Normal-to-relaxor ferroelectric phase transition and electrical properties in Nb-modified 0.72BiFeO3-0.28BaTiO3 ceramics

Preparation and electrical properties of sintered bodies consisting of monophase cubic spinel oxides, Cr_XMn_1.5Co_(1.0-X)Ni_0.5O₄ (0 ≦ X ≦ 0.42), were investigated. Specimens with compositions within X = 0.42 were prepared as starting materials. The element of Cr was used to exchange Co³⁺ in octahedral sites (B sites) with Cr³⁺ so that the hopping mechanism can be discussed. The sintered bodies with mono cubic spinel structure were confirmed to be prepared by heat-treatment for 48 h in air at 1000 °C to convert them into a cubic spinel structure after sintering at 1400 °C. The semiconductive characteristics of the sintered bodies were determined as p-type because the Seebeck coefficients were all positive. The electrical conduction of the sintered bodies was concluded to be controlled by the small polaron hopping mechanism. In the region 0.1 ≦ X ≦ 0.42, the lattice constant increases and electrical conduction (σ) decreases linearly with increasing Cr concentration. The decrease in σ and the increase in the lattice constant corresponded to the increase in Cr concentration by which the jumping distance of electrons between Mn³⁺ and Mn⁴⁺ is lengthened.