Netiesinio dielektrinio jautrio tyrimai Pb(Mg1/3Nb2/3)O3 šeimos feroelektriniuose relaksoriuose

Abstract

Research of relaxor ferroelectrics is an attractive field. These materials are known to be among the best piezoelectrics for application reasons. However, this work is dedicated for a fundamental research of relaxor ferroelectrics. It is mainly focused on a research of macroscopic properties of Pb(Mg1/3Nb2/3)O3 based relaxor ferroelectrics. Pb(Mg1/3Nb2/3)O3 is named as the canonical relaxor ferroelectric by many authors. However, it happened historically, because it was one of the first relaxor ferroelectrics found. Since the beginning of the research of Pb(Mg1/3Nb2/3)O3 there were many debates if it has a phase transition or not. It does show anomalies in temperature dependence of refraction index, dielectric permittivity, velocity of sound, etc. However, clear structural changes of lattice were not observed, by microscopic research techniques, as the structure remains averagely cubic in broad temperature range. Furthermore, since the beginning of research of relaxor ferroelectrics, there were attempts to apply Landou-Ginzburg-Devonshire phenomenological theory of phase transitions on temperature behavior of Pb(Mg1/3Nb2/3)O3. As well, there were attempts to write different theoretical models onto the unique behavior of relaxor ferroelectrics. It is widely accepted, that polar nano regions are responsible for the unique behavior of relaxor ferroelectrics. Those are regions of tens of nanometers of size, which depends on temperature, pressure and various additions in the crystal. Within one region all dipoles tends to orient the same direction, so sometimes polar nano regions are interpreted as superdipoles or dipole clusters, that may have flipping, breathing or both dynamical properties in electric field and non-zero temperature. Polar nano regions appear in the relaxor ferroelectrics due to an internal disordered charge distribution. However, universal theoretical model of relaxor ferroelectrics is still not found and after each attempt many debates and rhetorical questions remain. This work contributes to the knowledge of Pb(Mg1/3Nb2/3)O3 based relaxor ferroelectrics. It confirms that enhancing chemically ordered regions do not weaken relaxor behavior or promote a spontaneous phase transition. In order to find out more about relaxor ferroelectrics in general and to analyze them from a different perspective a non-linear susceptometer was built. The measurement method allows to measure temperature behavior of higher order susceptibilities of material without influencing internal electric fields of the sample. That method may give useful data on phase transitions occurring in the material. As well, it is more sensitive to changes of dipolar dynamics of the sample. Thanks to this unique measurement technique a phase transition was found in 0.83Pb(Mg1/3Nb2/3)O3 – 0.17PbTiO3 single crystal. As well, it opened new perspective of observation of dipolar dynamics of Mn-doped 0.9Pb(Mg1/3Nb2/3)O3 – 0.1PbTiO3 ceramics. PbTiO3 is normal ferroelectric material and many authors try to find occurrence of phase transition threshold of relaxor – normal ferroelectric solid solutions, that is called morphotropic phase boundary. This region of phase diagram is important for application reasons, as solid solutions of this specific mixture has best piezoelectric and tunability properties. Finally, this work definitely gives contribution to fundamental knowledge of Pb(Mg1/3Nb2/3)O3 based relaxor ferroelectrics. Furthermore, it opens doors to a different perspective to these materials, by introducing a variant of non-linear susceptometer and various application examples of it