Relaxor Ferroelectrics: The origin of chemically- and electrically-induced transition from ferroelectric to relaxor state in complex perovskite ceramics

Department of Materials Science and EngineeringSince the first discovery of relaxor ferroelectrics (relaxors), a lot of dedication have been fulfilled to extend the knowledge and understanding for relaxor features. Even though the true origins of their high performances are not fully understood, relaxors have been widely used due to their versatile ferroelectric/piezoelectric properties and temperature stability. To figure out the true origin of relaxor features still remains extremely hard since various results indicate that the causes of relaxation processes are appearing in atomic level scale with an application of electric field. Various consensual models, describing correlations between relaxor features and causes, have been made thanks to a lot of dedications. The existence of polar nanoregions (PNRs) is considered as a key role to carry out relaxor features. It leads to frequency dispersive dielectric maxima, which is the most important features introducing relaxor ferroelectrics. Therefore, a proper understanding of PNRs is a key to unveiling all the existing questions in relaxor ferroelectrics. Presently, lead-free relaxors become essential since we are under the pressure to find replacements for lead-containing relaxors in response to environmental regulations. However, extensive researches indicate that commonly accepted concepts from lead-based relaxors are not always applicable to lead-free relaxorsfor example, the double dielectric permittivity maxima and a deviation of temperature between depolarization (Td) and a transition from a ferroelectric-to-relaxor transition (TF-R). This work contains dedications to explain major relaxor enigmas as followings, 1) The relaxation processes mainly affect to the frequency dispersive dielectric response were investigated using canonical relaxor Pb0.92La0.08(Zr0.65Ti0.35)0.98O3 (PLZT) and Pb(Sc1/2Ta1/2)O3 (PST). It was able to deconvolute the plots, revealing three distinct processes, namely, slow, intermediate, and fast relaxtor, in terms of their characteristic relaxation time. 2) Lead-free (1-x)Bi1/2Na1/2TiO3-xBaTiO3 (BNT-xBT) system with artificially made deficiencies were investigated to define the correlation between the degree of random-field and relaxor features. The random-field is considered as the origin of creation of PNRs caused from random charges and/or random cation vacancies, which is well an accepted model for both lead-based and lead-free relaxors. 3) The conceptually suggested two-step process for a transition from a ferroelectric-to-relaxor state is observed in PLZT by electric-field induced in-situ monitoring methods such as an electrocaloric effect measurement, neutron diffraction and transmission electron microscopy. 4) The correlation between temperature-dependent and electric-field-induced transition from a ferroelectric-to-relaxor state, related to a deviation between the Td and the TF-R, is defined using ferroelectric PIC151, lead-based relaxor PLZT and lead-free BNT-BT. The major contribution of this dissertation is to expand our insight into what truly happens in relaxors. New systematic approaches will be discussed in detail in the body of this dissertation.clos