Structure, Non-stoichiometry, Valence of Ions, Dielectric and Magnetic Properties of Single-Phase Bi0.9La0.1FeO3-δ Multiferroics
The structure, microstructure, valence states, non-stoichiometry, dielectric and magnetic properties of lanthanum-modified multiferroics have been studied by X-ray diffraction, thermogravimetric, iodometric titration, SEM, XPS, dielectric spectroscopy and magnetic methods. Multiferroics of bismuth ferrite have been obtained by a rapid liquid phase sintering method under different pressures, Р, for compacting stoichiometric mixture of precursors. On the basis of the experimental data, the molar formulas of real BiFeO3-δ and Bi0.9La0.1FeO3-δ structures have been determined. The real structure contains Bi3+, La3+, Fe3+, Fe2+ and O2- ions as well as cation V(c) and anion V(a) vacancies. The optimal temperature regimes of the rapid liquid phase sintering method for obtaining the single-phase Bi0.9La0.1FeO3-δ, whose composition corresponds to the concentration region of destruction of a spin cycloid, have been defined. It has been established that oxygen non-stoichiometry δ and concentration of Fe2+ strongly depend on the pressure P. The initial dielectric permittivity of the Bi0.9La0.1FeO3-δ multiferroics can be controlled and changed by the pressure P more than 5000 times. The correlations between the composition, structure, non-stoichiometry, dielectric and magnetic properties in the BiFeO3-δ and Bi0.9La0.1FeO3-δ have been established. In the pure BiFeO3-δ at room temperature, rhombohedral distortions of a hexagonal structure with indications of a ferromagnetic double exchange have been detected. The magnetic structure of the single-phase Bi0.9La0.1FeO3-δ is homogeneous with a large value of coercitivity, НС ≥ 10 kОе, at room temperature. An analysis of the magnetic properties indicates the appearance of a weak ferromagnetism due to the destruction of the spin cycloid by lanthanum ions.