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, Ba6Sc1-xInxNb9O30 (x = 0.25, 0.5, 0.75), which display relaxor properties were characterised using high- and low-temperature dielectric spectroscopy. All powders were synthesised at 1250 degrees C (24 h) + 1350 degrees C (6.5 h), while the pellets were further sintered at 1350 degrees C (12 h). 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 increase 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.