Electrical properties of rutile-type FeTiMO₆ (M = Ta,Nb)

Various electrical properties of polycrystalline rutile-type compounds FeTiTaO₆ and FeTiNbO₆ were studied between room temperature and ∼750 K. The main purpose of this investigation was to analyse to what degree the giant relaxor-type dielectric constant ϵ(ω), with broad and dispersive maxima in ϵ(ω), suggested to be dictated by the presence of polar nanodomains due to the variety of heterovalent cations, is affected by extrinsic properties such as grain boundary and sample-electrode interfacial effects. For FeTiNbO₆, the voltage dependence of the magnitude of capacitance C _p (ω) in the whole frequency range, pointing to Schottky-type barriers, is strong indication of a considerable influence by electrode contact effects. For FeTiTaO₆, the dependence of C _p (ω) on the preparation conditions, including the cooling rate, points to the effect of microstructural properties, likely in part by oxygen deficiency leading to inhomogeneities at grain boundaries and resulting in high-capacitive layers. Values of DC conductivity σ _DC could be determined for the bulk, grain boundaries and electrode contacts in certain temperature ranges. The bulk contribution of FeTiTaO₆ to σ _DC is characterized by activation energy E _A  = 0.35 eV and σ _DC(295 K) ∼ 3 × 10^− 6 Ω^− 1cm^− 1 and for FeTiNbO₆ by E _A  = 0.31 eV and σ _DC(295 K) ∼ 7 × 10^− 5 Ω^− 1cm^− 1. Grain boundaries and electrode contacts are related to much lower σ _DC and higher E _A values, i.e. they exhibit higher resistivities by interlayer effects. The characteristic features of the frequency dependence of AC conductivity of both oxides are marked by grain boundary and electrode effects. Relaxation processes were established from loss data, being likely due to these effects. The thermopower is negative showing weak variation with temperature and pointing to a charge transfer polaron-hopping mechanism in the bulk of both compounds. The Mössbauer spectrum of FeTiNbO₆ shows besides the Fe^3 + component a trace of Fe^2 +.