Multiferroic Properties Of Ba₄R₂,Fe₂(Nb_xTa_1-x)₈O₃₀（R=Nd,Sm）Ceramics

Multiferroic materials with coexistence of at least two ferroic orders (ferroelectric, ferromagnetic, or ferroelastic) have attracted particular attention both in science and technology, because of their fascinating physics and their potential applications in multifunctional devices. Introducing magnetic ions into compounds with filled tetragonal tungsten bronze (TTB) structure is one of the important way to explore new multiferroics. In the present thesis, Ba4R2Fe2(NbxTa1-x)g03o (R=Nd, Sm) with TTB structure were prepared by standard solid state reaction process. The effects of magnetic cations and ionic substitution at B-sites on their structure and properties, together with the origin of ferromagnetism, were investigated. The effects of heat-treatment in different atmospheres on the dielectric, conductive and ferroelectric properties of Ba4Nd2Fe2Nb8O30ceramics were studied.In the Ba4Nd2Fe2(NbxTa1-x)8O30(x=0,0.5,0.7,0.9and0.95) system, the ceramics with composition x=0.9and0.95show diffuse-type ferroelectric phase transition with Tc above the room temperature (306K for x=0.9and320K for x=0.95). Magnetic hysteresis loops for x=0.7to0.95at room temperature were obtained and ascribed to the filled tungsten bronze structure phase. Hence the Ba4Nd2Fe2(NbxTa1-x)8O30ceramics with composition x=0.9and0.95are room temperature single-phase multiferroics. With the increasing of x, the ferroelectric transition becomes less diffusive and Tc shifts to higher temperature.In the Ba4Sm2Fe2(NbxTa1-x)8O30(x=0,0.5,0.7,0.95,1) system, the ceramics with composition x=0and0.5are paraelectrics at room temperature, while the ceramics display a ferroelectric nature for x=0.7,0.95and1with Tc=348K,395K and418K respectively. For all the compositions, magnetic hysteresis loops were obtained and ascribed to the tungsten bronze structure phase. With the increasing of x, the remnant moments increase. The Ba4Sm2Fe2(NbxTa1-x)8O30ceramics with composition x=0.7,0.95and1are room temperature single-phase multiferroics.It was found that oxygen defects play a very important role on the dielectric, conductive and ferroelectric properties of Ba4Nd2Fe2Nb8O30ceramics. The dielectric relaxation above Tc can be ascribed to the trap-controlled ac conduction around doubly ionized oxygen vacancies. The dc conductivity of the N2-annealed and O2-annealed samples are attributed to the long-range motion of the V-, and that of the as-sintered sample is considered to be governed by the electronic and oxygen-vacancy ionic mixed conduction mechanism. The annealing process greatly affects the concentration and distribution of the oxygen defects. Low concentration and random distributed oxygen vacancies are propitious to the domain switching, while high concentration and allied oxygen defects hinder the domain-wall movement.