Study On Dielectric Properties And Defect Chemistry Of La And Tb Co-doped BaTiO₃ Ceramics

Barium titanate?Ba Ti O₃?is well known to play an important role in electronic ceramics due to its excellent dielectric performance.Rare earth elements are rich in China and usually trivalent in compounds.The atomic number?AN?of Terbium?Tb?is 65,lying at between Gd and Dy in AN.Tb is a special rare-earth element.In compounds,Tb exists as Tb3+?4f8?or as metastable Tb4+?4f7?.However,researchers seldom considered the presence of Tb4+ in Ba Ti O₃-based ceramics.In this work,Ba Ti O₃ ceramics co-doped with La/Tb and those with the self-compensation mode of Tb were prepared by a mixed oxide method.The crystal structure,dielectric properties,and defect chemistry were investigated by X-ray diffraction?XRD?,Raman spectroscopy,scanning electron microscopy?SEM?,dielectric measurements,electron paramagnetic resonance?EPR?,and X-ray photoelectron spectroscopy?XPS?.1)Dielectric properties and defect chemistry of La/Tb co-doped Ba Ti O₃ceramicsThe nominal(Ba_1-x La_x)(Ti_1-x Tb_x)O₃?0.03 ? x ? 0.20?dielectric ceramics?BLTT?were prepared.The solid solution limits of Tb in BLTT were x = 0.15.BLTT had a tetragonal perovskite structure when x ? 0.03 and a cubic perovskite structureand when0.05 ? x ? 0.15.The dielectric-peak temperature?Tm?decreased linearly at a rate of-19 °C/%?La-Tb?.BLTT with x = _0.07 can be considered as a promising dielectric for X7 U applications because of its higher room-temperature permittivity??? = 2450?and lower loss?tan ? = 0.029?.The study on the defect chemistry indicates that the point defects in BLTT are Ba-site La3+?·BaLa?,Tb3+?·BaTb?,Ti-site Tb3+?'TiTb?,Tb4+??TiTb?,and Ti vacancies?''TiV?.Ba-site La3+ and Ti-site Tb3+ are predominant.The EPR and XPS results give the evidence of the predominant occupations of Tb4+ ions at Ti sites.The existence of both a small number of metastable Tb4+ ions at Ti sites and predominant La3+-Tb3+ complexes at double sites is responsible for the lower tan ? of BLTT.The incorporation of the extra Tb ions in(Ba_1-x La_x)(Ti_1-x Tb_x)O₃?x = 0.03?cangreatly enhance the ceramic relative density from 84 % to 96 %,accompanied by improvement in dielectric permittivity of BLTT.2)The exploration on the self-compensation mode of Tb in Ba Ti O₃ ceramics(Ba_1-x Tb_x)(Ti_1-x Tb_x)O₃?0.05 ? x ? 0.20?ceramics?BTTT?with a self-compensation mode were prepared.BTTT had a tetragonal perovskite structure.The solid solution limit of Tb in BTTT was determined to be x = 0.12 and the variation in unit cell volume?V0?with x satisfied the Vegard's law.The study on the defect chemistry indicates that in the case of Ba/Ti = 1,a complete self-compensation mode of Tb3+ in Ba Ti O₃ could not be formed like Eu3+ or Dy3+,but Tb exhibited a self-adjustable amphoteric behavior with the mixed valence states of Tb3+/Tb4+ to preserve lattice electroneutrality.The room temperature resistivity??RT?showed a linear fall from 1.0 × 108 to 1.7 × 107 ? cm with x owing to a gradually enhanced donor effect of Tb3+ ions at Ba sites.The x = 0.05 BTTT sample exhibited higher room-temperature permittivity???RT = 1190?and lower loss?tan ? < 0.02?,and its ??was independent of frequency in the frequency range 102 to 105 Hz.This ceramic is a promising candidate for X5 S dielectric application.3)The effect of sintering temperature on solid solution and dielectric properties of(Ba_0.93La_0.07)(Ti_0.93Tb_0.07)O₃ ceramics(Ba_0.93La_0.07)(Ti_0.93Tb_0.07)O₃ ceramics?BLTT7?were prepared under different sintering temperatures?Ts?.The results showed that as Ts was increased from 1100 to1300 °C,La ions were firstly incorporated into the perovskite lattice to form La-rich core grains,forming a so-called “core-shell” structure,in which no O-vacancy defects were detected.Tb ions in the shell gradually diffused into the Ti site as mixed Tb3+/Tb4+.BLTT7 sintered at Ts = 1300 °C satisfied the X8 P dielectric specification.BLTT7 sintered at Ts = 1400 °C satisfied the X7 U specification,but the higher sintering temperature of Ts = 1400 °C led to the collapse of “core-shell” structure.