Magnetically ordered relaxor ferroelectric lead iron tungstate and related materials: synthesis, structure properties

摘要

A complete ceramic solid solution between relaxor ferroelectric Pb(Fe2/3W1/3)O3 (PFW) and normal ferroelectric PbTiO3 (PT), [(1-x)PFW-xPT] has been synthesized by a modified B-site precursor method and characterized by X-ray diffraction, differential scanning calorimetry, and dielectric measurements. It was found that, with the increase of PT content, the perovskite structure gradually changes from the pseudo-cubic to a tetragonal phase at room temperature, accompanied by the transformation from the relaxor ferroelectric behaviour of PFW to a normal ferroelectric state in the binary system. A phase diagram between PFW and PT has been established, which displays a morphotropic phase boundary (MPB) within the composition interval 0.25  x 0.35. The electrical transport properties of the polycrystalline dielectric/ferroelectric ceramics of (1-x)PFW-xPT have been studied with the help of complex ac impedance spectroscopic measurements combined with electric modulus formalism analysis. In particular, the resistivity, capacitance (or dielectric constant) and electric modulus at different temperatures have been analysed, and the electric contributions from each microstructural component (phase) have been assigned, which reveals that the ferroelectric behaviour of the system is dominated by the bulk (grain) phase of the ceramic materials. Single crystals of the complex perovskite solid solution (1-x)PFWxPT have been synthesized by the high temperature solution growth method using PbO as flux, and characterized by X-ray diffraction, dielectric and magnetic measurements. The relaxor ferroelectric behaviour of the crystals (x  0.27) was fitted to the Vogel-Fulcher relaxation model. The macroscopic polarization induced under an alternating electric field was investigated and the influence of the PT component on the relaxor and ferroelectric behaviour was revealed in the PFWPT single crystals. Two types of magnetic ordering have been observed in the temperature dependence of magnetization in the crystals with x  0.27. Weak low-temperature ferromagnetism was found to be enhanced by the addition of ferroelectric PT up to x = 0.27. The intrinsic relations between the perovskite structure, composition, magnetic ordering, and ferroelectric relaxation was discussed. The 57Fe-enriched PFW and 0.75PFW0.25PT ceramic materials have been further investigated by Mössbauer spectroscopy. The influences of temperature and PT component on the spectra are discussed.