Formation of Solid Solutions in BaTiO₃-BaNi_1/3Ta_2/3O₃ and BaTiO₃-BaMg_1/3Ta_2/3O₃ Systems and Their Dielectric

Abstract

Specimens were prepared from BaCO₃, TiO₂, MgO, NiO and Ta₂O₅ with high purity. They were finally fired at 1400°C for 10-20 hrs. in air and were left to cool in the furnace. By X-ray powder diffraction analysis for them all of specimens showed single phase patterns with perovskite structure. Their lattice constants calculated from (422) reflections explained that a normal solid solution series was formed within the whole composition range in the system BaTiO₃-BaNi_1/3Ta_2/3O₃, however, an abnormal one was obtained in the system BaTiO₃-BaMg_1/3Ta_2/3O₃. On the X-ray powder diagrams the super lattice reflections of 1:2 cation order were seen in BaMg_1/3Ta_2/3O₃, whereas they were not clear in BaNi_1/3Ta_2/3O₃.
It is assumed that in solid solutions with BaMg_1/3Ta_2/3O₃ contents greater than 50 mole%, one Mg²⁺ layer and two Ta⁵⁺ layers through the octahedrally coordinated positions are alternately stacked along a [111] direction in the perovskite cell and the layers are partially occupied by Ti⁴⁺ ions without breaking the layer structure. In solid solutions with BaMg_1/3 Ta_2/3O₃ contents smaller than 50 mole%, as well as in the system BaTiO₃-BaNi_1/3Ta_2/3O₃, the arrangements of the cations in the octahedrally coordinated positions are regarded to be at random. Thus the system BaTiO₃-BaMg_1/3Ta_2/3O₃ may be considered as a rare case in which a solid solution series could be formed in a relatively wide composition range between a simple perovskite-type compound ABO₃ and a complex one A (B′B″)O₃ having an ordered arrangement of the composit cations.
The observed dielectric properties showed no essential difference between both systems. The peaks of the dielectric constants of the solid solutions did not shift appreciably from 120°C, but their heights were rapidly decreased with increasing contents of BaB_1/3Ta_2/3O₃ (B=Ni, Mg). Above the peak-temperatures the dielectric constants obeyed the Curie-Weiss law. Those dielectric properties are not regarded to be intrinsic to those solid solutions but can be attributable to the precipitated BaTiO₃ from the solid solutions in the preparation process. An exponential dependence of the dielectric constants on the content of BaB_1/3Ta_2/3O₃ (B=Ni, Mg) may be explained in an analogous way to the dependence of magnetic permeability on the content of ferromagnetic inclusions dispersed in a nonmagnetic cementing material.