Ergodic polar cluster state and its dynamics in K and Nb modified BaTiO₃

Ferroelectric to relaxor crossover of (KxBa1-x)(NbxTi1-x)O-3 [KBNTx]; (x = 0, 0.05, 0.075 and 0.150) system has been investigated by exploring its average and local structure using X-ray diffraction, Raman scattering and dielectric spectroscopy. X-ray diffraction confirmed all compositions' overall cubic (Pm-3m) structure. Dielectric spectroscopy clearly showed ferroelectric to relaxor crossover at x = 0.150. In the Raman spectra of doped compounds, three new modes of chemical origin that are absent in BaTiO3 are observed at frequencies similar to 120 cm(-1), 830 cm(-1) and 850 cm(-1) at room temperature. The temperature-dependent Raman scattering revealed the existence of ergodic polar regions by the presence of first-order ferroelectric mode at 302 cm(-1) [B-1/E(TO3)/E(LO2)], along with two static disorder-related modes 220 and 550 cm(-1) for T > T-c in the paraelectric phase. Moreover, Raman studies also elucidate that the decreased size of ergodic polar regions is responsible for relaxor crossover at x = 0.150 composition with the increased broadening of 302 cm(-1) modes with an increase of K and Nb concentration. Further, the polar dynamic investigations indicated that the interactions among polar nano regions slow down nano-polar dynamics without freezing or glass transition. Identifying polar regions and their dynamics in the present work will significantly impact the understanding of the tunability of K and Nb-modified BaTiO3 lead-free ferroelectrics.