Influence of Jahn-Teller active Mn³⁺ ions on electrical and dielectric properties, thermopower and Mössbauer spectra of rutile-type Fe_1-xMn_xNbTiO₆ (0 ≤ x ≤ 0.9)

Various electrical and dielectric properties were measured on rutile-type compositions Fe_1-xMn_xNbTiO₆ (0 ≤ x ≤ 0.9) between ≈ 100 K and 750 K using impedance spectroscopy. DC conductivity σ_DC shows Arrhenius behavior for the bulk above room temperature (RT) for all x values. Starting from x = 0, there is a considerable change in activation energy E_A and σ_DC(300 K) between x = 0.1 and 0.2 with increase from E_A ≈ 0.3–0.6 eV, accompanied by a fall in σ_DC(300 K) from ≈10⁻⁴ to 10⁻⁸ Ω⁻¹cm⁻¹; for x > 0.2 no considerable further variation with rising x is established. For x ≤ 0.1, below RT Mott's variable range hopping T^−1/4 law is obeyed for σ_DC. The known relaxor-type behavior of the dielectric constant ε′ for x = 0 with very high peaks at 500–600 K for low frequencies (163 Hz-6 kHz) changes with rising x to consecutively lower values in ε′ with disappearance of the peaks and with the largest values at the highest applied temperatures; the relaxor-type behavior can originate from a combination of bulk, grain boundary and sample-electrode effects. Below RT, a dramatic decrease in ε′ is noted for low x values, resulting finally at ≈ 100 K for compositions of any x in the bulk value of ε’ < 15. The thermopower above RT is negative, hence n-type conduction occurs and charge transport is attributed to small polaron hopping. ⁵⁷Fe Mössbauer parameters exhibit some irregularities between x = 0.05 and 0.2, ascribed predominantly to the influence of local distortions by Jahn-Teller active Mn³⁺ ions.