Fehr, K. T.; Hochleitner, Rupert; Schmidbauer, E.
Electrical properties of rutile-type FeTiMO6 (M = Ta,Nb).
In: Journal of Electroceramics, Vol. 29, Nr. 4: S. 240-249
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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 Ïμ(ω), with broad and dispersive maxima in Ïμ(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 Fe3+ component a trace of Fe2 + .