Study On Dielectric Properties And Defect Chemistry Of R/Ca ?R=Tb,Ho?and Pr/Ce Co-doped BaTio₃ Ceramics

Five series of ceramics with nominal formula?Ba1-_xHo_x??Ti1-_x Ho_x?O₃?BHTH?,Ba1-_x/2Ti1-_x/2Tb_x O₃?BTTb5?,?Ba₁–_x Pr_x?(Ti₁–_y–_x/4Ce_y)O₃?BPTC?,?Ba₁–_xTb_x?(Ti₁–_x/2Ca_x/2)O₃?BTTC?,and?Ba₁–_xHo_x?(Ti₁–_x/2Ca_x/2)O₃?BHTC?,were prepared using the conventional solid-state reaction method.The purpose of study on BHTH and BTTb5 with self-compensation modes is to lay a solid foundation on deep study on BPTC,BTTC,and BHTC.The structure,microstructure,density,solid solubility,valence states of co-dopants,dielectric properties,site occupation,and point defect chemistry were investigated using X-ray diffraction?XRD?,scanning electron microscopy?SEM?,Raman spectroscopy?RS?,X-ray photoelectron spectroscopy?XPS?,high-resolution transmission electron microscopy?HRTEM?,energy-dispersive X-ray?EDX?spectroscopy,photoluminescence?PL?,electron paramagnetic resonance?EPR?,and dielectric measurements.BHTH ceramics were prepared at a sintering temperature of T_s=1400°C.The result showed that under the condition of Ba/Ti=1,Ho³⁺in the BaTiO₃ lattice prefered a self-compensation mode with a slight Ba-site preference.BHTH ceramics showed a fine-grained microstructure and the solubility limit of Ho³⁺in BHTH was determined by XRD to be x=0.03.An abnormal phenomenon was observed,i.e.that the Curie temperature?T_C?of BHTH shifted towards high temperatures with increasing x.The relationship between dielectric properties and defect chemistry in BHTH was discussed.Ba_1-x/2Ti_1-x/2Tb_x O₃?x=0.05?ceramics?BTTb5?were prepared at T_s=1400°C for t=6?24h and at T_s=1300?1480°C for t=24 h,respectively.The results showed that the optimum preparation condition of BTTb5 was determined as T_s=1400°C for t=12 h because of higher densification and lower dielectric loss.As the sintering time increased,more Ti-site Tb⁴⁺ions are reduced to Ba-site Tb³⁺.The higher sintering temperature is apt to ceramic densification and the incorporation of more Tb⁴⁺ions into Ti-sites.Fine-grained(Ba_1–xPr_x)(Ti_1–y–x/4Ce_y)O₃?BPTC:x,y=0.03?0.05?ceramics were prepared at T_s=1450°C.The results showed that BPTC ceramics had a pseudo-cubic perovskite structure with a diffused high-permittivity peak around room temperature.Five BPTC compositions met Y5V dielectric specifications,in which the x=y=0.04 ceramic exhibited advantageous Y5V characteristics,such as higher room-temperature permittivity(?'_RT¹1000)and lower dielectric loss?tan?=0.027?.The defect chemistry of BPTC ceramics was discussed from EPR results in combination with ionic radii.New-type BTTC dielectric ceramics with nominal formula of?Ba₁–_xTb_x?(Ti₁–_x/2Ca_x/2)O₃?x=0.01–0.20?were prepared at T_s=1400oC.The results showed that the solubility limit of Tb/Ca in BTTC was determined by XRD to be x=0.12 and BTTC exhibited tetragonal perovskite structures.An abnormal phenomenon was discovered that with increasing x from 0.01 to 0.04 the dielectric peak associated with the orthorhombic-tetragonal phase transition point shifted to low temperatures.The mixed valence states of Tb³⁺/Tb⁴⁺and mixed Ba/Ti occupancy of Tb/Ca²⁺in the perovskite lattice were confirmed.BTTC with x=0.03 and 0.04 satisfied X5R and X4P specifications,respectively,and exhibited some advantageous properties such as higher relative density??_r=91 and 93%?,lower loss?tan?=0.016 and 0.014?,higher permittivity,and frequency stability in the range of 1–10⁷ Hz.The point defect chemistry was discussed and the real formula of BTTC was suggested.By comparing BaTiO₃ ceramics containing Tb or co-doped with Tb and other rare-earth ions?La,Dy?in many existed works in the dielectric field,the synergistic effect of Tb and Ca in BaTiO₃ is more significant and helpful to improve the temperature stability of Tb-related BaTiO₃ ceramics or to reduce the dielectric loss in the working temperature region.A small number of Ti-site Tb⁴⁺in BTTC played a decisive role in the low dielectric loss.Nominal?Ba₁–_xHo_x?(Ti₁–_x/2Ca_x/2)O₃?BHTC?with high relative density??_r=⁹5%?was prepared at T_s=1400oC.The results showed that the solubility limit of Ho/Ca in BHTC was determined by XRD to be x=0.10 and BHTC with x?0.10 exhibited tetragonal perovskite structures.The unit-cell volume?V₀?increased linearly and the tetragonal degree?c/a?exhibited a downward trend with the increase of x from 0.01 to 0.15.Ho³⁺and Ca²⁺ions in the perovskite lattice dominantly substituted on the Ba-sites and Ti-sites,respectively.BHTC with x=0.06 and0.08 exhibited some advantageous properties such as higher relative density??_r=91 and 93%?,higher permittivity(?'_RT=2980 and 2730),lower loss?tan?=0.016-0.035,–75oC²00oC?,and Y5V specifications.The dielectric-peak temperature of BHTC shifted towards low temperatures with increasing x from 0.01 to 0.03 and T_C shifted towards high temperatures in the x range of0.01?x?0.10.The point defect chemistry of BHTH was disccussed.In conclusion,BaTiO₃-based ceramics satisfying Y5V,X5R and X4P specifications were achieved in this study successfully.Three formulations have been successfully explored and can be used as alternative options for industrial applications.In Tb/Ca co-doped BaTiO₃ ceramics,the existence evidence of variable valence Tb ions was found by XPS.The complex site occupations of Pr/Ce co-doped BaTiO₃ ceramics were discussed.The evidences of complex site occupations of Tb/Ca co-doped BaTiO₃ ceramics were found directly.