Abstract:
The search for new functional materials is inextricably linked with modification of the properties of already known materials with optimal parameters. Thus, sodium niobate is used as a basis for complex lead-free ferroelectric oxides. This work is devoted to the study of the structure and dielectric properties of sodium niobate ceramics doped with Bi and Fe in concentrations of 10, 20 and 30 mol.%, in a wide range of frequencies (1–10$^6$ Hz) and temperatures (30–650$^\circ$C). It was found that with an increase in the impurity concentration, the average grain size decreases from 5–10 $\mu$m to 0.5–2 $\mu$m, the heterogeneity of the microstructure increases, and the grain shape becomes closer to cubic. At the same time, the volume of the unit cell increases with an unchanged symmetry of the crystal lattice $(Pmc2_1)$. The method of dielectric spectroscopy revealed the presence of two relaxation processes corresponding to ionic thermal polarization and conductivity relaxation, which affects the temperature dependence of the permittivity. The doping leads to an increase in the temperature region of phase transitions and a shift of $T_m$ by approximately 150$^\circ$C to lower temperatures.
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