirect energy band gap in the high entropy ceramic compound concerning to homologue BTO/STO single-phase. The dielectric spectra show a broad peak in the ϵ’ and tan , which are shifted in temperatures with increasing frequencies due to a relaxor ferroelectric transition typical of the diffuse phase transitions (DPT). The relaxor behavior is unexpected since the global crystal structure is centrosymmetric, implying the rise of polar nanoregions (PNRs). These PNRs coexisting with non-polar regions were observed by piezo force microscopy, PFM. Furthermore, the slim polarization loop confirms the relaxor behavior for BPSREKTO with RE= La3+, Nd3+, and Sm3+, respectively. These ferroelectric features make this rare-earth entropy ceramic materials very good candidates for high-energy storage.