Preparation of dense BaPb0.75Bi0.25O3 ceramic by controlling the defect structure

The improvement of the sintering properties of a superconducting BaPb0.75Bi0.25O3 ceramic by the introduction of point defects was examined. In order to introduce Ha vacancies into the structure, a mixture of BaCO3, Pb3O4 and Bi2O3, with Ba-deficient nominal composition, was used as a starting material. The density of the specimen increased by using this nominally Ba-deficient mixture, However, it was revealed that the grain size in the sintered material was not affected by the amount of the nominal Ha deficiency and that a second phase, composed mainly of Hi and oxygen, was generated at grain boundaries, It was concluded that Ha vacancies could not be formed in BaPb0.75Bi0.25O3 lattice and that the apparent high density and electrical resistivity, which was as high as 10(-2) Omega.cm at room temperature, is attributed to the second phase formation at grain boundaries. When sintering under low oxygen partial pressure, the density of the superconducting BaPb0.75Bi0.25O3 ceramic also increased. However, it was observed that the grain size of the BaPb0.75 Bi0.25O3 ceramic increased without the generation of a second phase at grain boundaries while sintering under reducing atmosphere. The dependence of the grain size on the oxygen partial pressure led to the suggestion that enhanced grain growth is governed by the increase of volume diffusion due to the introduction of oxygen vacancies, After annealing under oxygen to remove oxide ion vacancies, the specimen showed an electrical resistivity at room temperature as low as 10(-3) Omega.cm and a sharp superconducting transition at 11.5 K.