THERMAL EXPANSION, ELECTROTRANSPORT AND DIELECTRICAL PROPERTIES OF Bi4(Ti,Nb,Fe)3O12 SOLID SOLUTIONS

Abstract

The Bi4Ti3–2xNbxFexO12 (0.05 ≤ x ≤ 0.15) titanates were prepared using ceramic method. Their structure and physicochemical properties were studied. Compounds were p-type semiconductors, which electrical conductivity, Curie temperature, dielectric constant, and dielectric losses decreased, but lattice constants, thermo-EMF coefficient and sinterability did not changed at partial replacing of titanium by niobium and iron. Introduction of niobium and iron oxides into ceramics based on layered Bi4Ti3O12 lead to the sharp decreasing in its grains size. The transition of ceramics from ferroelectric to paraelectric state was accompanied by a stepwise increase in linear thermal expansion coefficient (LTEC) (from (9.63–9.81)·10–6 K–1 to (12.71-14.67)·10–6 K–1 and by decrease in the activation energy of DC electrical conductivity (from 1.13–1.52 eV to 0.72–0.99 eV). Electrical resistivity of ceramics was determined by electrical resistivity of grains, relaxation processes in it were of a non-Debay type, while the activation energy of relaxation for Bi4Ti3–2xNbxFexO12 titanates, found from impedance spectroscopy (1.01–1.05 eV), was close to the value activation energy of their intragrain conductivity (0.85–0.97 eV). The frequency dependences of AC electrical conductivity obeyed Jonscher’s power law σ ~ νn, where n < 1 and increased with temperature, which indicated that charge transfer in the ceramic was accomplished by ions translating over small distances, which was accompanied by small polarons hopping charge transfer.