Structural and electrical properties of the 2Bi2O3·3ZrO2 system

Powder mixtures of α-Bi2O3 (bismite) and monoclinic m-ZrO2 (baddeleyite) in the molar ratio 2:3 were mechanochemically and thermally treated with the goal to examine the phases, which may appear during such procedures. The prepared samples were characterized by X-ray powder diffraction, differential scanning calorimetry (DSC), electrical measurements, as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The mechanochemical reaction leads to the gradual formation of a nanocrystalline phase, which resembles δ-Bi2O3, a high-temperature Bi2O3 polymorph. Isothermal sintering in air at a temperature of 820 °C for 24 h followed by quenching to room temperature yielded a mixture of ZrO2-stabilized β-Bi2O3 and m-ZrO2 phases, whereas in slowly cooled products, the complete separation of the initial α-Bi2O3 and m-ZrO2 constituents was observed. The dielectric permittivity of the sintered samples significantly depended on the temperature. The sintered and quenched samples exhibited a hysteresis dependence of the dielectric shift, showing that the ZrO2-doped β-Bi2O3 phase possess ferroelectric properties, which were detected for the first time. This fact, together with Rietveld refinement of the β-Bi2O3/m-ZrO2 mixture based on neutron powder diffraction data showed that ZrO2-doped β-Bi2O3 has a non-centrosymmetric structure with image as the true space group. The ZrO2 content in the doped β-Bi2O3 and the crystal chemical reasons for the stabilization of the β-Bi2O3 phase by the addition of m-ZrO2 are discussed.