Structure,Magnetic And Dielectric Properties Of Rare Earth Double Perovskite And Perovskite-like Ceramics

As an important kind of giant dielectric constant materials, rare earth double perovskites have drawn much scientific attention, but the physical nature of their dielectric behavior has always been controversial. In the present thesis, the structure, together with magnetic, dielectric and transport properties of rare earth double perovskite ceramics and perovskite-like ceramics were investigated systematically. The relationship between physical properties and structure was discussed deeply, and the physical nature of dielectric relaxation and giant dielectric response was revealed.Both Nd2NiMn06and Sm2NiMnO6ceramics were ferromagnetism with the monoclinic symmetry, which were similar with La2NiMno6ceramics. The ferromagnetic originated from the ordered alignment Ni2+-O-Mn4+superexchange interaction. The<Ni-O-Mn> bond angle decreased with the reduced lanthanide ionic radius (RLn) in Ln2NiMnO6(Ln=La, Nd and Sm) ceramics, resulting in the decrease of the Curie temperature TC.The gaint dielectric constant was a common feature of Ln2NiMnO6(Ln=La, Nd and Sm) ceramics, and the dielectric constant ε’monotonically decreased with decreasing RLn. The hopping of electrons between Ni2+and Mn4+cations, which was equivalent to the reversal of polar region, contributed to the gaint dielectric constant. The similar variation tendency of magnetic and dielectric properties with RLn indicated that the structural origins of the magnetic and dielectric response should be closely linked. The magnetodielectric effects observed in Nd2NiMnO6ceramics at room temperature further conformed the coupling between magnetic and dielectric response.In both Nd2CoMnO6and Sm2CoMnO6ceramics, the ordering of Co2+/Mn4+and disordering of Co3+/Mn3+contributed to two magnetic transition temperatures. The <Ni/Co-O-Mn> bond angle played an important role in magnetic properties of Ln2MMn06(Ln=La, Nd and Sm, M=Ni and Co) ceramics. Ferromagnetic Curie temperature TC generally decreased with decreasing<Ni/Co-O-Mn> bond angles, which depended on RLn-Meanwhile, as RLn decreased, the electron hopping between Co2+and Mn4+was more suppressed and larger activation energy was needed, thus the dielectric constant ε’decreased monotonically. Tc and ε’exhibited same variation tendency with the variation of RLn, and both of them had been correlated with <Co-O-Mn>, demonstrating a certain extent coupling between magnetic and dielectric response.There was only one dielectric relaxation with strong frequency dependence in La2CuTiO6ceramics at200-500K, and a giant dielectric constant step (ε’～10,000) was observed. The dielectric relaxation in the present ceramics should be attributed to the mixed-valent structure (Cu+/Cu2+and Ti3+/Ti4+), which was induced from the oxygen vacancy. The concentration of oxygen vacancies decreased after the oxygen annealing, and fewer dipoles were produced by suppressed mixed-valent structure, leading to the weakening of dielectric response in La2CuTiO6ceramics.Two dielectric relaxations following Arrhenius law were observed in La2CuSnO6ceramics at180-300K and300-550K, respectively. Both of them were thermal activated process. The low-temperature relaxation was nearly a Debye relaxation with the intrinsic nature, which probably attributed to the charge hopping between Cu2+and Cu3+ions. The high-temperature relaxation was closely related to the thermal activated hopping process of electrical conduction, since the dielectric relaxation activation energy was similar to that of grain interiors conductivity and the variation of dielectric behaviors and ac conductivities were the same.The charge ordering La1.5Sr0.5CoO4ceramics demonstrated a thermally activated dielectric relaxation with activation energy of0.44eV, which originated from the Co2+-Co3+mixed-valent structure, i.e., dipolar effect associates with localized charge carriers (polarons) hopping between spatially fluctuating lattice potentials. The main charge carrier in La1.5Sr0.5CoO4was the holes located at Co3+ions. They moved around in the form of small polarons under applied electric field because they were bound to Co3+ions. Therefore, the intersite hopping of polarons under ac electric field contributed to dielectric relaxation, while the transport of polarons through Co2+-O-Co3+linked path under dc electric field produced dc conduction. The dielectric constant and ac conductivities of La1.5Sr0.5CoO4ceramics increased after annealing in O2atmosphere, while opppsite effect was obtained after annealing in N2atmosphere. This abnormal phenomenon should be attributed to the variation of concentration of small polarons, which is due to the capability of K2NiF4-type arrangements for accommodating extra oxygen in the interstitials of the structure.