Dielectric And Magnetic Properties Of Ba₆MNb₉O₃₀(M=Fe,Co,Ni) Ceramics With Tungsten Bronze Structure

Recently.tungsten bronze ceramics have attracted much scientific attention because of the possible multiferroic characteristics.In the present work.the dielectric. ferroelectric and magnetic properties of Ba₆MNb₉O₃₀(M=Fe.Co.Ni) ceramics with tungsten bronze structure have been investigated together with the physical nature and the structural origin,and the following primary conclusions have been obtained.The tetragonal tungsten bronze structure in space group P4bm is determined with the lattice parameters:a=b=12.597(6)(?),c=3.989(1)(?).V=633.082(8)(?)³ for Ba₆FeNb₉O₃₀.Dielectric characteristics of Ba₆FeNb₉O₃₀ ceramics have been evaluated over a broad temperature and frequency range.An extremely high dielectric constant with strong frequency dispersion is observed,and the dielectric abnormity at low temperature is similar to those of the relaxor dielectric behaviour and should originate from the random distribution of Fe³⁺ and Nb⁵⁺ in the center of the oxygen octahedra.While the dielectric abnormity at high temperature is related to the high conductivity.The nonlinear magnetic hysteresis curve is observed in the present ceramics at 5 K.which is related to magnetic ions(Fe³⁺) in tungsten bronze structure.The tetragonal tungsten bronze structure in space group P4bm is determined for Ba₆CoNb₉O₃₀,A broad dielectric peak with strong frequency dispersion is observed at 140 K-225 K.which well follows the Vogel-Fulcher relationship and indicates that the relaxor behavior is analogous to a spin glass with polarization fluctuations above a static freezing temperature.The P-E hysteresis loops varying with temperature are consistent with this dielectric relaxation process and provide the further evidence on the relaxor ferroelectric nature.The nonlinear magnetic hysteresis curve is observed in Ba₆CoNb₉O₃₀ ceramics at 5 K.indicating the soft magnetic behavior which is related to the tetragonal tungsten bronze crystal structure containing magnetic ions of Co³⁺.Ba₆NiNb₉O₃₀ forms the tetragonal tungsten bronze major phase combined with minor amount of Ba_1.04NbO₃.A broad dielectric relaxation peak with strong frequency dispersion is observed.An unobvious dielectric abnormity which associates with some inhomogeneity is observed.P-E hysteresis loops reveal the existence of ferroelectricity,the remnant polarization(2P_r) and electric field(2E_c) are 2.08μC/cm² at 31.22 kV/cm in Ba₆NiNb₉O₃₀.The nonlinear magnetic hysteresis curve is observed in Ba₆NiNb₉O₃₀ ceramics at 5 K.indicating the soft magnetic behavior which is related to the tetragonal tungsten bronze crystal structure containing magnetic ions of Ni³⁺.