Critical Properties Of Potassium Tantalate Niobate Crystal Near The Curie Temperature

The ferroelectric materials are those with excellent piezoelectric, electro-optic, pyroelectric and electrocaloric effects, which have been widely applied to fabricate the ultrasonic transducer, electro-optical modulator, pyroelectricity sensor and energy storage. In the past half a century, solid-solution perovskite ferroelectrics have attracted great attention of researchers because of their best piezoelectric, electrostrictive and electro-optic effects near the phase-transition boundary. It have been suggested that these excellent functional properties of solid-solution ferroelectrics have close connections to the inner polar nanoregions(PNRs) near the phase-transition boundary. Thus, clarifying the characteristic properties of PNRs and the relationship between them and the macroscopic properties of materials have been important topics in the ferroelectrics research and will supply possible ways to realize the artificial modulation of ferroelectric properties, new ferroelectric materials and new functional properties exploration. KTa1-x Nbx O3(KTN) crystal is a kind of solid-solution ferroelectrics with typical perovskite structure. It is a proper object to do research about PNRs with the advantages of clear site-occupation, single source of polarization contribution and without nonequilibrium local field. At present, the research about PNRs mostly concentrates on the origin and evolution aspects. The relationship between PNRs and macroscopic properties and the PNRs modulation are seldom reported, which is exactly needed to understand for realizing the real value of PNRs research.The evolution of polarization in the para-ferroelectric phase-transition process of KTN on both sides of the Curie temperature has been investigated at first. Above the Curie temperature, the origin and evolution of PNRs, influence of material composition on PNRs, were systematically discussed. Below the Curie temperature, the spontaneous polarization in tetragonal KTN was found to exhibit good recovery property. Then the asymmetric polarization distribution mechanism was properly proposed for KTN crystal to explain the above phenomena in tetragonal phase combing the composition fluctuation and Landau phase-transition theories. The asymmetric distribution of Nb5+ ions results in the asymmetric potential well of Gibbs free energy, giving prior orientation of local polarization, supplying the recovery force to polarization after applying the electric field.The influence mechanisms of PNRs on the refractive index of KTN were investigated at first. Then the impact of PNRs on the quadratic electro-optic coefficients of KTN was analyzed with the above mechanism. The temperature dependence of quadratic electro-optic coefficients near the Curie point was researched by the Mach-Zehnder interferometer system and influence of PNRs in this process was discussed. In the following, the frequency dependence of quadratic electro-optic coefficients near the Curie point was also researched by the Senarmont compensator method system and frequency response of PNRs in this process was analyzed. At last, the redistribution of orientation density of PNRs was proposed to understand the contribution mechanism of PNRs to electro-optic effect.The influence of PNRs on the electrostrictive effect and electro-mechanical coupling phenomenon in KTN crystal was researched. A rare electro-mechanical coupling resonance was observed in paraelectric KTN above Curie temperature by the Senarmont compensator method, which is merely induced by an AC electric field 10 k V/cm. In the following, the origin of this electro-mechanical coupling resonance was evidenced to be the giant electrostrictive effect in KTN near the Curie temperature, having nothing to do with the piezoelectric effect which is the normal origin of electro-mechanical coupling in ferroelectric phase. At last, the physical origin of giant electrostrictive effect in KTN was investigated and found to be the local strain-field of PNRs and their collectively disorder-order reorientations.The non-random orientation of PNRs in KTN was investigated. At first, the anomalous birefringence associated with the anisotropic distribution of PNRs was observed in the KTN above Curie temperature and it shows good ergodicity in the repetitive temperature increasing and decreasing process. Then the mechanism of anisotropic distribution of PNRs for the anomalous birefringence in KTN was properly proposed. At last, the nonuniform of local composition is proved to be the origin of non-random orientation of PNRs, which finally induces the anisotropic distribution of PNRs. This mechanism can also be applied in the anomalous birefringence phenomenon in other solid-solution ferroelectrics after being properly extended.