Electrical properties and Mossbauer spectra of rutile-type Fex/2Ta(Nb)x/2Ti1-xO2 (x=0.05, 0.1) ceramics

A variety of electrical quantities of rutile-type Fex/2Ta(Nb)(x/2)Ti1-xO2 (x = 0.05, 0.1) ceramics were measured between room temperature and a parts per thousand... 760 K. One aim was to study the dielectric constant epsilon(omega) (omega is angular frequency = 2 pi f) and dielectric dissipation factor tan delta and to compare these quantities with those of rutile-type Inx/2Nbx/2Ti1-xO2 (0 a parts per thousand currency signaEuro parts per thousand x a parts per thousand currency signaEuro parts per thousand 0.1) (INT) ceramics, known to exhibit very high epsilon(omega) and low tan delta, that are believed to originate from a mechanism designated as electron-pinned defect dipole clusters. If a special kind of clusters gives rise to high epsilon(omega) values, Fe3+ at the place of In3+ may lead to a variation of this kind of clusters, acting as electrical dipoles, and result possibly in different epsilon(omega) values; in addition, the processing technique may have a marked effect. For all samples (prepared in air) we observed rather low epsilon(omega) < 100 up to a parts per thousand... 500 K between 162 Hz and 6 kHz and with further rising temperatures an increase up to a parts per thousand... 10(3) at a parts per thousand... 770 K; tan delta < 2 up to a parts per thousand... 550 K increasing up to a parts per thousand... 8 at a parts per thousand... 750 K. The electrical DC conductivity sigma(DC) is low, it exhibits Arrhenius behaviour for the bulk of Fe0.05Ta0.05 T0.9O2 with activation energy of E-A = 1.07 eV and sigma(DC)(500 K) a parts per thousand... 2x10(-8) Omega(-1)cm(-1); the AC conductivity sigma(AC)(omega) shows in part approximately UDR power law frequency dependence. From low epsilon(omega) for all compositions we infer the presence of a different kind of clusters or no clusters at all relative to INT ceramics; as high epsilon(omega) is suggestedly associated with a certain concentration of oxygen vacancies, they may not be present in our case. Samples prepared under less oxidizing condition (CO2) or reducing atmosphere (7%H-2/CO2) exhibit considerably increased sigma(DC) with sigma(DC)(300 K) up to a parts per thousand 10(-5) Omega(-1)cm(-1) and low epsilon(omega); Fe-57 Mossbauer spectra showed in this case the presence of mixed Fe2+/Fe3+ cations.