Structural, electrical and relaxor properties of Sc-In solid solution in tetragonal tungsten bronze ceramics

The electrical properties of a family of tetragonal tungsten bronze (TTB) materials, Ba₆Sc_1-xIn_xNb₉O₃₀ (x = 0.25, 0.5, 0.75), which display relaxor properties were characterised using high-temperature impedance spectroscopy and low-temperature dielectric spectroscopy. All powders were synthesized at 1250°C (24hours) + 1350°C (6.5hours), while the pellets were further sintered at 1350°C (12hours). At high temperature, the electrical conductivity in all samples is dominated by a single response, attributed to the bulk response, while the subtle asymmetry of the imaginary impedance peak suggests a minor contribution of the grain boundary response. In general, the Sc-In solid solutions samples exhibit similar electrical behaviour to the Sc compound, x = 1 reported previously (Ceram. Int. 2016, 42, 11810–11821). At low temperature all compounds exhibit frequency dependent relaxor behaviour. With increasing the content of In, the dielectric peaks are more asymmetric and the curves flatten more at temperatures above the transition temperature. The dipole freezing temperature, T_VF, determined by Vogel-Fulcher (VF) analysis does not simply increases linearly as a function of M³⁺ (Sc³⁺,In³⁺) cation size, the extrapolation of VF-like curves results in considerably different values when compared to those found previously, since the acquired dielectric data are very sensitive to preparative conditions.