Crystal structure, dielectric and thermal properties of cobalt doped bismuth tantalate pyrochlore

A phase-pure pyrochlore of the Bi1.49Сo0.8Ta1·6O7.0 composition (space group Fd-3m:2, 10.54051 (3) Å, Z = 8 (No. 227)) obtained by the solid-phase synthesis was characterized. It was found that the pyrochlore structure is disordered, bismuth ions are shifted to the 96 g position, and oxygen ions replace the 48f and 32e positions. Co(II) cations are characterized by a mixed placement in the 96 g and 16b positions. The samples have a dense, low-porosity microstructure. According to high-temperature X-ray diffraction data, cobalt pyrochlore is stable up to a temperature of 1080 °C. With a further temperature increase, it thermally dissociates with the formation of CoTa2O6. The thermal expansion coefficient increases uniformly and weakly from 3.4 to 7.5·10−6 °С−1 in the range of 30–1020 °C. The electrical properties of the pyrochlore were studied by impedance spectroscopy. No ionic processes in the sample were detected. The Bi1·49Co0·8Ta1·6O7.0 is weakly dependent on temperature and frequency. At RT, the permeability varies in the range of 25–28. The dielectric losses tangent decreases with increasing frequency and at a frequency of 106 Hz is equal to ∼0.005. The conductivity activation energy in the range of 22–450°С is 0.58 eV.

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