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München, Germany

Fehr K.Th.,Ludwig Maximilians University of Munich | Hochleitner R.,Mineralogische Staatssammlung | Schmidbauer E.,Ludwig Maximilians University of Munich
Journal of Electroceramics | Year: 2012

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 + . © 2012 Springer Science+Business Media New York. Source


Fehr K.T.,Ludwig Maximilians University of Munich | Hochleitner R.,Mineralogische Staatssammlung | Schmidbauer E.,Ludwig Maximilians University of Munich
Journal of Electroceramics | Year: 2013

Electrical properties of rutile-type Fe0.9W 0.05TiMO6TiMO6(M=Ta,Nb) ceramics were measured at and above room temperature and the results are compared with those gained previously on rutile-type relaxor ferroelectrics FeTiMO6(MTa,Nb). The aliovalent W6 cationsin the current compounds might change the suggested polar nanodomains, giving rise to very high dielectric constant ε(ω), and further electrical quantities can possibly shed additional light on the nature of the mechanism leading to extraordinary values in ε(ω). In part similar electrical data were established such as very high ε(ω) but also different results were noted. Apart from ε(ω), the electrical response was analysed by measuring losses, dissipation factor δ, DC conductivity σDC and AC conductivity σAC(ω) using impedance spectroscopy, and thermopower; the results are discussed in conjunction with literature data. The role of grain boundaries and sample-electrode processes was investigated in particular with respect to the sample capacitance. Eventual microstructural local inhomogeneities were checked by means of 57Fe Mössbauer spectroscopy. For both compounds, the temperature dependence of bulk σDC showed Arrhenius behaviour with activation energy EA∼ 0.35 eV and σDC (295 K) ∼ 5× 105Ω-1cm -1; grain boundaries exhibited slightly higher EA but the value of σDC was a factor of up to ∼ 10 lower at all temperatures. From σAC(ω) data, a power law frequency dependence of grain boundary conductivity was derived. Relaxation processes were established from loss and δ data. The thermopower is negative and varies weakly with temperature, pointing to long-range charge transfer by a hopping-type mechanism of electron polarons. © 2013 Springer Science+Business Media New York. Source


Fehr K.T.,Ludwig Maximilians University of Munich | Gunther A.,Ludwig Maximilians University of Munich | Hochleitner R.,Mineralogische Staatssammlung | Schmidbauer E.,Ludwig Maximilians University of Munich
Journal of Electroceramics | Year: 2014

Several electrical quantities of rutile-type Fe1-x Mn x TiTaO6 (0 ≤ x ≤ 0.3) ceramics were measured between room temperature and ∼ 770 K. One aim of the investigation was to study differences to data known from rutile-type FeTiTaO6 ceramics, showing giant relaxor ferroelectric-type constant (ω) (ω is angular frequency). FeTiTaO6 (x = 0) samples showed, as expected, capacitance C p (ω) peaks at 500-600 K using Ag-paint contacts, however upon application of Pt-paint, C p (ω) increased steadily up to the maximum employed temperature of ∼ 770 K. For x = 0.02 (Ag-paint), C p (ω) peaks were detected pointing to relaxor ferroelectric behaviour. For x > 0.02 (Ag-paint), no C p (ω) peaks were observed; C p (ω) increased for all frequencies steadily with temperature up to maximum values at the highest applied temperature. Complex plane impedance plots were characterized for each composition by two semicircular arcs due to bulk and grain boundary charge processes. A marked separation of arcs was observed for FeTiTaO6 (x = 0) and 0.02; for x ≥ 0.05 this fact became much reduced. The derived DC conductivity σ DC for bulk conduction showed Arrhenius behaviour for all compositions with activation energy E A ∼ 0.4-0.7 eV and σ DC (300 K) ∼ 10-9 - 10-6Ω -1cm-1 depending on x. For x ∼ 0.05, the frequency dependence of AC conductivity σ AC (ω) was weak at and above room temperature, in contrast for x < or > 0.05 a distinct dispersion was measured showing in part UDR-type power law dependence. 57Fe Mössbauer spectra consisted of one experimental doublet due to Fe3+; each spectrum could be adequately fitted using one doublet with Lorentzian line shape; there was enhanced line width for x = 0.05, pointing to atomic or nanoscale inhomogeneity. © 2013 Springer Science+Business Media New York. Source


Fehr T.,Ludwig Maximilians University of Munich | Hochleitner R.,Mineralogische Staatssammlung | Laumann A.,Ludwig Maximilians University of Munich | Schmidbauer E.,Ludwig Maximilians University of Munich | Schneider J.,Ludwig Maximilians University of Munich
Physics and Chemistry of Minerals | Year: 2010

Mineralogical analysis, electrical conductivity and thermopower are reported for monocrystalline heterosite (Fe3+, Mn3+)PO4 with the orthorhombic olivine-type structure. The 57Fe Mössbauer spectrum could be adequately described using two Fe3+ doublets. By impedance spectroscopy (20 Hz-1 MHz) the electrical DC conductivity σDC and AC conductivity σAC were determined parallel ({double pipe}) and perpendicular to the [001] direction (space group Pnma) in the range ~160-440 K. The graph log σDC-1/T shows a slightly bent curve in both directions with activation energies of EA ~0.30 and ~0.15 eV in the high and low temperature ranges, respectively. The reduced EA is associated with electronic conduction; σDC {double pipe} [001] follows Mott's T1/4 variable range hopping law at lower temperatures with hopping between localized levels. The values of σAC are increased relative to σDC at high frequencies and low temperatures, obeying Jonscher's universal dynamic response law; for σAC {double pipe} [001], the variation with temperature of the frequency exponent is in fair agreement with the model of small polaron hopping. The absolute thermopower Θ is negative and low between ~295 and ~440 K, Θ does hardly vary with temperatures in both directions; the temperature independency of Θ {double pipe} [001] is consistent with the small polaron hopping model. © Springer-Verlag 2009. Source


Fehr K.T.,Ludwig Maximilians University of Munich | Gunther A.,Ludwig Maximilians University of Munich | Hochleitner R.,Mineralogische Staatssammlung | Schmidbauer E.,Ludwig Maximilians University of Munich
Journal of Electroceramics | Year: 2015

Electrical properties of rutile-type Fe0.9 W0.1TiMO6 (M = Ta,Nb) ceramics were studied. The results are compared to those observed previously on rutile-type relaxor ferroelectric-like FeTiMO6 and Fe0.9 W0.05TiMO6 (M = Ta,Nb) ceramics. In part high dielectric constant ε(ω) (ω is angular frequency) is observed at ≈ 500–600 K, with peak ε(ω) up to ≈ 2.3 × 104 for the frequency of 162 Hz (Ag-paint contacts), decreasing with rising frequency. Some quantities might also be of interest in terms of solid oxide fuel cell anode materials for which similar rutile-type oxide solid solutions were proposed in the literature. Apart from the study of ε(ω), the electrical response is analysed by measuring DC conductivity σDC, AC conductivity σAC(ω), losses ε”(ω) and dissipation factor tan δ, using impedance spectroscopy. For both compounds, the temperature dependence of bulk σDC shows Arrhenius behavior with activation energy of EA ≈ 0.2–0.35 eV and σDC(295 K) ≈ 10−3–10−5 Ω−1 cm−1 depending on preparation conditions and composition; grain boundaries exhibit slightly higher EA, the values of σDC are a factor of up to ≈ 10 lower at all temperatures. From σAC(ω) data, a power law frequency dependence of conductivity by grain boundaries is derived in some instances. Relaxation processes are established from ε”(ω) and tan δ data. Thermopower data reveal different bulk conduction mechanisms for samples prepared in air or reducing atmosphere. Eventual microstructural local inhomogeneities and possible mixed valences Fe2+/Fe3+ are analysed by means of 57Fe Mössbauer spectroscopy. © 2015 Springer Science+Business Media New York Source

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