Study On Valence-state Behavior,Dielectric Properties And Defect Chemistry Of (Ba_1-xEu_x)(Ti_1-yM_y)O₃（M=Cr,Co,Tm） Ceramics

Barium titanate（BaTiO₃）ceramics co-doped with rare-earth Eu and transition-metal Cr/Co,with double rare earths Tm and La,Eu,Dy,were prepared using a mixed-oxides method.The structure,microstructure,site occupations and valence-variable phenomenon of doping ions,defect chemistry,and dielectric properties of these ceramics were studied using X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy-dispersive X-ray spectroscopy（EDXS）,X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy（RS）,photoluminescence（PL）,fluorescence spectroscopy（FS）,electron paramagnetic resonance（EPR）,and dielectric measurements.The dielectric materials satisfying Y5R,Y7R and X8R specifications were successfully achieved.Nominal(Ba_1-xTm_x)Ti_1-x/4O₃withBa-sitesubstitution（BTmT）,Ba(Ti_1–xTm_x)O_3-x/2-x/2 with Ti-site substitution（BTTm）,and(Ba_1-x/2Tm_x/2)(Ti_1-x/2Tm_x/2)O₃（BTmTTm）ceramics with self-compensation mode were explored to study the site-occupation preference of Tm³⁺in BaTiO₃.Tm³⁺ions in BTTm and BTmTTm cannot completely enter the BaTiO₃ lattice,accompanied by appearance of a secondary phase of Ba₆Tm₂Ti₄O₁₇.The solubility limit of Tm³⁺ions in BTmT is x=0.02,and these ceramics exhibit a tetragonal perovskite structure.The above results indicated that Tm³⁺ions prefer Ba sites in the BaTiO₃ lattice and show a slight amphoteric behavior.(Ba_1-xRE_x)(Ti_1-xTm_x)O₃（RE=Eu,La,Dy）ceramics are abbreviated BLTTm,BETTm,and BDTTm,respectively.The solubility limit of La/Tm in BLTTm was determined to be x=0.08.The dielectric thermal behavior of BLTTm with x=0.05satisfies the X6T specification,with a very low tanδ（<0.02）below 100°C.Tm³⁺ions in BLTTm exhibit an asymmetric amphoteric behavior.The solubility limit of Eu/Tm and Dy/Tm in the BaTiO₃ lattice is lower（x=0.03）,and BETTm and BDTTm with x≤0.03 have a tetragonal perovskite structure.Eu³⁺and Tm³⁺exhibit a slight amphoteric behavior.(Ba_1-xEu_x)(Ti_1-xCr_x)O₃（BETC）and(Ba_1-xEu_x)(Ti_1.01-xCr_x-0.01)O₃（BETC1）ceramics exhibit single-phase perovskite structures and the mixed valence states of Eu³⁺/Eu²⁺and Cr³⁺/Cr⁵⁺.Co-doping with Eu and Cr greatly inhibits the grain growth of BaTiO₃ ceramics,resulting in a fine-grained microstructure（0.7μm）.Two permittivity peaks of BETC with x≥0.02 are characteristic of an inhomogeneous dopant concentration distribution forming a core-shell structure.For BETC1,the mixed valence states and site occupations of Eu/Cr were analyzed to elucidate an expansion in the unit-cell volume,very high Eu/Cr ratio（=3.5）in coarser grains,regular diffuse phase transition（DPT）behavior,a linearly decreased dielectric peak temperature（T_m）at a rate of-24°C/at.%（Eu/Cr）,higher room-temperature dielectric permittivity(ε′_RT=4550),and lower dielectric loss（tanδ=0.0196）in BETC1,as well as the destruction of the core-shell structure in BETC.For the(Ba_1-xEu_x)(Ti_1-xCo_x)O₃（x=0.01⁰.05）（BEuTCo）and(Ba_1-xEu_x)(Ti_1.01-x.01-x Co_x-0.01)O₃（x=0.03）（BEu3TCo2）ceramics,BEuTCo has a tetragonal perovskite structure for x≤0.03 and a pseudo-cubic structure for x≥0.04.The mixed valence states of Eu³⁺/Eu²⁺and Co³⁺/Co⁴⁺were confirmed.BEuTCo ceramics with x=0.02and 0.03 satisfy temperature-stable Y7R and Y5R dielectric specifications,respectively.BEu3TCo2 satisfies the X8R dielectric specification.Co-doping with Eu and Co greatly inhibits the grain growth of BaTiO₃ ceramics,resulting in a fine-grained microstructure.