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 tan??<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?0.7?m?.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(??_RT=4550),and lower dielectric loss?tan?=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.