Structure and Magnetic Properties of a Nanosized Iron-Doped Bismuth Titanate Pyrochlore

The nanosized (50-70 nm) pyrochlore Bi_1.5Fe_0.5Ti₂O_7-δ was prepared by a coprecipitation technique. Characterization of Bi_1.5Fe_0.5Ti₂O_7-δ was carried out by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Raman spectroscopy, Mössbauer spectroscopy, and magnetic susceptibility measurements. The study of Fe doping in Bi₂Ti₂O₇ was performed by means of density functional theory (DFT) calculations. The nanosized Bi_1.5Fe_0.5Ti₂O_7-δ sample crystallizes in the structural type of pyrochlore (Fd3̅m). The distribution of Fe atoms over the sites of Bi and Ti was studied from DFT simulations and then confirmed by the XRD analysis and Mössbauer method. The local distribution, electronic structure, and magnetic behavior of nanosized Bi_1.5Fe_0.5Ti₂O_7-δ are determined by the local microstructure of the metastable nanosized sample. Based on the examination of the Mössbauer spectrum of the Bi_1.5Fe_0.5Ti₂O_7-δ nanopowder, the following states of oxidation were revealed for iron atoms: Fe⁴⁺ in the titanium sites with a fraction of ∼5.7% and two states of Fe³⁺ (in the Bi and Ti sites) with different geometries of the oxygen surrounding. The ratio of Fe³⁺ distributed over the sites correlates well with the distribution in the ceramic sample. The presence of Fe⁴⁺ was found only in the nanosized Bi_1.5Fe_0.5Ti₂O_7-δ. The experimental effective magnetic moment of Fe atoms in the nanosized Bi_1.5Fe_0.5Ti₂O_7-δ appeared noticeably lower than that in the ceramic sample. The temperature dependence of μ_eff within the temperature range of 50-300 K is adequately described by the model of coexistence of Fe³⁺ and Fe⁴⁺ and the existence of clusters.