Electrical properties, thermopower and Mössbauer spectroscopy of rutile-type relaxor ferroelectric-like Fe_0.9 W_0.1TiMO₆ (M = Ta,Nb) ceramics

Electrical properties of rutile-type Fe_0.9 W_0.1TiMO₆ (M = Ta,Nb) ceramics were studied. The results are compared to those observed previously on rutile-type relaxor ferroelectric-like FeTiMO₆ and Fe_0.9 W_0.05TiMO₆ (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 σ_DC, AC conductivity σ_AC(ω), losses ε”(ω) and dissipation factor tan δ, using impedance spectroscopy. For both compounds, the temperature dependence of bulk σ_DC shows Arrhenius behavior with activation energy of E_A ≈ 0.2–0.35 eV and σ_DC(295 K) ≈ 10⁻³–10⁻⁵ Ω⁻¹ cm⁻¹ depending on preparation conditions and composition; grain boundaries exhibit slightly higher E_A, the values of σ_DC are a factor of up to ≈ 10 lower at all temperatures. From σ_AC(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.